This work has been supported by the Swedish Research Council, Carl Trygger&#180;s Foundation for Scientific Research and the Spanish Ministry of Economy and Competitiveness under the project CICYT DPI2014-55932-C2-2-R. Thanks to Sannarpsgymnasiet for letting us try the RL with students.

NOTE: The laser used in this experiment is a class IV laser delivering up to 1 W of visible laser radiation. All personnel present in the laser laboratory must have conducted adequate laser safety training.
1. Hands-On Experimental Protocol
<ol>
	<li>Safety
	<ol>
		<li>Make sure everyone in the lab is aware that a laser will be turned on.</li>
		<li>Turn on the laser warning lamp in the lab.</li>
		<li>Check that no watch or metal rings are worn and put on the laser goggles.</li>
		<li>Check that the four light absorbing boards, closest to the experiment, are in place.</li>
		<li>Check the space between the laser and the absorbing board for obstacles. Also check that the space between the trapping cell and the beam block is free from objects.</li>
	</ol>
	</li>
	<li>Prepare the software and the experiment.
	<ol>
		<li>Turn on the lab computer. Wait until it is ready to operate.</li>
		<li>Open the Remote Startup folder from the desktop and click the icon Main1806.vi. Run the program by pressing the arrow in the top left corner. 
		NOTE: This opens the control program (e.g., Labview) shown in Figure 2 and Figure 3 and automatically turns on both the power supply for the laser and the electric field. All buttons referenced from now on in this section refer to those that appear in these figures.</li>
		<li>Under &#8220;EJS variables&#8221;, mark the checkbox named &#8220;Laser Remote Enable2&#8221; power and set &#8220;laser current2&#8221; to 25 so that the laser power slide to the right ends up at 25%. Observe the laser beam using alignment laser goggles to make sure that the beam ends up in the beam dump. If not, adjust the position of the beam dump.</li>
		<li>Check Drops2 and move the tip of the droplet dispenser until the droplets are falling into the laser beam. Do this by adjusting the translation stage marked with letter A in Figure 4. For that purpose, gently turn the driving screws at the base of the translation stage until the desired position is reached.
		<ol>
			<li>If no drops are coming, apply some pressure in the syringe until a droplet is shown in the tip of the dispenser. Wipe it off carefully (fragile tip) using a paper with acetone. The droplets should now start coming. When this occurs, start over from point 1.2.4.</li>
		</ol>
		</li>
		<li>Raise the laser power to about 66% using the Laser Current 2 input field and trap a droplet. Uncheck Drops2 as soon as a droplet is trapped. 
		NOTE: Figure 5 shows a droplet captured in the experimental environment. The lower green dot corresponds to the real droplet, while the upper one is its reflection on the glass of the cell in which the droplet is located. From this moment on, it will be The trapped droplet is now imaged onto the PSD.</li>
	</ol>
	</li>
	<li>Determine the size of a droplet.
	<ol>
		<li>Adjust the laser power until the PSD position is as close as possible to zero. 
		NOTE: As droplets can be trapped below or above previous trapping positions, depending on the laser power or the size/weight. This step is performed to move the droplet image to the center of the PSD.</li>
		<li>Observe the diffraction pattern created on the screen (see Figure 1). Take a picture with the web camera that is positioned to observe the screen from underneath. 
		NOTE: The pattern is caused by laser light diffracted by the trapped droplet.</li>
		<li>Use the picture to determine distances from the line marked 1 to two arbitrary minima in the image. The distance is positive if it is further from the droplet than the line marked 1, else negative. Then, add 40 cm to both distances. Call the shortest a1, and the longest a2. Use Equation 1 to calculate the size of the droplet: 
		<img alt="Equation 1" src="/files/ftp_upload/58699/58699eq1.jpg" />(1) 
