Non-spherical particles for optical trap assisted nanopatterning.

Optical trap assisted nanopatterning is a laser direct-write technique that uses an optically trapped microsphere as a near-field objective. The type of feature that one can create with this technique depends on several factors, one of which is the shape of the microbead. In this paper, we examine how the geometry of the bead affects the focus of the light through a combination of experiments and simulations. We realize nanopatterning using non-spherical dielectric particles to shape the light-material interaction. We model the resulting nanoscale features with a finite difference time domain simulation and obtain very good agreement with the experiments. This work opens the way to systematic engineering of the microparticle geometry in order to tailor the near-field focus to specific nanopatterning applications.

Measuring thermal rupture force distributions from an ensemble of trajectories.

Rupture, bond breaking, or extraction from a deep and narrow potential well requires considerable force while producing minimal displacement. In thermally fluctuating systems, there is not a single force required to achieve rupture, but a spectrum, as thermal forces can both augment and inhibit the bond breaking. We demonstrate measurement and interpretation of the distribution of rupture forces between pairs of colloidal particles bonded via the van der Waals attraction. The otherwise irreversible bond is broken by pulling the particles apart with optical tweezers. We show that an ensemble of the particle trajectories before, during and after the rupture event may be used to produce a high fidelity description of the distribution of rupture forces. This analysis is equally suitable for describing rupture forces in molecular and biomolecular contexts with a number of measurement techniques.

Micromechanics of magnetorheological suspensions.

We apply optical trapping techniques in a magnetorheological (MR) suspension, allowing us to directly measure the mechanical properties of single dipolar chains, such as the rupturing tensions and strains under tensile and lateral deformations. Our results are in excellent agreement with calculations of the rupturing tensions using a self-consistent point dipole model of the particle interaction that accounts for induction and multibody effects along the chain. Additionally, we observe the annealing of chain defects under an applied stress, such as the inclusion of neighboring particles into the chain. The micromechanical properties of single chains offers important insight into magnetorheology and electrorheology, especially the yield stress behavior.