First passage analysis of a self-propelled particle moving through a medium of passive crowders

What is it about?

We performed experiments to study the stochastic transport of a self-propelled camphor boat, driven by Marangoni forces (forces due to surface tension gradient), through a crowd of passive paper discs floating on water. We analyze the statistics of the first passage times of the active particle to travel from the center of a circular container to its boundary. The mean first passage times rise monotonically as a function of the covered area fraction of the passive paper discs, but the fluctuations in first passage times show a non-monotonic behavior – being higher at low and high value of the packing fraction of passive crowders, compared to intermediate values. The reason is traced to an interplay of two distinct sources of fluctuations – one intrinsic to the dynamics of the camphor boat, while the other due to the crowding of passive particles.

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Why is it important?

In the biological context of living cells, active components often have to move from one part of the cell to another. They have to move amidst many other passive, and active cellular components. Often reaching a target zone for the first time may be of interest. Motivated by such general situations, we have done experiments on a simple representative physicochemical system. We study a chemically active camphor particle that travels through a crowd of passive paper discs, and we characterize the statistics of stochastic times involved in the process of transport from the center to the periphery of a confined region. The fields of `active transport' and `first passage' are both of contemporary interest; this work is an instance that combines the two. The first passage of active particles has got recent theoretical attention, while experiments on this topic had been lacking.

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