Colloidal gels are induced by short-range attraction between particles. Often, they are under shear and/or compressional stress and the gel structure gets broken or compressed. We measured the mechanical properties of the networks induced by depletion attraction, and determine the relationship between the strength of the depletion attraction and the magnitude of the compressional modulus. We also developed a non-linear poroelastic model that quantitatively predicts their deformation kinetics.

Mechanical Properties of Colloidal Gels

We sheared a colloidal glass, and found that the resulting  state of matter shows several striking similarities to that of a supercooled colloidal liquid: i) an MSD that is linear in time at long times, ii) strongly cooperative dynamics, iii ) heterogeneous dynamics for times shorter than the diffusion time but homogeneous dynamics for longer times. In contrast to the situation in a supercooled liquid, however, we find strongly cooperative microscopic dynamics also in the dynamically homogeneous regime for the shear-melted glass. This is a direct effect of the high packing fraction characterizing the glassy state. The highly driven dynamics in the shear-melted glass can occur only by cooperative rearrangements involving several neighbors. Thus, shear-melting albeit similar to the volume fraction controlled glass-liquid transition, leads to an entirely different state of matter.

Dynamics of Shear Melted Colloidal Glasses

We investigated viscoelastic properties of monomolecular layers of polymers on the air/water interface, as probed principally by surface light scattering. The method is a non-invasive one that makes use of spontaneous capillary waves, induced by density fluctuations within liquids under thermal equilibrium.  Thus, the power spectra of scattered light from the interface provide the propagation rate and the damping coefficient. Analysis is based on the resonant mode-coupling of lateral and transverse waves that are recast into the lateral storage modulus and the corresponding loss modulus. By virtue of the two-dimensional character of the monolayers, many intriguing observations have been made with respect to amphiphilic properties and chain architecture of homopolymers and copolymers. Close connection and correspondence between the static properties of polymer monolayers and their rheological behavior have been established.

2D Viscoelasticities of Monolayers at the Air/Water Interface

Dynamic properties of cholesterol-mixed DLPC monolayers at the vacuum/water interface were investigated with full atomic molecular dynamics simulations for 10ns. The effect of cholesterol concentration and the initial distribution on the dynamic properties such as the spinning and wobbling rotational diffusion constants and the lateral diffusion constant were investigated. Anomalous sub-diffusion in the lateral diffusion in the monolayer system was analyzed in terms of fractal dimensions. Therefore, the lipid monolayer system may be described as a partially linked two-dimensional network maintained up to a certain relaxation time. Due to these temporary linkages, lipid molecules diffuse cooperatively, which results in the sub-diffusion in short times. The nature of the linkage may be related to the dipolar interaction between zwitterionic head groups and the intra-molecular tail frictional interaction.

Environmentally Friendly Polyurethane Foams

Molecular Dynamics of Lipid Monolayers at the Air/Water Interface 

An environmentally friendly processing method of polyurethane foam was proposed and evaluated for the application of thermal insulation. For the production of polyurethane foam, chlorofluorocarbon (CFC) gases

were eliminated to minimize environmental destruction. Carbon dioxide gas was used as a blowing agent instead of the CFC gas. Ultrasonic excitation was also applied to the mixture of polyol and diisocyanate to increase the rate of nucleation and decrease the thermal conductivity.

We utilized chemically inert glass capillaries based microfluidic device to generate monodisperse emulsions, colloids, or liposomes. This technique can be used to encapsulation of drug, cell or active biomaterials in lipid vesicle or bio gels for drug delivery and bioreactor applications.

Microfluidics

We synthesized fluorescently-labeled polymer particles in a density- and index- matched organic solvent. These particles can be used to study model atomic systems.

Colloidal particle synthesis

We have produced monodisperse liquid crystal elastomer (LCE) colloidal particles.  The LCE particles are composed of siloxane polymer backbone, mesogen on the side and crosslinkers. LCE materials are provided by the New Liquid Crystal Materials Facility (NLCMF).

Colloidal Liquid Crystal Elastomers