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&amp;nbsp;</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...

Watch this Scientific Journal Video about Safe Experimentation in Optical Levitation of Charged Droplets Using Remote Labs at JoVE.com

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

Taking Advantage of Reduced Droplet-surface Interaction to Optimize Transport of Bioanalytes in Digital Microfluidics

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of &#8220;lab-on-a-chip&#8221; platforms. Often, droplet motion relies on the wetting of a surface, directly associated with the application of an electric field; surface interactions, however, make motion dependent on droplet contents, limiting the breadth of applications of the technique.
Some alternatives have been presented to minimize this dependence. However, they rely on the addition of extra chemical species to the droplet or its surroundings, which could potentially interact with droplet moieties. Addressing this challenge, our group recently developed Field-DW devices to allow the transport of cells and proteins in DMF, without extra additives.
Here, the protocol for device fabrication and operation is provided, including the electronic interface for motion control. We also continue the studies with the devices, showing that multicellular, relatively large, model organisms can also be transported, arguably unaffected by the electric fields required for device operation.

The protocol for fabrication and operation of field dewetting devices (Field-DW) is described, as well as the preliminary studies of the effects of electric fields on droplet contents.

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of &#8220;lab-on-a-chip&#8221; ...

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 ...

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the Earth's atmosphere. So-called 'solar fuel' devices, currently developed for terrestrial applications in the quest for realizing a sustainable energy economy on Earth, provide promising alternative systems to existing air-revitalization units employed on the International Space Station (ISS) through photoelectrochemical water-splitting and hydrogen production. One obstacle for water (photo-) electrolysis in reduced gravity environments is the absence of buoyancy and the consequential, hindered gas bubble release from the electrode surface. This causes the formation of gas bubble froth layers in proximity to the electrode surface, leading to an increase in ohmic resistance and cell-efficiency loss due to reduced mass transfer of substrates and products to and from the electrode. Recently, we have demonstrated efficient solar hydrogen production in microgravity environment, using an integrated semiconductor-electrocatalyst system with p-type indium phosphide as the light-absorber and a rhodium electrocatalyst. By nanostructuring the electrocatalyst using shadow nanosphere lithography and thereby creating catalytic 'hot spots' on the photoelectrode surface, we could overcome gas bubble coalescence and mass transfer limitations and demonstrated efficient hydrogen production at high current densities in reduced gravitation. Here, the experimental details are described for the preparations of these nanostructured devices and further on, the procedure for their testing in microgravity environment, realized at the Bremen Drop Tower during 9.3 s of free fall.

Efficient solar-hydrogen production has recently been realized on functionalized semiconductor-electrocatalyst systems in a photoelectrochemical half-cell in microgravity environment at the Bremen Drop Tower. Here, we report the experimental procedures for manufacturing the semiconductor-electrocatalyst device, details of the experimental set-up in the drop capsule and the experimental sequence during free fall.

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the ...

Chemistry

Measuring the Interaction Force Between a Droplet and a Super-hydrophobic Substrate by the Optical Lever Method

The goal of this paper is to investigate the interaction force between droplets and super-hydrophobic substrates in the air. A measurement system based on an optical lever method is designed. A millimetric cantilever is used as a force sensitive component in the measurement system. Firstly, the force sensitivity of the optical lever is calibrated using electrostatic force, which is the critical step in measuring interaction force. Secondly, three super-hydrophobic substrates with different grid fractions are prepared with nanoparticles and copper grids. Finally, the interaction forces between droplets and super-hydrophobic substrates with different grid fractions are measured by the system. This method can be used to measure the force on the scale of sub-micronewton with a resolution on the scale of nanonewton. The in-depth study of the contact process of droplets and super-hydrophobic structures can help to improve the production efficiency in coating, film and printing. The force measurement system designed in this paper can also be used in other fields of microforce measurement.

The protocol aims to investigate the interaction between droplets and super-hydrophobic substrates in the air. This includes calibrating the measurement system and measuring the interaction force at super-hydrophobic substrates with different grid fractions.

The goal of this paper is to investigate the interaction force between droplets and super-hydrophobic substrates in the air. A measurement system based on ...