		where, x is the vertical distance from the droplet to the screen (x&#160;= 23.5 cm), &#955; is the wavelength of the laser light (&#955; = 532 nm) and &#916;n is the number of fringes (integer) between the two minima used in the calculation. 
		NOTE: When the droplet is imaged in the middle of the PSD, the distance (x), from the droplet to the screen is 23.5 &#177; 0.1 cm. A more detailed explanation of the process can be found in the work of J. Swithenbank et al. 6.</li>
	</ol>
	</li>
	<li>Determine the polarity of the charge of the droplet.
	<ol>
		<li>Choose the tab run to the right of EJS variables and set the E-Field DC control2 to +2 V (see Figure 3). Be careful, since the voltage on the electrode is now 200 V. 
		NOTE: The polarity of the droplet charge is determined by observing how the droplet respond to an applied vertical electric field. A sketch of how the electric field is applied can be seen in Figure 6</li>
	</ol>
	</li>
	<li>Determine the charge of the droplet 
	NOTE: To calculate the charge of the droplet, it is necessary first to measure the size of the droplet. The weight of the droplet can then be determined since the density of the liquid is known. Figure 7&#160;describes the procedure schematically.
	<ol>
		<li>Set the E-field DC control2 to zero.</li>
		<li>Estimate and note an average value for the position of the droplet by the PSD Normalize Position trace in the Chart Waveform.</li>
		<li>Note the value of the laser power. This value will be FRad1 in Equation 2.</li>
		<li>Set the E-field DC control2 between +1 and +5 Volts or -1 and -5 Volts so that the drop moves upwards. The droplet is now at a new position. Slowly reduce the laser power until the droplet is back in its original position as noted in Step 1.5.2. Write down the new laser power (FRad2). 
		&#8203;If the droplet is lost, check Drops2 and start over from Step 1.2.4.</li>
		<li>Use the following procedure to calculate the charge. First, calculate the force from the electric field: 
		<img alt="Equation 2" src="/files/ftp_upload/58699/58699eq2.jpg" />(2)</li>
		<li>Determine the absolute charge using the expression 
		<img alt="Equation 3" src="/files/ftp_upload/58699/58699eq3.jpg" />(3) 
		Here d is the distance between the electrodes and U is the applied voltage.</li>
	</ol>
	</li>
</ol>
2. Remote Experimentation Protocol
<ol>
	<li>Access the remote laboratory.
	<ol>
		<li>Open UNILabs webpage on a web browser: https://unilabs.dia.uned.es/</li>
		<li>Select the desired language if needed. The option is found at the first item of the menu under the header.</li>
		<li>Log in with the following data: 
		Username: test 
		Password: test 
		NOTE: The login frame is under the news and introduction info of the webpage.</li>
		<li>In the course area, next to the login area, left click on the logo of the University of Gothenburg (GU).</li>
		<li>Click on Optical Levitation to access the material of this experiment.</li>
		<li>Access the remote laboratory by clicking on Remote Laboratory of Optical Levitation. After that, ensure that the main frame of the webpage show the user interface of the remote laboratory, as shown in Figure 8.</li>
	</ol>
	</li>
	<li>Connect to the Optical Levitation laboratory. 
	NOTE: All the instructions here refer to Figure 8.
	<ol>
		<li>Click on the Connect button. If the connection is successful, the button text will change to Connected. 
		NOTE: When a user connects to the remote laboratory, it emits an acoustic signal that warns other people in the surrounding area that someone will power on and manipulate the laser remotely.</li>
		<li>Click on Tracking droplets and check that the PSD data is being received. 
		NOTE: As there are no droplets captured at this point, the value obtained is not relevant.</li>
		<li>Click on General view to identify all elements of the setup: the laser, the droplet dispenser, the trapping cell and the PSD.</li>
	</ol>
	</li>
	<li>Trap a droplet. 
	NOTE: All the instructions here refer to Figure 8.
