Vikram Prasad

Confocal microscopy of colloids

Colloids have increasingly been used to characterize or mimic many aspects of atomic and molecular systems. With confocal microscopy these colloidal particles can be tracked spatially in three dimensions with great precision over large time scales. This review discusses equilibrium phases such as crystals and liquids, and non-equilibrium phases such as glasses and gels. The phases that form depend strongly on the type of particle interaction that dominates. Hard-sphere-like colloids are the simplest, and interactions such as the attractive depletion force and electrostatic repulsion result in more non-trivial phases which can better model molecular materials. Furthermore, shearing or otherwise externally forcing these colloids while under microscopic observation helps connect the microscopic particle dynamics to the macroscopic flow behaviour. Finally, directions of future research in this field are discussed.

Three-dimensional confocal microscopy of colloids

Confocal microscopy is used in the study of colloidal gels, glasses, and binary fluids. We measure the three-dimensional positions of colloidal particles with a precision of approximately 50 nm (a small fraction of each particles radius) and with a time resolution sufficient for tracking the thermal motions of several thousand particles at once. This information allows us to characterize the structure and the dynamics of these materials in qualitatively new ways, for example, by quantifying the topology of chains and clusters of particles as well as by measuring the spatial correlations between particles with high mobilities. We describe our experimental technique and describe measurements that complement the results of light scattering.

Two-Particle Microrheology of Quasi-2D Viscous Systems

We study the spatially correlated motions of colloidal particles in a quasi-2D system (human serum albumin protein molecules at an air-water interface) for different surface viscosities eta s. We observe a transition in the behavior of the correlated motion, from 2D interface dominated at high eta s to bulk fluid dependent at low eta s. The correlated motions can be scaled onto a master curve which captures the features of this transition. This master curve also characterizes the spatial dependence of the flow field of a viscous interface in response to a force. The scale factors used for the master curve allow for the calculation of the surface viscosity eta s that can be compared to one-particle measurements.

Immersion of charged nanoparticles in a salt solution/air interface.

Electrostatic interactions strongly affect the immersion depth of nanoparticles into an interface. We prove this statement by measuring the diffusion constant of charged nanoparticles at a sodium chloride solution/air interface. Interfacial diffusion of nanoparticles slows down with increasing ionic strength of the sodium chloride solution. Hydrodynamic calculations are used to estimate the immersion depth from the diffusion constant, suggesting that nanoparticles with a carboxylate surface are only slightly immersed into a bare air/water interface. With increasing molarities of sodium chloride, the immersion depth increases to complete immersion for a 10(-2) molar solution. Our experiments show that the location of nanoparticles at interfaces is determined by an intricate interplay between the electrostatic properties of the solution/air interface, the solution/solid interface, and the classical contact angle.

Flow fields in soap films: Relating viscosity and film thickness

We follow the diffusive motion of colloidal particles in soap films with varying h / d, where h is the thickness of the film and d is the diameter of the particles. The hydrodynamics of these films are determined by looking at the correlated motion of pairs of particles as a function of separation R. The Trapeznikov approximation [A. A. Trapeznikov, (Butterworths, London, 1957), p. 242] is used to model soap films as an effective two-dimensional (2D) fluid in contact with bulk air phases. The flow fields determined from correlated particle motions show excellent agreement with what is expected for the theory of 2D fluids for all our films where 0.6 < or = h / d < or = 14.3 , with the 2D shear viscosity matching that predicted by Trapeznikov. However, the parameters of these flow fields change markedly for thick films (h / d > 7 + or - 3) . Our results indicate that three-dimensional effects become important for these thicker films, despite the flow fields still having a 2D character.