Portable Optical Tweezers Kit
Chapter 6: Experiments
Page 66
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6.4. Manipulating a Dairy Cream Particle in a Cream/Water
Emulsion
Mix a drop of dairy cream with sufficient water so that the solution is still slightly milky.
Now, create a finished sample for the optical tweezers as described in Section 6.1, above.
Observe the solution under the optical tweezers.
Exercise
What do you observe? Try to bring a particle into the beam path. What happens? Then,
try to track the particle by changing the z- position. Where is it? Switch the laser off and
observe the particle. What is happening here?
Solution
In this part of the experiment, a sample must first be created out of a cream/water
emulsion. If one attempts to trap the cream particles with the laser, it is observed that they
disappear from the focus and can no longer be clearly seen on the monitor (see Figure
51). The explanation for this observation can be found by examining more closely the
cream/water emulsion. Cream consists primarily of fat, which collects on the surface when
mixed with water. The cream particles, which are trapped by the laser, are therefore
located on the surface of the water. However, the laser focus is located at a deeper level,
so when the cream particles are trapped, they are pulled down into the emulsion. This
effect can be observed when the particle at which the laser is directed is tracked with the
aid of the z-axis screw.
Cream Particles in a Cream / Water Emulsion
If a cream particle, which is located in the optical trap, has been brought into focus with
the z-axis screw and thus can clearly be seen on the monitor, the laser can be switched
off and the particle observed. Switching off the laser causes the cream particle to no longer
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