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%