Investigating the dynamics of asymmetric absorbing microclusters trapped in air using photophoretic forces

Pahi, Anita (2025) Investigating the dynamics of asymmetric absorbing microclusters trapped in air using photophoretic forces. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

This thesis investigates the dynamics of asymmetric absorbing microclusters levitated in air by photophoretic forces. By combining carefully designed optical trapping experiments with phenomenological modeling, we study a series of novel motional features that arise from the interplay between particle asymmetry resulting in anisotropic drag and light-induced forces. Collectively, these results advances photophoretic trapping as a versatile platform for probing and controlling microscopic motion in air. At the core of this work is the observation of rich spontaneous dynamics of trapped microclusters. The trapped microclusters interact with the optical field in ways that give rise to a variety of spontaneous dynamical states. These dynamical states can be broadly classified into two categories: rotating and non-rotating states. Within the rotating category, we identify two distinct types of motion: (i) pure self-rotation of the clusters about their own axis, and (ii) dual-spin dynamics, where self-rotation is accompanied by orbital motion around a fixed point. In contrast, the non-rotating states correspond to clusters that remain stably trapped without exhibiting any angular motion. In both the rotating cases, the rotational dynamics are coupled with oscillations along the beam axis, with rotational frequency inversely correlated to axial oscillation amplitude, where one grows at the expense of other. This difference in rotational states of clusters having the same mass highlights morphology and not mass as the decisive factor in determining motional state. Furthermore, at lower trapping powers, we observe non-rotating states dominated by thermal fluctuations, pointing to a transition between deterministic and stochastic regimes. These findings demonstrate that particle geometry and beam configuration can be harnessed to engineer specific dynamical modes, opening pathways to the realization of optically driven micromachines in air. Beyond these rotational and orbital states, we reveal the coexistence of active and diffusive dynamics in single large asymmetric microclusters which are non-rotating. Along the longitudinal axis, parallel to beam, there occur irregular stochastic kicks originating from the interplay between gravity and photophoretic forces, producing bimodal position distributions and near-ballistic mean-squared displacement scaling. Transverse motion, by contrast, shows conventional diffusive dynamics. To describe this dichotomy, we introduce a two-dimensional Langevin model incorporating anisotropic drag and an additional stochastic force along the longitudinal direction. Our work represents, to the best of our knowledge, the first attempt to use photophoretically trapped systems into a stochastic dynamical framework, and reproduces the essential experimental features with qualitative accuracy. The work also establishes a new platform for studying non-equilibrium statistical mechanics in air, where abrupt, fluctuation-driven events fundamentally shape the dynamics. Finally, we extend these insights to driven dynamics by externally modulating the trap. A striking finding is that driving the particle along one axis (transverse to the beam propagation direction) induces pronounced oscillatory motion along the orthogonal axis (longitudinal to the beam), often with greater amplitude than in the driven direction itself. This motional coupling arises from anisotropic drag coefficients, which leads to overdamped motion along one axis while allowing large resonance-like responses in the other. The position probability distribution function along longitudinal direction undergoes a clear frequency-dependent transition – from bimodal at low driving frequencies to Gaussian at high frequencies – demonstrating controllable switching between coherent and stochastic regimes. Unlike the spontaneous behaviors of the first two studies, this work shows that inherent couplings can be deliberately exploited for active control, offering a handle to tune system dynamics via external modulation. Taken together, the studies presented here provide a coherent picture of the spontaneous and driven dynamics of photophoretically trapped microclusters. They highlight how asymmetry acts both as a source of complex motion and as a lever for control, establishing photophoretic trapping as a powerful experimental platform for studying out-of-equilibrium phenomena in air. Beyond their fundamental importance, these findings suggest promising applications in designing active micromachines, fluctuationdriven devices, and Brownian engines operating with efficiencies beyond equilibrium limits.

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