	<ol>
		<li>Once the remote laboratory is connected, click on the Trapping droplets button to visualize the pipette and the droplet dispenser nozzle.</li>
		<li>Click on the Turn on laser button to establish the connection to the laser. 
		NOTE: The laser is started manually and independently of the rest of the instruments because it can damage the environment if it is not correctly aligned.</li>
		<li>Set the laser power around the first quarter of the control strip, which is situated under the Turn on laser button. Wait until the green light is visible.</li>
		<li>Check the laser alignment. 
		NOTE: If the laser is correctly aligned, a thin green light beam will be seen. Otherwise, a scattered green spot will be perceived. In case of incorrect alignment, shut down the system, and contact the lab maintenance services. To contact the maintenance services, click on the icon that represents a speech bubble, located in the upper left corner of UNILabs webpage. Then click on the Admin user message, write down the message at the bottom describing the problem and press Send. This usually does not happen, since all the optics are fixed.</li>
		<li>Increase the laser power to 3/4 of the bar. 
		NOTE: A power of 60% (550 mW) is enough to capture and keep a droplet levitated.</li>
		<li>Press the Start drops button to turn on the droplet dispenser.</li>
		<li>Watch the webcam image and wait until a flash is produced. At that moment, a droplet has been captured. Check the webcam image again and verify that a droplet is levitating in the center of the trapping cell. Press the Stop drops button to turn off the droplet dispenser. 
		NOTE: Optionally, it is possible to obtain a larger droplet by catching several of them and waiting for them to merge with the one already captured. It is necessary to bear in mind that if several are caught, the droplet mass increases so that the laser power may not be enough to keep it levitated.</li>
	</ol>
	</li>
	<li>Determine the size of a droplet. 
	NOTE: All instructions here refer to Figure 9.
	<ol>
		<li>Press the Sizing droplets button to observe the diffraction pattern formed by the trapped droplet.</li>
		<li>Follow the same procedure as in the hands-on experimentation protocol (Step 1.3) to determine the size of the droplet by means of the diffraction pattern.</li>
	</ol>
	</li>
	<li>Determining the droplet charge polarity. 
	NOTE: All instructions here refer to Figure 10.
	<ol>
		<li>Click on the Tracking droplets button to view the PSD graph and the webcam view of the pipette.</li>
		<li>Click on the Electric Field tab at the bottom left of the user interface.</li>
		<li>Set the DC voltage to 100 V. To do this, click on the numeric field to the right of the DC (V) label and enter the value 100.</li>
		<li>Check the PSD graph showing the position of the droplet and observe whether the droplet moves upwards or downwards when the electrical field is applied. 
		NOTE: The polarity of the plates is arranged so that if a positive voltage is applied, a negatively charged droplet will move downwards and a positively charged droplet will move upwards.</li>
		<li>Now change the value of the electric field and check that the droplet moves in the opposite direction; for this purpose, enter -100 in the DC (V) numeric field.</li>
	</ol>
	</li>
	<li>Determine the charge of the droplet. 
	NOTE: All instructions here refer to Figure 10.
	<ol>
		<li>Having a droplet trapped, click on the Tracking droplets view.</li>
		<li>Select the Electric Field menu.</li>
		<li>Set the DC electric field to zero with the DC (V) numeric field.</li>
		<li>Estimate and note an average value of the droplet position given by the chart and note the laser power.</li>
		<li>Set the DC electric field to a value between +500 V and -500 V to make the droplet change its position.</li>
		<li>Reduce or increase the laser power with the slider until the droplet is back in its original position and write down the new value of the laser power.</li>
