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42898-G7
Mobility of Microparticles in Solid-Stabilized Emulsions
During the period of 9/1/2006-8/31/2007, we continued our pioneering work of using Pickering emulsions as a novel experimental model to investigate the mobility of microparticles at liquid-liquid interfaces, which has important physical, chemical, and biological applications. We find that the dynamics of microparticles at oil-water interfaces depend strongly on the oil phase viscosity, particle size, and particle wettability. We have performed the experiments on sulfate-treated polystyrene (S-PS) particles of 0.2 micron and 1.1 micron, carboxylate-treated polystyrene (C-PS) particles of 0.2 micron, and amine-treated polystyrene (A-PS) particles of 0.2 micron. The sulfate-treated polystyrene particles are relatively hydrophobic whereas the carboxylate-treated and amine-treated ones are relatively hydrophilic. The polydimethylsiloxane (PDMS) viscosity changes from 5 to 60K cSt, varying from pure viscous to viscoelastic. The diffusion of the sulfate-treated polystyrene particles at the oil-water interfaces follows reasonably well of the Stokes-Einstein relationship (even the relationship is for particles dispersed in the bulk oil phase) until the oil phase becomes viscoelastic. The elasticity of the oil phase significantly hinders the diffusion of the sulfate-treated polystyrene particles but not the carboxylate-treated and amine-treated ones, likely due to the fact that the former immerse more into the oil phase because their relative hydrophobicity.
After obtaining a good fundamental understanding of the mobility of particles at the oil-water interfaces, we have moved the project to the next level – validating microrheology at liquid-liquid interfaces. Rheology, a study of deformation and flow of matter, is a powerful tool to measure transport properties of materials and of critical importance in various natural and industrial processes. Traditional rheological characterization is performed experimentally using controlled-stress or controlled-strain rheometers. Alternatively, microrheology, the experimental technique based on tracking colloidal particles in materials, has received more and more attentions due to the advantages of minute sample requirement, capability of studying local viscoelastic properties and heterogeneities in the materials, and wide experimental frequency range. So far, particle tracking microrheology technique has been applied only to three-dimension (3-D) bulk systems. Here we focus on developing and understanding microrheology of a model two-dimension (2-D) system, specifically, a liquid-liquid interface. We find that the apparent loss modulus, storage modulus, and relaxation time of the oil-water interfaces obtained from singe-particle microrheology depend strongly on the surface nature of the tracer particles, especially when the oil phase is viscoelastic: experiments using the relatively hydrophilic tracer particles show that the interface is only dissipative without detectable elasticity, whereas experiments using the relatively hydrophobic tracer particles suggest that although the interface is still dominated by the dissipative component the elasticity also plays an important role and prolongs the relation time. One message to be delivered here is that single-particle microrheology of liquid-liquid interfaces is more sensitive to the nature of tracer particles (e.g. hydrophobic/hydrophilic) than is 3-D microrheology. We are currently in the process of developing two-particle interfacial microrheology and comparing with the results from one-particle interfacial microrheology.
The accomplished work significantly contributes to the fundamental understanding of the dynamics of particles at liquid-liquid interfaces and pioneers the development of microrheology at liquid-liquid interfaces. The project has led to four peer-reviewed manuscripts published and two invited journal submission under review. The principle investigator (PI) or the graduate students made 11 presentations, including 4 invited talks, which acknowledged the support of this grant.
