Safe Experimentation in Optical Levitation of Charged Droplets Using Remote Labs

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.