Theory of microdroplet and microbubble deformation by Gaussian laser beam

TL;DR

This paper develops a theoretical framework for understanding how Gaussian laser beams deform fluid microdroplets and microbubbles, highlighting differences due to fluid properties and beam parameters, with implications for optical manipulation.

Contribution

It presents a comprehensive theory for linear deformation of fluid microparticles by Gaussian laser beams, comparing different fluid systems and analyzing dynamic responses.

Findings

Droplets deform more than bubbles due to lensing effects.

Optical contrast significantly affects deformation shapes.

Surface oscillations are underdamped for water droplets, overdamped for oil-emulsions.

Abstract

The theory for linear deformations of fluid microparticles in a laser beam of Gaussian profile is presented, when the beam focus is at the particle center as in optical trapping. Three different fluid systems are considered: water microdroplet in air, air microbubble in water, and a special oil-emulsion in water system used in experiments with optical deformation of fluid interfaces. We compare interface deformations of the three systems when illuminated by a wide (compared to particle radius) and narrow laser beams and analyse differences. Deformations of droplets are radically different from bubbles under otherwise identical conditions, due to the opposite lensing effect (converging and diverging, respectively) of the two; a droplet is deformed far more than a bubble, cetera paribus. Optical contrast is found to be of great importance to the shape obtained when comparing the relatively low-contrast oil-emulsion system to that of water droplets. We finally analyse the dynamics of particle motion when the laser beam is turned on, and compare a static beam to the case of a short pulse. The very different surface tension coefficient implies a very different time scale for dynamics: microseconds for the water-air interface and tens of milliseconds for the oil-emulsion. Surface oscillations of a water microdroplet are found always to be underdamped, while those of the oil-emulsion are overdamped; deformations of a microbubble can be either, depending on physical parameters.