Multicolor Fluorescence Detection for Droplet Microfluidics Using Optical Fibers

Fluorescence assays are the most common readouts used in droplet microfluidics due to their bright signals and fast time response. Applications such as multiplex assays, enzyme evolution, and molecular biology enhanced cell sorting require the detection of two or more colors of fluorescence. Standard multicolor detection systems that couple free space lasers to epifluorescence microscopes are bulky, expensive, and difficult to maintain. In this paper, we describe a scheme to perform multicolor detection by exciting discrete regions of a microfluidic channel with lasers coupled to optical fibers. Emitted light is collected by an optical fiber coupled to a single photodetector. Because the excitation occurs at different spatial locations, the identity of emitted light can be encoded as a temporal shift, eliminating the need for more complicated light filtering schemes. The system has been used to detect droplet populations containing four unique combinations of dyes and to detect sub-nanomolar concentrations of fluorescein.

Multicolor fluorescence detection in droplet microfluidics typically involves bulky and complex epifluorescence microscope-based detection systems. Here we describe a compact and modular multicolor detection scheme that utilizes an array of optical fibers to temporally encode multicolor data collected by a single photodetector.

Fluorescence assays are the most common readouts used in droplet microfluidics due to their bright signals and fast time response. Applications such as ...

Bioengineering

High Throughput Analysis of Liquid Droplet Impacts

Experimental studies of liquid drop impacts on surfaces are often restricted in their scope due to the large range of possible experimental parameters such as material properties, impact conditions, and experimental configurations. Compounding this, drop impacts are often studied using data-rich high-speed photography, so that it is difficult to analyze many experiments in a detailed and timely manner. The purpose of this method is to enable efficient study of droplet impacts with high-speed photography by using a systematic approach. Equipment is aligned and calibrated to produce videos that can be accurately processed by a custom image processing code. Moreover, the file structure setup and workflow described here ensure efficiency and clear organization of data processing, which is carried out while the researcher is still in the lab. The image processing method extracts the digitized outline of the impacting droplet in each frame of the video, and processed data are stored for further analysis as required. The protocol assumes that a droplet is released vertically under gravity, and impact is recorded by a camera viewing from side-on with the drop illuminated using shadowgraphy. Many similar experiments involving image analysis of high-speed events could be addressed with minor adjustment to the protocol and equipment used.

This protocol enables efficient collection of experimental high-speed images of liquid drop impacts, and speedy analysis of those data in batches. To streamline these processes, the method describes how to calibrate and set up apparatus, generate an appropriate data structure, and deploy an image analysis script.

Experimental studies of liquid drop impacts on surfaces are often restricted in their scope due to the large range of possible experimental parameters ...

Aqueous Droplets Used as Enzymatic Microreactors and Their Electromagnetic Actuation

For the successful implementation of microfluidic reaction systems, such as PCR and electrophoresis, the movement of small liquid volumes is essential. In conventional lab-on-a-chip-platforms, solvents and samples are passed through defined microfluidic channels with complex flow control installations. The droplet actuation platform presented here is a promising alternative. With it, it is possible to move a liquid drop (microreactor) on a planar surface of a reaction platform (lab-in-a-drop). The actuation of microreactors on the hydrophobic surface of the platform is based on the use of magnetic forces acting on the outer shell of the liquid drops which is made of a thin layer of superhydrophobic magnetite particles. The hydrophobic surface of the platform is needed to avoid any contact between the liquid core and the surface to allow a smooth movement of the microreactor. On the platform, one or more microreactors with volumes of 10 &#181;L can be positioned and moved simultaneously. The platform itself consists of a 3 x 3 matrix of electrical double coils which accommodate either neodymium or iron cores. The magnetic field gradients are automatically controlled. By variation of the magnetic field gradients, the microreactors' magnetic hydrophobic shell can be manipulated automatically to move the microreactor or open the shell reversibly. Reactions of substrates and corresponding enzymes can be initiated by merging the microreactors or bringing them into contact with surface immobilized catalysts.