		<li>Follow the procedure described in Step 1.5.5 to calculate the droplet charge.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Ashkin, A., Dziedzic, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+levitation+by+radiation+pressure.">Optical levitation by radiation pressure.</a> Applied Physics Letters. 19, 283-285 (1971).</li><li>Roosen, G., Imbert, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+levitation+by+means+of+two+horizontal+laser+beams:+A+theoretical+and+experimental+study.">Optical levitation by means of two horizontal laser beams: A theoretical and experimental study.</a> Physics Letters. 59 (1), 6-8 (1976).</li><li>Heradio, R., de la Torre, L., Galan, D., Cabrerizo, F. J., Herrera-Viedma, E., Dormido, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Virtual+and+remote+labs+in+education:+A+bibliometric+analysis.">Virtual and remote labs in education: A bibliometric analysis.</a> Computers &amp; Education. 98, 14-38 (2016).</li><li>Isaksson, O., Karlsteen, M., Rostedt, M., Hanstorp, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+optical+levitation+system+for+a+physics+teaching+laboratory.">An optical levitation system for a physics teaching laboratory.</a> American Journal of Physics. 8810, 88-100 (2018).</li><li>Galan, D., Isaksson, O., Rostedt, M., Enger, J., Hanstorp, D., de la Torre, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+remote+laboratory+for+optical+levitation+of+charged+droplets.">A remote laboratory for optical levitation of charged droplets.</a> European Journal of Physics. 39 (4), 045301 (2018).</li><li>Swithenbank, J., Beer, J., Taylor, D., Abbot, D., Mccreath, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+laser+diagnostic+technique+for+the+measurement+of+droplet+and+particle+size+distribution.">A laser diagnostic technique for the measurement of droplet and particle size distribution.</a> 14th Aerospace Sciences Meeting, Aerospace Sciences Meetings. , (1976).</li><li>Christian, W., Esquembre, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modeling+physics+with+easy+java+simulations.">Modeling physics with easy java simulations.</a> The Physics Teacher. 45, 475-480 (2007).</li><li>de la Torre, L., Sanchez, J., Heradio, R., Carreras, C., Yuste, M., Sanchez, J., Dormido, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Unedlabs+-+an+example+of+ejs+labs+integration+into+moodle.">Unedlabs - an example of ejs labs integration into moodle.</a> World Conference on Physics Education. , (2012).</li><li>Chaos, D., Chacon, J., Lopez-Orozco, J. A., Dormido, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Virtual+and+remote+robotic+laboratory+using+ejs,+matlab+and+labview.">Virtual and remote robotic laboratory using ejs, matlab and labview.</a> Sensors. 13, 2595-2612 (2013).</li><li>Lundgren, P., Jeppson, K., Ingerman, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Lab+on+the+web-looking+at+different+ways+of+experiencing+electronic+experiments.">Lab on the web-looking at different ways of experiencing electronic experiments.</a> International journal of engineering education. 22, 308-314 (2006).</li><li>Ivanov, M., Chang, K., Galinskiy, I., Mehlig, B., Hanstorp, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+manipulation+for+studies+of+collisional+dynamics+of+micron-sized+droplets+under+gravity.">Optical manipulation for studies of collisional dynamics of micron-sized droplets under gravity.</a> Optics Express. 25, 1391-1404 (2017).</li><li>Galinskiy, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+of+particle+motion+in+optical+tweezers+embedded+in+a+Sagnac+interferometer.">Measurement of particle motion in optical tweezers embedded in a Sagnac interferometer.</a> Optics express. 23, 27071-27084 (2015).</li><li>Polat, M., Polat, H., Chander, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrostatic+charge+on+spray+droplets+of+aqueous+surfactant+solutions.">Electrostatic charge on spray droplets of aqueous surfactant solutions.</a> Journal of Aerosol Science. 31, 551-562 (2000).</li></ol>

