Research

We have a wide range of research area and interests in the space of colloidal science and emulsion polymer. Here we list a subset of research thrusts.

Synthesizing large-sized polystyrene colloidal particle

Highly monodisperse polystyrene (PS) microspheres in the size range of 1 to 10 μm were synthesized. The particle size distribution (PSD) of the these microspheres obtained was investigated. The PS microspheres with coefficient of variation (CV) values all less than 10% analyzed by scanning electron microscopy.

Synthesizing sub-micron colloidal particles by emulsion polymerization

Uniform polymer particles are synthesized by emulsion polymerization. This technique is able to adjust particle diameters (50 - 300 nm), glass transition temperatures (-50 to 100 degree Celsius), and volume fractions (< 50%). Part of EPI's latex library is listed here.

Active Brownian Particles for Active Colloidal Suspension

Active matter systems are made up of units that transfer other type of energy into mechanical work. Many new collective behaviors were found in these system, such as swarming, clustering, and motility induced phase separation. By studying the behaviors that emerge, new frameworks of statistical physics of out-of-equilibrium systems can be built.

In our lab, we study the collective behavior of two dimensional dynamic clustering by induced-charge electrophoresis driven active particles. we also study the Non-Boltzmann statistical physics of an active particle in optical trap.

Investigating the Internal Structure Development of a Drying Latex Film

Latex is a colloidal system of polymer particles suspended in water. During drying of latex, particles distribute inhomogeneously in space, compromising the final quality of the coating. To understand the mechanism of drying inhomogeneity, we integrate Optical Coherence Tomography (OCT) with gravimetric and video analysis (called “OCT-Gravimetry-Video” method) to monitor the internal structure, the water evaporation rate and the appearance of a latex coating simultaneously. Hard polystyrene particles with two different particle sizes (135 nm and 53 nm in diameter) were used as model latexes. Cracks and drying fronts were recorded by video. Packing of particles, debonding of film and shear bands were observed in reflection image and speckle variance image of OCT. Drying rate was constant and close to that of deionized water before cracking or debonding. The effects of particle size on the packing and cracking phenomena were profound and unexpected. This method provides a non-invasive and non-destructive approach to study the drying process of latex, especially with the OCT imaging on the cross-section of coating that traditional methods can hardly achieve.

Probing Stress and Strain Distribution in Drying Latex Films

Stress relaxation of drying colloids attracts our interest because the final mechanical properties of materials depend on colloidal structure. We use oil droplets as probes to visualize how local stress develops in the colloids near close packing. The colloids are condensed due to uniaxial evaporation, and the drying pressure compresses both colloids and oil droplets. The deformation of oil-colloids composite is observed under confocal fluorescence microscopy. During slow drying process, viscosity of the colloids is negligible, and the composite is under a hydrostatic state. For each moment, surface tension of oil-suspension interface and elasticity of the colloids are assumed to balance. The surface tension is measured separately and assumed as a constant during evaporation because we add saturated surfactant to the colloids. By knowing surface tension, strain of the composite, and packing fraction, the osmotic pressure of colloids is obtained. We find the pressure measured for poly (methyl methacrylate) colloids agrees with hard-sphere theory. We hope to apply the oil-colloids composite to study stress relaxation in soft colloids which cannot be predicted by theories.

Page updated

Report abuse