Optical Force Measurements In Concentrated Colloidal Suspensions

Abstract

This work concerns the construction and testing of an optical tweezers-based force transducer, and its application to a hard-sphere colloidal system. A particle in an optical trap forward-scatters a fraction of the trapping light, which is collected in order to give high-resolution information on the trapped particle’s position relative to the trap centre. The system is then calibrated to convert particle displacements to forces. The colloid used in this study is a density- and refractive index-matched suspension of PMMA particles, radius 860 ± 70nm, with volume fractions in the range φ = 40 → 62%. Passive microrheological measurements have yielded information about rearrangements in a tracer’s cage of nearest neighbours, as well as highly localised measurements of the high-frequency viscosity, where the presence of the colloidal host causes around a tenfold increase compared to the bare solvent case. Measurements have also demonstrated the effect of sample history on local short-time self-diffusion coefficient, with perturbations caused by translating a particle within the sample taking up to an hour to relax in a φ = 58% sample. The high resolution particle tracking offered by this technique has also allowed for the first measurement of structure at a shorter lengthscale than the ‘dynamic cage size’ observed using other experimental techniques. In addition, active measurements have shown the emergence of a yield stress on the order of 5Pa as the volume fraction approaches the glass transition at φ ≈ 58%.