Trapping of fluids in air - photophoresis

The photophoretic effect occurs at absorbing media. In contrast to optical tweezers, in which transparent objects get trapped in a Gausian shaped intensity profile by counterbalancing gradient forces and scattering forces, absorbing objects will be repelled by regions with high intensities. The reason for this phenomenon is the significant higher scattering forces. If an absorbing object is illuminated from one side the absorption at this side leads to an inhomogeneous heating. The illuminated side gets hotter than the not illuminated side. Furthermore the surrounding molecules at the illuminated side of the absorbing object gain a higher velocity and collide more often with the object than molecules at the colder side. The resulting momentum transfer applies a movement away from high towards lower light intensities. The influences of the photophoretic effect acting on absorbing solids are well investigated. To demonstrate the potential to trap also absorbing liquids we developed the photophoretic trampoline, which is an impressing example for applying photophoresis on absorbing liquids.

Photophoretic trampoline

The impact of the photophoretic effect at absorbing solids is well investigated. The transfer of this concept onto absorbing airborne droplets was firstly demonstrated by our group. In contrast to solid absorbing objects droplets suffer from heat transport mechanisms, which are much stronger, compared to solid objects. Convection as well as evaporation leads to a significant reduction of the deposited heat at the illuminated side. At the end the photophoretic effect acting on absorbing liquids is much weaker.

Nevertheless to prove an interaction between high light intensities and absorbing droplets we created a plane horizontal light sheet with moderate intensity. The droplets were generated by utilizing a printer cartridge able to release one droplet with a fixed volume at a certain time. On their way down the droplets pass the light sheet. By slightly increasing the light sheets intensity eventually an interaction can be observed. The interaction is depicted by changing the droplets trajectory after passing the light sheet. At certain intensity the falling droplets bounce on the light sheet. By adjusting the inclination of the light sheet droplets can be sorted by their size, which is an important application.

The approach of the photophoretic trampoline can be extended in that way that absorbing droplets can be completely immobilized. With a suitable beam shaping process a light cage or bottle beam can be created. Such an intensity profile features an area with very low intensity surrounded by an area of very high intensity. Diffractive optical elements (DOE´s) are able to shape the light beam in such an advanced way. In addition we used holographic optical tweezers (HOT) whereby a spatial light modulator was used to shape an incoming light field by applying computer generated holograms (CGH´s) to create several optical traps.