The authors have nothing to disclose.

This work presents a setup for carrying out a modern physics experiment in which droplets are optically levitated. The experiment can be performed either in a traditional hands-on way or remotely. With the remote system establishment, students and researchers all over the world can get access to the experimental set-up. This also guarantees the users&#8217; safety, since they do not need to be in presence of the high-power laser and electric fields required for the experiment. In addition, the users can interact with the...

The fact that light carries momentum was first suggested by Kepler when he explained why the tail of a comet always points away from the sun. The use of a laser to move and trap macroscopic objects was first reported by A. Ashkin and J. M. Dziedzic in 1971 when they demonstrated that it is possible to levitate micrometer sized dielectric objects1. The trapped object was exposed to an upward directed laser beam. Part of the laser beam was reflected on the object which imposed a radiation pressure on it that was sufficient to counterbalance gravity. Most of the light, however, was refracted through the dielectric object. The change of the direction of the light causes a recoil of the object.&#160; The net effect of the recoil for a particle placed in a Gaussian beam profile is that the droplet will move towards the region of highest light intensity2. Hence, a stable trapping position is created in the center of the laser beam at a position slightly above the focal point where radiation pressure balances gravity.
Since the optical levitation method allows small objects to be trapped and controlled without being in contact with any objects, different physical phenomena can be studied using a levitated droplet. However, the experiment presents two limitations to be reproduced and applied at schools or universities since not all institutions can afford the required equipment and since there are certain risks in the hands-on operation of the laser.
Remote laboratories (RLs) offer online remote access to the real laboratory equipment for experimental activities. RLs first appeared at the end of the 90s, with the advent of the Internet, and their importance and use have been growing over the years, as the technology has progressed and some of their major concerns have been solved3. However, the core of RLs has remained the same over time: the use of an electronic device with Internet connection to access a lab, and control and monitor an experiment.
Due to their remote nature, RLs can be used to offer experimental activities to users without exposing them to the risks that may be associated with the realization of such experiments. These tools allow students to spend more time working with laboratory equipment, and hence develop better laboratory skills. Other advantages of RLs are that they 1) facilitate for handicapped people to perform experimental work, 2) expand the catalog of experiments offered to students by sharing RLs between universities and 3) increase the flexibility in scheduling laboratory work, since it can be performed from home when a physical laboratory is closed. Finally, RLs also offer training in operating computer-controlled systems, which nowadays are an important part of research, development and industry. Therefore, RLs cannot only offer a solution to both the financial and safety issues that traditional labs present, but also provide more interesting experimental opportunities.
With the experimental setup used in this work, it is possible to measure the size and charge of a trapped droplet, investigate the motion of charged particles in electric fields and analyze how a radioactive source can be used to change the charge on a droplet4.
In the experimental setup presented, a powerful laser is directed upwards and focused into the center of a glass cell4. The laser is a 2 W 532 nm diode-pumped solid-state laser (CW), where usually about 1 Watt (W) is used. The focal length of the trapping lens is 3.0 cm. Droplets are generated with a piezo droplet dispenser and descend through the laser beam until they are trapped just above the focus of the laser. Trapping occurs when the force from the upward directed radiation pressure is equal to the downward directed gravitational force. There is no upper time limit observed for trapping. The longest time a droplet has been trapped is 9 hours, thereafter, the trap was turned off. The interaction between the droplet and the laser field produces a diffraction pattern which is used to determine the size of the droplets.
The droplets emitted from the dispenser consist of 10% glycerol and 90% water. The water part quickly evaporates, leaving a 20 to 30 &#181;m sized glycerol droplet in the trap. The maximum size of a droplet that can be trapped is about 40 &#181;m. There is no evaporation observed after about 10 s. At this point, all water is expected to have evaporated. The long trapping time without any observable evaporation indicates that there is minimal absorption and that the droplet essentially is at room temperature. The surface tension of the droplets makes them spherical. The charge of the droplets generated by the droplet dispenser depends on the environmental conditions in the laboratory, where they most commonly become negatively charged. The top and the bottom of the trapping cell consists of two electrodes placed 25 mm apart. They can be used to apply a vertical electric direct current (DC) or alternating current (AC) field over the droplet. The electric field is not strong enough to create any arcs even if 1000 volts (V) is applied over the electrodes. If a DC field is used, the droplet moves up or down in the laser beam to a new stable equilibrium position. If an AC field is applied instead, the droplet oscillates around its equilibrium position. The magnitude of the oscillations depends on the size and charge of the droplet, on the intensity of the electric field, and on the stiffness of the laser trap. An image of the droplet is projected onto a position-sensitive detector (PSD), which allows users to track the vertical position of the droplet.
This work presents a successful initiative of modernizing teaching and research using Information and Communication Technologies through an innovative RL on optical levitation of charged droplets which illustrates modern concepts in physics. Figure 1 shows the architecture of the RL. Table 1 shows the possible injuries that lasers can cause according to their class; In this setup, a Class IV laser has been used, which is the most dangerous one. It can operate with up to 2.0 W of visible laser radiation, so the safety provided by the remote operation is clearly suitable for this experiment. The optical levitation of charged droplets RL was presented in the work of D. Galan et al. in 20185. In this work, it is demonstrated how it can be used online by teachers who want to introduce their students to modern concepts of physics without having to be concerned about the costs, the logistics or the safety issues. Students access the RL through a web portal called University Network of Interactive Laboratories (UNILabs - https://unilabs.dia.uned.es) in which they can find all the documentation regarding the theory related to the experiment and the use of the experimental setup by means of a web application. By using the concept of a remote laboratory, experimental work in modern physics that requires costly and dangerous equipment can be made available to new groups of students. Furthermore, it enhances the formal learning by providing traditional students with more laboratory time and with experiments that normally are inaccessible outside research laboratories.

