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Discussion on a mechanical equilibrium condition of a sessile drop on a smooth solid surfaceWe present a method for the control of small droplets based on the thermal Marangoni effect using laser heating. With this approach, droplets covering five orders of magnitude in volume (ϳ1.7 L to 14 pL), immersed in decanol, were moved on an unmodified polystyrene surface, with speeds of up to 3 mm/s. When two droplets were brought into contact, they spontaneously fused and rapidly mixed in less than 33 ms. This optically addressed microfluidic approach has many advantages for microfluidic transport, including exceptional reconfigurability, low intersample contamination, large volume range, extremely simple substrates, no electrical connections, and ready scaling to large arrays.

We report 3-D imaging of density in a supersonic expansion using beam-deflection optical tomography. Quantitative high-resolution images with absolute accuracy of 3%, dynamic range of 500:1, and spatial resolution to within a factor of 1.7 of the diffraction limit were produced with a He-Ne laser and simple apparatus. Theory shows that the spatial frequency content of beam-deflection measurements is well suited for tomographic reconstruction. The theory for the diffraction-limited resolution for tomography is presented.

We apply optical manipulation to prepare lipid bilayers between pairs of water droplets immersed in an oil matrix. These droplet pairs have a well-defined geometry allowing use of droplet shape analysis to perform quantitative studies of the dynamics during the bilayer formation and to determine time-dependent values for the droplet volumes, bilayer radius, bilayer contact angle, and droplet centerline approach velocity. During bilayer formation, the contact angle rises steadily to an equilibrium value determined by the bilayer adhesion energy. When there is a salt concentration imbalance between the droplets, there is a measurable change in droplet volume. We present an analytical expression for this volume change and use this expression to calculate the bilayer permeability to water.