Parameter exploration of optically trapped liquid aerosols

TL;DR

This study investigates the dynamics of optically trapped liquid aerosols in air, revealing that a simple harmonic oscillator model with Faxén's correction effectively describes their Brownian motion, and explores how experimental parameters influence damping and stability.

Contribution

It provides a parameter study of aerosol Brownian motion near critical damping, demonstrating the applicability of a harmonic oscillator model with Faxén's correction and analyzing how trap parameters affect system behavior.

Findings

System described by harmonic oscillator with Faxén's correction.

Easy transition between over- and under-damped motion via trap parameters.

Stable aerosol trapping possible in under-damped regime.

Abstract

When studying the motion of optically trapped particles on the $\mu s$ time scale, in low viscous media such as air, inertia cannot be neglected. Resolution of unusual and interesting behaviour not seen in colloidal trapping experiments is possible. In attempt to explain the phenomena we use power spectral methods to perform a parameter study of the Brownian motion of optically trapped liquid aerosol droplets concentrated around the critically damped regime. We present evidence that the system is suitably described by a simple harmonic oscillator model which must include a description of Fax\'{e}n's correction, but not necessarily frequency dependent hydrodynamic corrections to Stokes' law. We also provide results describing how the system behaves under several variables and discuss the difficulty in decoupling the parameters responsible for the observed behaviour. We show that due to the relatively low dynamic viscosity and high trap stiffness it is easy to transfer between over- and under-damped motion by experimentally altering either trap stiffness or damping. Our results suggest stable aerosol trapping may be achieved in under-damped conditions, but the onset of deleterious optical forces at high trapping powers prevents the probing of the upper stability limits due to Brownian motion.