Electric control of droplets in microfluidic devices.
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Biomedical subjects
Zhengdong Cheng
Publications and source records attributed to Zhengdong Cheng.
Crystallization kinetics of thermosensitive colloids probed by transmission spectroscopy.
The kinetics of crystallization of poly-N-isopropylacrylamide (PNIPAM) particles has been investigated using the UV-visible transmission spectroscopy. Since the particle size decreases with the increase in temperature, microgel dispersions of different volume fractions have been obtained by varying the temperature of a single sample. It is found that the rates of the change in crystallinity, the average crystallite size, and the number density of crystallites at the most rapid stage over a certain time interval at various temperatures can be described by the power-law relations. At 19 degrees C, the PNIPAM system behaves as a hard sphere system under microgravity. The hard sphere theory based on Monte Carlo simulation has been used as a reference point to compare with conventional hard spheres, soft spheres, and PNIPAM spheres.
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Periodic oscillation of a colloidal disk near a wall in an optical trap.
Colloidal disks can be stably trapped using optical tweezers. However, when the tweezers press the disk against an opposing wall, we observe an instability leading to periodic motion which we model using coupled nonlinear equations. The resulting "switchback" oscillation involves combined orientational and translational motion of the disk. This observation reveals a new degree of freedom in colloidal architectures, that is, the ability to drive translational motion from a static light field energy source.
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Dynamics of pattern coarsening in a two-dimensional smectic system.
We have followed the coarsening dynamics of a single layer of cylindrical block copolymer microdomains in a thin film. This system has the symmetry of a two-dimensional smectic. The orientational correlation length of the microdomains was measured by scanning electron microscopy and found to grow with the average spacing between +/-1/2 disclinations, following a power law xi2(t) approximately t(1/4). By tracking disclinations during annealing with time-lapse atomic force microscopy, we observe dominant mechanisms of disclination annihilation involving tripoles and quadrupoles (three and four disclinations, respectively). We describe how annihilation events involving multiple disclinations result in similarly reduced kinetic exponents as observed here. These results map onto a wide variety of physical systems that exhibit similarly striped patterns.
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Nature of the divergence in low shear viscosity of colloidal hard-sphere dispersions.
Measurements of the low-shear viscosity eta(o) with a Zimm-Crothers viscometer for dispersions of colloidal hard spheres are reported as a function of volume fraction phi up to 0.56. Nonequilibrium theories based on solutions to the two-particle Smoluchoski equation or ideal mode coupling approximations do not capture the divergence. However, the nonhydrodynamic contribution to the relative viscosity Deltaeta(o) is correlated over a wide range of volume fractions by the Doolittle and Adam-Gibbs equations, indicating an exponential divergence at phi(m)=0.625+/-0.015. The data extend the previously proposed master curve, providing a test for improved theories for the many-body thermodynamic and hydrodynamic interactions that determine the viscosity of hard-sphere dispersions.
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Crystallization kinetics of hard spheres in microgravity in the coexistence regime: interactions between growing crystallites.
The hard sphere disorder-order transition serves as the paradigm for crystallization. However, measurements of the crystallization kinetics for colloidal hard spheres in the coexistence regime are incomplete for early times and are affected by sedimentation. We use time resolved Bragg light scattering to characterize crystal nucleation and growth in a microgravity environment on the space shuttle. In contrast to the classical picture of the nucleation and growth of isolated crystallites, we find substantial coarsening of growing crystallites. We also observe dendritic growth and face-centered cubic as the stable structure.
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