Paper Number
SC44 

Session
Suspensions and Colloids

Title
Bond strength in colloidal depletion gels from direct measurement of thermal rupture force distributions

Presentation Date and Time
October 8, 2014 (Wednesday) 5:15

Track / Room
Track 1 / Commonwealth A

Authors (Click on name to view author profile)

1. Whitaker, Kathryn A. (University of Delaware, Chemical and Biomolecular Engineering)
2. Hsiao, Lilian C. (University of Michigan, Chemical Engineering)
3. Solomon, Michael J. (University of Michigan, Chemical Engineering)
4. Furst, Eric M. (University of Delaware, Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Kathryn A. Whitaker¹, Lilian C. Hsiao², Michael J. Solomon², and Eric M. Furst^1
1Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716; ²Chemical Engineering, University of Michigan, Ann Arbor, MI

Speaker / Presenter 
Whitaker, Kathryn A.

Text of Abstract
Stability and shelf life of colloidal depletion gels is industrially relevant for manufacturing materials such as paints, foods, pharmaceuticals, and personal care products. The rheology of gels, including their elastic modulus and yield stress, is a multi-scale function of colloidal interactions and microstructure. We recently formulated a new model depletion gel to directly measure and relate the particle interactions, gel structure, and macroscopic rheology in a single system. Predicting the macroscopic rheology of depletion gels has often relied on theoretical calculations to estimate the interactions. This new model system provides sufficient index of refraction contrast between the particles and the solvent to utilize optical trapping in order to directly measure the interparticle forces between pairs of particles in a dilute suspension. Fluorescent polyhydroxystearic acid (PHSA) stabilized polymethyl methacrylate (PMMA) particles were suspended in a solution of 37% by mass cyclohexane and 63% by mass cyclohexyl bromide (CHB). Non-adsorbing polystyrene depletant (Mw = 900,000 g/mol, c = 0-10 mg/mL, c/c* = 0.0-0.93) was added to the solvent to induce an interparticle attraction. This talk focuses on the direct measurement of colloidal rupture forces that constitute the "bonds" between particles in a gel. Optical tweezers are used to bring a pair of particles into contact and subsequently pull them apart. Thermal rupture force distributions are calculated by averaging many approach and retraction bond rupturing cycles between pairs of particles [1]. The largest forces from each distribution agree well with the forces predicted by a combined DLVO and Asakura-Oosawa potential. Verification of the interparticle potential is a first step toward connecting the microscopic properties with some of the measurable bulk properties of the gels such as the modulus, stability, coarsening, and yield stress. [1] Swan, J., Shindel, M. and Furst, E. M., Phys. Rev. Lett., 109, 198302 (2012).