Autocorrelation and relaxation time measurements on metal oxide core: Dielectric shell beads in an optical trap

Abstract

Optical Tweezers are capable of trapping individual particles of sizes that range from micrometers to sub micrometers. One can compute the trap strength experienced by a particle by analyzing the fluctuations in the position of the trapped particle with time. It is reported that the trap strength of a dielectric bead increases linearly with increase in the power of the trapping laser. The situation with metallic particles, however, is strongly dependent on the particle size. Available literature shows that metallic Rayleigh particles experience enhanced trap strengths when compared to dielectric particles of similar sizes due to a larger polarizability. On the contrary, micrometer sized metallic particles are poor candidates for trapping due to high reflectivity. We report here that commercially available micrometer sized metal oxide core-dielectric shell (core-shell) beads are trapped in a single beam optical tweezer in a manner similar to dielectric beads. However as the laser power is increased these core-shell beads are trapped with a reduced corner frequency, which represents a lowered trap strength, in contrast to the situation with ordinary dielectric beads. We attribute this anomaly to an increase in the temperature of the medium in the vicinity of the core-shell bead due to an enhanced dissipation of the laser power as heat. We have computed autocorrelation functions for both types of beads at various trapping laser powers and observe that the variation in the relaxation times with laser power for core-shell beads is opposite in trend to that of ordinary dielectric beads. This supports our claim of an enhanced medium temperature about the trapped core-shell bead. Since an increase in temperature should lead to a change in the local viscosity of the medium, we have estimated the ratio of viscosity to temperature for core-shell and dielectric beads of the same size. We observe that while for ordinary dielectric beads this ratio remains a constant with increasing laser power, there is a decrease for core-shell beads. We plan to extend this work towards studying the hydrodynamic correlations between a pair of trapped beads where one of the beads acts as a heat source.