Microdroplet apparatus

Ashkin and Dziedzic (1977) used the radiation pressure force of a laser beam to levitate microdroplets with the apparatus presented in Fig. 15. A polarized and electro-optically modulated laser beam illuminated the particle from below. The vertical position of the particle was detected using the lens and split photodiode system shown. When the particle moved up or down a difference signal was generated then a voltage proportional to the difference and its derivative were added, and the summed signal used to control an electro-optic modulator to alter the laser beam intensity. Derivative control serves to damp particle oscillations, while the proportional control maintains the particle at the null point. 

D.B. Wallace, Method and apparatus for forming microdroplets of liquids at elevated temperatures. U.S. patent 5,415,679, 1995. 

Surface-induced layering corresponds to oscillations of the disjoining pressure in the molecular range of thickness (see also chapter 1). Well-known saw-tooth force profiles are obtained with surface forces apparatus (SFA) setups. In the case of microdroplets, where the thickness is free to adjust, an unstable part in the disjoining pressure behaves just like the usual pressure at three dimensions a Maxwell construction leads to a horizontal part in n(f), corresponding to the coexistence of films with different thickness (see chapter 3). 

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