Osmotic Equilibrium of Colloidal Suspensions under Potential Forces
Abstract
Colloidal particles in suspension reach an equilibrium density distribution under external potential forces, such as gravity. In the absence of particle interactions the density distribution is known to follow a Boltzmann distribution of the potential energy profile. In the presence of particle interactions, however, the particle number density profile depends on the details of the colloidal interactions between the particles. According to the equation developed by Einstein for the osmotic equilibrium of a colloidal suspension under a potential force, the colloidal equation of state can be determined from the density distribution if the potential force profile is known. Based on this principle, Jean-Baptist Perrin used the equilibrium sedimentation density distribution of the colloidal particles under gravity to determine the Avogadro number and won the Nobel Prize for Physics in 1926. While sedimentation under gravity is easy to understand conceptually, the process for the particle distribution to attain equilibrium is very slow; it could take more than a few days for nano-sized particles to reach sedimentation equilibrium. In comparison, it is relatively fast and easy to determine colloidal equation of state by the use other known potential forces, such as the optical gradient forces or dielectrophoresis, to create density distributions.
This talk will begin with an introduction of Einstein’s theory and a brief history of Perrin’s experiment. The speaker will then demonstrate how one could use the their ideas and take advantage of the advanced optical imaging techniques to determine colloidal osmotic equation of state by external potential forces produced by electromagnetic fields to determine the colloidal equation of state as well as to predict phase transitions when the colloidal interactions are varied by changing ionic screening or by polymer-induced depletion attraction. Possibilities of extending Einstein’s original idea for equilibrium systems to non-equilibrium systems such as a suspension of active particles, or the drying process of colloidal particles during film formation will also be briefly discussed.
About the speaker
Prof. Daniel Ou-Yang received his PhD in physics from University of California at Los Angeles in 1985. He was a postdoctoral fellow at the University of Pennsylvania and Exxon Corporate Research Laboratory from 1985 to 1988. He joined Lehigh University as an assistant professor of physics in 1988, and became full professor in 2000. Currently, he is also Director of Lehigh’s Emulsion Polymers Institute. In 1999-2000, he worked as a visiting research scientist at the CNRS Laboratory for Dynamics of Complex Fluids in France.
Prof. Ou-Yang conducts research in experimental soft condensed matter physics that spans the topics of polymers, colloids and cellular biophysics. His research activities include the studies of ultrasound induced reorientation of gold nanodisks, polymer adsorption at colloidal surfaces, complexation of cyclodextrin with hydrophobic molecules in aqueous environment, structure and interactions of colloidal particles, active microrheology of polymer solutions and gels, quantitative study of electrophoresis and dielectrophoresis; in vitro cytoskeleton networks and biological cells at subcellular levels. To conduct experimental research, his laboratory develops novel capabilities of oscillatory optical tweezers and optical bottles and combines optical manipulation capabilities with advanced optical fluorescence imaging and spectroscopic techniques for understanding the interactions and dynamics of colloidal particles and biological materials at nano-meter scale.
