Investigating the coupled dynamics of asymmetric microclusters in a photophoretic trap

Sahoo, Kirty Ranjan (2025) Investigating the coupled dynamics of asymmetric microclusters in a photophoretic trap. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Kirty Ranjan Sahoo (20MS014)) 20MS014_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (10MB)

Abstract

Optical tweezers (OT) work e!ciently in liquids due to high viscosity, which naturally damps particle motion. However, trapping particles in air is challenging because of low viscosity and high di”usivity, which require steep potential confinement. Photophoretic forces, arising from temperature gradients on absorbing particles interacting with surrounding gas molecules, o”er a viable alternative. These forces are five orders of magnitude stronger than radiation pressure and enable stable trapping using a loosely focused upward-propagating Gaussian beam that levitates the particle vertically and confines it radially. This thesis investigates the fluctuation dynamics of photophoretically trapped absorbing microclusters in air under both spontaneous and driven conditions. Chapter 1 outlines the basics of photophoresis, types of photophoretic forces, trapping mechanisms, and the Langevin dynamics theoretical framework. Chapter 2 presents experimental studies on the spontaneous dynamics of large, asymmetric carbon microclusters. We observe irregular jumps along the axial (z) direction, leading to a bimodal position distribution and near-ballistic mean squared displacement, mimicking active Brownian motion. Meanwhile, the transverse (x) dynamics remain di”usive. A 2D-Langevin model with experimentally extracted parameters qualitatively captures these behaviours. In the next work, we explore the response of these microclusters to periodic modulation along the x-axis. Due to dynamic coupling between x and z degrees of freedom, axial motion exhibits a resonance peak, while the x response shows flay-to-decaying behaviour with frequency. The z-position distribution transitions from bimodal to Gaussian with increasing frequency. The previously used Langevin-based model, modified to include a sinusoidal driving term in potential, reproduces these experimental trends but highlights the need for improved modelling of irregular kicks. Together, these results reveal complex, frequency-dependent dynamics in photophoretic traps and provide insights for future studies of nonequilibrium systems in air.

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