Levich Institute Seminar Announcement, 09/13/2005

Tuesday, 09/13/2005
2:00 PM
Steinman Hall, Room #312
(Chemical Engineering Conference Room)

Professor Eric Furst
University of Delaware
Chemical Engineering Department

"Laser Tweezer Microrheology and Micromechanics of Colloidal Suspensions "


The micromanipulation and dynametrical capabilities of laser tweezers provide the ability to probe the mechanics and interactions of colloidal materials at nanometer to micrometer lengthscales. I will present two recent applications of laser tweezers to investigate the mechanical properties and microrheology of colloidal suspensions. First, the mechanics of model colloidal aggregates will be discussed. We have developed experimental approaches to measure the micromechanics of aggregates that mimic the stress-bearing backbone of strong particulate gels. Using time-shared and scanning optical traps, backbone mimics are directly assembled from individual particles into precisely-controlled aggregate geometries far from the sample interface. The aggregates are then subjected to mechanical loads up to tens of piconewtons. Our approach enables us to bridge macroscopic rheology of colloidal gels to the underlying microstructural response, and furthermore, provides critical insight into the nanoscale near-contact interactions between particles. Second, I will demonstrate adaptations of these methods to study the microrheology of suspensions using single probe particles in a bath of refractive index-matched particles. Laser trapped probes are subjected to steady uniform flows, enabling measurements of the suspension microviscosity as a function of bath particle volume fraction and flow velocity. The results agree with bulk rheology at low shear rates; however, at high shear rates, the microviscosity exhibits a pronounced shear-thinning behavior. The onset of this non-linear response occurs at lower volume fractions than bulk rheology. Using confocal microscopy and fluorescent PMMA dispersions, we demonstrate that the microrheology is consistent with the development of an anisotropic nonequilibrium pair distribution function between the probe and bath particles with a denser region at the leading surface of the probe and a wake trailing it. Both the non-linear response and underlying microstructure are in good agreement with recent theory [Squires, T. M. and Brady, J. F., Phys. Fluids 17, 073101, 2005.]



Complex fluid rheology, microrheology, optical trapping, colloidal dispersions, tissue engineering