<table>
	<tbody>
		<tr>
			<td>GEM 532</td>
			<td>Laser Quantum</td>
			<td></td>
			<td>Green laser with adjustable power between 50 mW and 2 W</td>
		</tr>
		<tr>
			<td>Lateral Effect Position Sensor</td>
			<td>THOR Lab</td>
			<td>PDP90A</td>
			<td>PSD to sensor the position of the droplet in the pipette</td>
		</tr>
		<tr>
			<td>Advanced Educational Spectrometer Kit, Metric</td>
			<td>THOR Lab</td>
			<td>EDU-SPEB1/M</td>
			<td>Mirrors and other elements to control the laser beam&#160;</td>
		</tr>
		<tr>
			<td>Pipette</td>
			<td>Self made</td>
			<td></td>
			<td>The chamber were the droplet is trapped was specially made for this setup</td>
		</tr>
		<tr>
			<td>AC/DC Power supply</td>
			<td>Keithley Instruments, Inc.</td>
			<td>2380-500-30</td>
			<td>A power supply to generate the electric field (0V - 500V DC)</td>
		</tr>
		<tr>
			<td>Power Distribution Unit</td>
			<td>APC</td>
			<td>AP7900</td>
			<td>A PDU to remotelly connect the lab instrumentation</td>
		</tr>
	</tbody>
</table>

safe experimentation in optical levitation of charged droplets using remote labs

The work presents an experiment that allows the study of many fundamental physical processes, such as photon pressure, diffraction of light or the motion of charged particles in electrical fields. In this experiment, a focused laser beam pointing upwards levitate liquid droplets. The droplets are levitated by the photon pressure of the focused laser beam which balances the gravitational force. The diffraction pattern created when illuminated with laser light can help measure the size of a trapped droplet. The charge of the trapped droplet can be determined by studying its motion when a vertically directed electrical field is applied. There are several reasons motivating this experiment to be remotely controlled. The investments required for the setup exceeds the amount normally available in undergraduate teaching laboratories. The experiment requires a laser of Class 4, which is harmful to both skin and eyes and the experiment uses voltages that are harmful.

When the laser beam is well aligned, and the bottom plate is clean, the drops are almost immediately trapped. When a droplet is trapped it can stay in the trap for several hours, giving plenty of time for investigations. The radius r of the droplets is in the range of 25 &#8804; r &#8804; 35 &#181;m and the charge has been measured between 1.1x10-17 &#177;1.1x10-18 C and 5.5x10-16 &#177;5.5x10-17 C. The size of the droplets stays, accor...

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Safe Experimentation in Optical Levitation of Charged Droplets Using Remote Labs

Optical levitation is a method for levitating micrometer-sized dielectric objects using laser light. Utilizing computers and automation systems, an experiment on optical levitation can be controlled remotely. Here, we present a remotely controlled optical levitation system that is used both for educational and research purposes.

The work presents an experiment that allows the study of many fundamental physical processes, such as photon pressure, diffraction of light or the motion ...

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7.7K Views.  UNED. Optical levitation is a method for levitating micrometer-sized dielectric objects using laser light. Utilizing computers and automation systems, an experiment on optical levitation can be controlled remotely. Here, we present a remotely controlled optical levitation system that is used both for educational and research purposes.

