Understanding how interactions, microstructure and microscopic mechanics determine the macroscopic properties and responses of complex fluids is a fundamental problem that impacts materials processing and development, biology and medicine. For particulate gels that occur in the manufacture of ceramic parts, coatings, mineral recovery and lubricant degradation, we seek to understand the microscopic origins of viscoelastic, yield, and non- linear behavior that ultimately affects processing and final properties, such as the thermal and mass transport characteristics.
The goal of this research is to explore the use of direct microscopic manipulation and imaging to understand the relationship between microscopic and macroscopic properties in particulate gels. The PI will develop and employ new experimental tools based on optical micromanipulation and visualization, including optical trapping in combination with video, fluorescence, and confocal microscopies. Because the techniques will allow investigation of the basic, microscopic mechanisms of rheological and mechanical behavior by quantifying struc- tural rearrangements, stresses and interactions in situ, they are powerful complements to scattering 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 compressive yield behavior in particulate gels, the PI will develop optical trapping and microscopy tech- niques to measure the bending stiffness, frequency response, and relaxation timescales of model aggregates of the gel backbone. With videomicroscopy, the PI will characterize particle 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. Using the experimental techniques designed for measuring gel backbone properties, the PI will measure the frequency response and rupturing mechanics of flocs formed during the aggregation process. Concurrent videomicroscopy and optical trapping will enable the PI to characterize and correlate mechanics and structural rearrangements in flocs that give rise to strain-hardening in fractal colloidal gels. The PI will measure mechanics between flocs to distinguish the contribution of internal and floc- floc mechanics in the rheology of gels.
3. Develop optical trapping and microscopy for dense, gelling suspensions. Establishing the relationship between individual aggregate mechanics, microrheology and rheology of particulate gels will require micromechanical measurements in bulk suspensions. The PI will develop the appropriate core-shell particles and experimental techniques to take advantage of simultaneous optical trapping and confocal microscopy in dense suspensions. This will enable the PI to directly visualize structure while inducing local deformation and stresses in the concentrated, bulk gel, ultimately providing a means of characterizing mechanisms of stress relaxation.
