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...

<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>

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...

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 ...

Our protocol can be used to visualize many fundamental physical concepts, such as photon pressure or how a charged particle moves in an electric field. Even more, all these tasks can be performed remotely. The main advantage is that we can visualize physical phenomena with our bare eye as the only detector, either directly using laser goggles, or on the computer screen using a web camera.The experimental system can, for instance, provide insight in atmospheric sciences by investigating correlations between liquid droplets or the chemical composition of a droplet using Raman Spectroscopy. The remote access protocol is applicable to a wide range of experiment types. The main experimental challenge is that we use high powered lasers.Therefore, strict laser safety regulations has to be applied. The whole idea with this experiment is to visualize concepts of physics that normally are demonstrated in black box experiment or through theoretical treatments. The experiment will be demonstrated by Oscar Isaksson and Andreas Johansson.They are both sharing their time between teaching in a local high school and doing research aiming for a PH.D.degree. Anytime powerful lasers are involved, safety has to be the top priority. These lasers delivers two Watt of visible laser radiation.Therefore, all personnel has to attend the Laser Safety Course. Begin by informing everyone in the lab area that a laser will be turned on. Then turn on the laser warning lamp inside the laboratory.Next, position the four light-absorbing boards and check that the space between the laser and the absorbing boards is free from obstacles. Also, check that the space between the trapping cell and the beam block is free from objects. Prior to starting the laser, also remove any watches and metal rings, and put on proper eye protection.Now, turn on the lab computer and wait until it is ready to operate. Open the Remote Start-up folder from the desktop and click the icon Main1806. VI.Run the program by pressing the arrow in the top left corner.Under EJS Variables, mark the checkbox named Laser Remote Enable 2 Power and set Laser Current 2 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 dom. If not, adjust the position of the beam dom. Next, check Drops2 in the program VI.Then, adjust the Translation Stage, by turning the driving screws at its base, to move the tip of the droplet dispenser until the droplets are falling into the laser beam.Back in the software, raise the laser power to about 66%using the Laser Current 2 input field to trap a droplet. As soon as a droplet is trapped, uncheck Drops2. After determining the size and polarity of the droplet by following the text protocol, determine the charge of the droplet by multiplying its known size with its density.Next, set the E-Field DC Control 2 to Zero and estimate and note an average value for the position of the droplet by the PSD Normalized Position Trace in the chart Waveform. Calculate the value of the laser power given as F-Read 1 in Equation 2. Now, set the E-Field DC Control 2 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. Write down the new laser power as F-Read 2.To access the Remote Laboratory, open the Unilab's webpage on a web browser. Once connected, select the desired language in the first item of the menu under the header. Then log in using the following data.In the course area, next to the log in area, left-click on the logo of the University of Gothenburg. Then, click on Optical Levitation to access the material of this experiment. Access the Remote Laboratory by clicking on Remote Laboratory of Optical Levitation.After that, ensure that the mainframe of the webpage looks like this and shows the user interface of the Remote Laboratory. Then, click on the Connect button. If the connection is successful, the button text will change to Connected.Next, click on Tracking Droplets and check that the PSD data is being received. Then click on General View to identify all elements of the set up:the laser, the droplet dispenser, the trapping cell, and the PSD. To trap a droplet, first click on the Trapping droplets button to visualize the pipette and the droplet dispenser nozzle.Then, click on the Turn on laser button to establish the connection to the laser. From here, 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.If the laser is correctly aligned, a thin green beam light will be seen. In case of incorrect alignment, please contact the maintenance services as described in the protocol. Once the laser is aligned, increase its power to three quarter of the bar.Then, click on the Start drops button to turn on the droplet dispenser. 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. Then, press the Stop drops button to turn off the droplet dispenser. To determine the size of the droplet, press Sizing droplets and follow the procedure in Section 8 of the accompanying text protocol.To determine the charge for your captured droplet, first click on the Tracking droplets view. Then, select the Electric field menu, and set the DC Electric field to Zero with the DC Voltage numeric field. Using the chart, estimate and note an average value of the droplet position and also note the laser power.Now, set the DC Electric field to a value between or 500 Volts to make the droplet change its position. Once the position changes, modify 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. Finally, use Equation 2 from the accompanying text protocol to calculate the droplet's charge.Shown here is a trapped droplet levitating. It's possible to see one of the droplets levitating inside of the cell of the setup. The green color is due to the laser and the sight of two dots, instead of one, is due to the reflection of the droplet on the glass of the cell.In this case, the upper point is the reflection and the lower point is the droplet. The most important thing when trying to trap is to have patience. A micrometer-sized droplet is supposed to land in a beam of light just slightly wider than the droplet itself.In order to get a more accurate value for the charge, a control loop can be used. While applying an electric field, the control loop will change the laser power until the droplet is back in its original position. Further developments from this experimental setup is to study droplet collisions with high-speed cameras, and also to investigate how the droplet behaves in a high-vacuum chamber.A Class 4 laser is used in this experiment. It is important to take all safety measures to protect the personnel in the lab and the environment.

safe-experimentation-optical-levitation-charged-droplets-using-remote

Research

JoVE Journal

Engineering

7.7K vistas.  UNED. Nuestro protocolo se puede utilizar para visualizar muchos conceptos físicos fundamentales, como la presión de fotones o cómo se mueve una partícula cargada en un campo eléctrico. Aún más, todas estas tareas se pueden realizar de forma remota. La principal ventaja es que podemos visualizar fenómenos físicos con nuestro ojo desnudo como el único detector, ya sea directamente usando gafas láser, o en la pantalla del ordenador usando una cámara web.El sistema experimental pued...