One and two point micro-rheology of hard sphere suspensions

Abstract

The material that is covered in this thesis concerns the calibration and application of a set of optical tweezers to be used for one- and two-point micro-rheology experiments on hard sphere colloidal suspensions. The colloidal suspensions that were used in this study were all quasi-monodisperse density- and refractive index-matched PMMA particles that had a radii, a = 0:90 ± 0:05μm or a = 0:86 ± 0:07 for one-point microrheology experiments and radii a = 0:90 ± 0:05μm or a = 0:133 ± 0:010μm for the two-point micro-rheology experiments. By collecting the forward scattered light from a single optically trapped particle the particle's displacements in time were used to determine passive microviscosity, η(Passive) μ , for colloidal suspension in the range of 0:10 < Ø < 0:57 and comparison with literature data has been made and agreement found. Actively dragging an optically trapped particle through suspensions with volume fractions of the same range has yielded the active microviscosities, η(Active) μ , for both high and low shear regimes, displaying shear thinning behaviour. Comparison to literature data has been made and agreement found as well. Collecting the forward scattered light from two optically trapped particles has been used to determine the cross-correlated motion of the two particles in bare solvent and in suspensions with volume fraction Ø = 0:02. The friction coefficients ξ1;1 and ξ1;2 were extracted from the cross-correlated motion of the particles and agreement was found with theoretical predictions for bare solvent only. The suspensions with volume fraction Ø = 0:02 were found to have a friction coefficient ξ1;1 that was greater than what theory predicted with the suspension with bath particles a = 0:90 ± 0:05μm had the greater magnitude. The magnitude ξ1;2 was found to decrease for the suspension with bath particles of radius a = 0:133 ± 0:010μm and to increase for the suspension with bath particles a = 0:90 ± 0:05μm.