Video: Safe Experimentation in Optical Levitation of Charged Droplets Using Remote Labs

Construction and Testing of Coin Cells of Lithium Ion Batteries

Rechargeable lithium ion batteries have wide applications in electronics, where customers always demand more capacity and longer lifetime. Lithium ion batteries have also been considered to be used in electric and hybrid vehicles1 or even electrical grid stabilization systems2. All these applications simulate a dramatic increase in the research and development of battery materials3-7, including new materials3,8, doping9, nanostructuring10-13, coatings or surface modifications14-17 and novel binders18. Consequently, an increasing number of physicists, chemists and materials scientists have recently ventured into this area. Coin cells are widely used in research laboratories to test new battery materials; even for the research and development that target large-scale and high-power applications, small coin cells are often used to test the capacities and rate capabilities of new materials in the initial stage. 
 In 2010, we started a National Science Foundation (NSF) sponsored research project to investigate the surface adsorption and disordering in battery materials (grant no. DMR-1006515). In the initial stage of this project, we have struggled to learn the techniques of assembling and testing coin cells, which cannot be achieved without numerous help of other researchers in other universities (through frequent calls, email exchanges and two site visits). Thus, we feel that it is beneficial to document, by both text and video, a protocol of assembling and testing a coin cell, which will help other new researchers in this field. This effort represents the "Broader Impact" activities of our NSF project, and it will also help to educate and inspire students.
In this video article, we document a protocol to assemble a CR2032 coin cell with a LiCoO2 working electrode, a Li counter electrode, and (the mostly commonly used) polyvinylidene fluoride (PVDF) binder. To ensure new learners to readily repeat the protocol, we keep the protocol as specific and explicit as we can. However, it is important to note that in specific research and development work, many parameters adopted here can be varied. First, one can make coin cells of different sizes and test the working electrode against a counter electrode other than Li. Second, the amounts of C black and binder added into the working electrodes are often varied to suit the particular purpose of research; for example, large amounts of C black or even inert powder were added to the working electrode to test the "intrinsic" performance of cathode materials14. Third, better binders (other than PVDF) have also developed and used18. Finally, other types of electrolytes (instead of LiPF6) can also be used; in fact, certain high-voltage electrode materials will require the uses of special electrolytes7.

A protocol to construct and test coin cells of lithium ion batteries is described. The specific procedures of making a working electrode, preparing a counter electrode, assembling a cell inside a glovebox and testing the cell are presented.

Rechargeable lithium ion batteries have wide applications in electronics, where customers always demand more capacity and longer lifetime.  Lithium ion ...

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves 2. This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks 3, negative refractive index materials 4, and lenses that resolve objects below the diffraction limit 5. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size 6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response 8. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber 9 with the Taylor-wire process 10, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 &deg;C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 &mu;m diameter indium wires separated by ~100 &mu;m, which exhibit a terahertz plasmonic response.

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam1. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec1, which has serious implications for biological matter2,3.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime4. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement5,6. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres7 and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

The following setup approach details low power optical trapping of dielectric nanoparticles using a double-nanohole in metal film.

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support1,2,3. In this context, metal nanowires are especially desirable for realizing dense routing networks4. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons5 in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires6 randomly distributed on a glass substrate7. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization8,9. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity5,9. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate9,10. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy9,11. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.

Silver nanowires can simultaneously transport electrons and optical information in the form of surface plasmons. A procedure is described here to realize such a shared circuitry and the limitations at propagating both information carriers are evaluated.

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information ...

Experiments on Ultrasonic Lubrication Using a Piezoelectrically-assisted Tribometer and Optical Profilometer

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at a frequency above the acoustic range (20 kHz). As a solid-state technology, ultrasonic lubrication can be used where conventional lubricants are unfeasible or undesirable. Further, ultrasonic lubrication allows for electrical modulation of the effective friction coefficient between two sliding surfaces. This property enables adaptive systems that modify their frictional state and associated dynamic response as the operating conditions change. Surface wear can also be reduced through ultrasonic lubrication. We developed a protocol to investigate the dependence of friction force reduction and wear reduction on the linear sliding velocity between ultrasonically lubricated surfaces. A pin-on-disc tribometer was built which differs from commercial units in that a piezoelectric stack is used to vibrate the pin at 22 kHz normal to the rotating disc surface. Friction and wear metrics including effective friction force, volume loss, and surface roughness are measured without and with ultrasonic vibrations at a constant pressure of 1 to 4 MPa and three different sliding velocities: 20.3, 40.6, and 87 mm/sec. An optical profilometer is utilized to characterize the wear surfaces. The effective friction force is reduced by 62% at 20.3 mm/sec. Consistently with existing theories for ultrasonic lubrication, the percent reduction in friction force diminishes with increasing speed, down to 29% friction force reduction at 87 mm/sec. Wear reduction remains essentially constant (49%) at the three speeds considered.

