EXPLORATORY: Microrheology of Particulate Gels

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Understanding how interactions, microstructure and microscopic mechanics determinethe macroscopic properties and responses of complex fluids is a fundamental problem thatimpacts materials processing and development, biology and medicine. For particulate gelsthat occur in the manufacture of ceramic parts, coatings, mineral recovery and lubricantdegradation, we seek to understand the microscopic origins of viscoelastic, yield, and non-linear behavior that ultimately affects processing and final properties, such as the thermaland mass transport characteristics. The goal of this research is to explore the use of direct microscopic manipulationand imaging to understand the relationship between microscopic and macroscopic propertiesin particulate gels. The PI will develop and employ new experimental tools based on opticalmicromanipulation and visualization, including optical trapping in combination with video,fluorescence, and confocal microscopies. Because the techniques will allow investigation of thebasic, microscopic mechanisms of rheological and mechanical behavior by quantifying struc-tural rearrangements, stresses and interactions in situ, they are powerful complements toscattering and rheological methods used by other groups.Specific research aims are:1. Directly measure the micromechanical properties of models of the gel back-bone. To understand fundamental mechanisms of gel elasticity, shear and compressiveyield behavior in particulate gels, the PI will develop optical trapping and microscopy tech-niques to measure the bending stiffness, frequency response, and relaxation timescalesof model aggregates of the gel backbone. With videomicroscopy, the PI will characterizeparticle rearrangement mechanisms that are relevant to creep, yield and non-linear be-havior. The PI will systematically vary interparticle interactions, particle concentration,and polydispersity, and characterize surface heterogeneity.2. Measure the mechanical properties of flocs and floc- floc interfaces. Usingthe experimental techniques designed for measuring gel backbone properties, the PI willmeasure the frequency response and rupturing mechanics of flocs formed during theaggregation process. Concurrent videomicroscopy and optical trapping will enable the PIto characterize and correlate mechanics and structural rearrangements in flocs that giverise to strain-hardening in fractal colloidal gels. The PI will measure mechanics betweenflocs to distinguish the contribution of internal and floc- floc mechanics in the rheologyof gels.3. Develop optical trapping and microscopy for dense, gelling suspensions.Establishing the relationship between individual aggregate mechanics, microrheologyand rheology of particulate gels will require micromechanical measurements in bulksuspensions. The PI will develop the appropriate core-shell particles and experimentaltechniques to take advantage of simultaneous optical trapping and confocal microscopyin dense suspensions. This will enable the PI to directly visualize structure while inducinglocal deformation and stresses in the concentrated, bulk gel, ultimately providing ameans of characterizing mechanisms of stress relaxation.