Simulation of light-absorbing microparticles in an optical landscape

Abstract

Simulating the dynamics of active particles play a key role in understanding the many behaviours active matter can exhibit. Experimental studies are more costly than simulations in this regard, as there is much work that needs to be performed with setups and observation time. Computer simulations are a powerful and costeffective supplements to experiments. One topic of study within active matter is light-absorbing microparticles which are commonly made of silica with a lightabsorbing metallic compound such as iron oxide or gold. One such microparticle is the Janus particle, a silica particle with a hemispherical coating of gold as the absorbing compound. When illuminated with a laser, the coating absorbs the light and heats up rapidly, generating a temperature gradient which allows the Janus particle to exhibit self-propulsion and clustering with other Janus particles due to thermophoresis and Brownian motion. In this thesis, I introduce a model which simulates light-absorbing microparticles with a desired distribution of iron oxide in an optical landscape. In particular, the case of an optical landscape characterized by a periodical sinusoidal intensity profile of a given spatial periodicity will be considered. The results show that for a hemispherical distribution (Janus particle) there is selfpropulsion originating at the side of the cap, with super-diffusive characteristics. When the laser periodicity is similar to the particle radius, it becomes confined between two high intensity peaks. A particle with uniform distribution diffuses with Brownian motion, with no self-propulsion. Clustering behaviour arises when multiple particles are in close proximity to each other, as observed in experiments. The agreement with experimental results opens up for the opportunity to simulate other light-absorbing particles with different distributions of absorbing compounds.