Lab-in-a-drop reaction systems allow the versatile implementation of complex reactions in a microfluidic scale. An automated actuation platform consisting of a 3 x 3 matrix of electromagnetic coils was developed and successfully used to merge two 10 &#181;L microreactors and thereby initiate an enzymatic reaction in the resulting liquid marbles.

For the successful implementation of microfluidic reaction systems, such as PCR and electrophoresis, the movement of small liquid volumes is essential. In ...

A Versatile Kit Based on Digital Microfluidics Droplet Actuation for Science Education

This paper describes an educational kit based on digital microfluidics. A protocol for luminol-based chemiluminescence experiment is reported as a specific example. It also has fluorescent imaging capability and closed humidified enclosure based on an ultrasonic atomizer to prevent evaporation. The kit can be assembled within a short period of time and with minimal training in electronics and soldering. The kit allows both undergraduate/graduate students and enthusiasts to obtain hands-on experience in microfluidics in an intuitive way and be trained to gain familiarity with digital microfluidics.

We describe an educational kit that allows users to execute multiple experiments and gain hands-on experience on digital microfluidics.

This paper describes an educational kit based on digital microfluidics. A protocol for luminol-based chemiluminescence experiment is reported as a ...

Induction of Microstreaming by Nonspherical Bubble Oscillations in an Acoustic Levitation System

When located near biological barriers, oscillating microbubbles may increase cell membrane permeability, allowing for drug and gene internalization. Experimental observations suggest that the temporary permeabilization of these barriers may be due to shear stress that is exerted on cell tissues by cavitation microstreaming. Cavitation microstreaming is the generation of vortex flows which arise around oscillating ultrasound microbubbles. To produce such liquid flows, bubble oscillations must deviate from purely spherical oscillations and include either a translational instability or shape modes. Experimental studies of bubble-induced flows and shear stress on nearby surfaces are often restricted in their scope due to the difficulty of capturing shape deformations of microbubbles in a stable and controllable manner. We describe the design of an acoustic levitation chamber for the study of symmetry-controlled nonspherical oscillations. Such control is performed by using a coalescence technique between two approaching bubbles in a sufficiently intense ultrasound field. The control of nonspherical oscillations opens the way to a controlled cavitation microstreaming of a free surface-oscillating microbubble. High-frame rate cameras allow investigating quasi-simultaneously the nonspherical bubble dynamics at the acoustic timescale and the liquid flow at a lower timescale. It is shown that a large variety of fluid patterns may be obtained and that they are correlated to the modal content of the bubble interface. We demonstrate that even the high-order shape modes can create large-distance fluid patterns if the interface dynamics contain several modes, highlighting the potential of nonspherical oscillations for targeted and localized drug delivery.

A fast and reliable technique is proposed to control the shape oscillations of a single, trapped acoustic bubble that is based on coalescence technique between two bubbles. The steady-state, symmetry-controlled bubble shape oscillations allow analysis of the fluid flow generated in the vicinity of the bubble interface.

When located near biological barriers, oscillating microbubbles may increase cell membrane permeability, allowing for drug and gene internalization. ...

Glass-Based Devices to Generate Drops and Emulsions

In this manuscript, three different step-by-step protocols to generate highly monodisperse emulsion drops using glass-based microfluidics are described. The first device is built for the generation of simple drops driven by gravity. The second device is designed to generate emulsion drops in a coflowing scheme. The third device is an extension of the coflowing device with the addition of a third liquid that acts as an electric ground, allowing the formation of electrified drops that subsequently discharge. In this setup, two of the three liquids have an appreciable electrical conductivity. The third liquid mediates between these two and is a dielectric. A&#160;voltage difference applied between the two conducting liquids creates an electric field that couples with hydrodynamic stresses of the coflowing liquids, affecting the jet and drop formation process. The addition of the electric field provides a path to generate smaller drops than in simple coflow devices and for generating particles and fibers with a wide range of sizes.

Here, a protocol to manufacture glass-based microfluidic devices used for generating highly monodisperse emulsions with controlled drop size is presented.

In this manuscript, three different step-by-step protocols to generate highly monodisperse emulsion drops using glass-based microfluidics are described. ...