We present a protocol for using a piezoelectrically-assisted tribometer and optical profilometer to investigate the dependence of ultrasonic wear and friction reduction on linear velocity, contact pressure, and surface properties.

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at ...

Writing Bragg Gratings in Multicore Fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...

The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various characteristic length scales can result in pressure fluctuations over a wide frequency range. This property of turbulence requires sensing devices to have sufficient sensitivity over a wide range of frequencies. Furthermore, the small characteristic length scales of turbulent structures require small sensing areas and the ability to place the sensors in very close proximity to each other. The complex geometries of the solid bodies, often including large surface curvatures or discontinuities, require the probe to have the ability to be set up in very limited spaces. The development of a remote microphone probe, which is inexpensive, consistent, and repeatable, is described in the present communication. It allows for the measurement of pressure fluctuations with high spatial resolution and dynamic response over a wide range of frequencies. The probe is small enough to be placed within the interior of typical wind tunnel models. The remote microphone probe includes a small, rigid, and hollow tube that penetrates the model surface to form the sensing area. This tube is connected to a standard microphone, at some distance away from the surface, using a "T" junction. An experimental method is introduced to determine the dynamic response of the remote microphone probe. In addition, an analytical method for determining the dynamic response is described. The analytical method can be applied in the design stage to determine the dimensions and properties of the RMP components.

Here, we present a protocol to measure, with high spatial resolution, the unsteady surface pressure in turbulent flows. This method demonstrates the construction of a remote microphone probe (RMP) and the determination of its frequency-dependent, complex transfer function. An analytical determination of the dynamic response is presented and validated.

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various ...

Using Synchrotron Radiation Microtomography to Investigate Multi-scale Three-dimensional Microelectronic Packages

Synchrotron radiation micro-tomography (SR&#181;T) is a non-destructive three-dimensional (3D) imaging technique that offers high flux for fast data acquisition times with high spatial resolution. In the electronics industry there is serious interest in performing failure analysis on 3D microelectronic packages, many which contain multiple levels of high-density interconnections. Often in tomography there is a trade-off between image resolution and the volume of a sample that can be imaged. This inverse relationship limits the usefulness of conventional computed tomography (CT) systems since a microelectronic package is often large in cross sectional area 100-3,600 mm2, but has important features on the micron scale. The micro-tomography beamline at the Advanced Light Source (ALS), in Berkeley, CA USA, has a setup which is adaptable and can be tailored to a sample&#39;s properties, i.e., density, thickness, etc., with a maximum allowable cross-section of 36 x 36 mm. This setup also has the option of being either monochromatic in the energy range ~7-43 keV or operating with maximum flux in white light mode using a polychromatic beam. Presented here are details of the experimental steps taken to image an entire 16 x 16 mm system within a package, in order to obtain 3D images of the system with a spatial resolution of 8.7 &#181;m all within a scan time of less than 3 min. Also shown are results from packages scanned in different orientations and a sectioned package for higher resolution imaging. In contrast a conventional CT system would take hours to record data with potentially poorer resolution. Indeed, the ratio of field-of-view to throughput time is much higher when using the synchrotron radiation tomography setup. The description below of the experimental setup can be implemented and adapted for use with many other multi-materials.

For this study synchrotron radiation micro-tomography, a non-destructive three-dimensional imaging technique, is employed to investigate an entire microelectronic package with a cross-sectional area of 16 x 16 mm. Due to the synchrotron&#39;s high flux and brightness the sample was imaged in just 3 min&#160;with an 8.7 &#181;m spatial resolution.

Synchrotron radiation micro-tomography (SR&#181;T) is a non-destructive three-dimensional (3D) imaging technique that offers high flux for fast data ...

Optical Trap Loading of Dielectric Microparticles In Air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.