Stretching and squeezing of sessile dielectric drops by the optical radiation pressure

TL;DR

This study numerically investigates how optical radiation pressure from a laser deforms sessile dielectric drops, revealing shape transitions, stability thresholds, and the potential for active droplet manipulation at microscale.

Contribution

It introduces a boundary integral method to analyze steady-state deformations of dielectric drops under optical radiation pressure, extending electrohydrodynamics concepts to optical manipulation.

Findings

Drop shape transitions from prolate to near-conical with increasing radiation pressure.

Above a threshold, optical cones become unstable and disrupt.

Optically squeezed drops form stable concave shapes called optical tori.

Abstract

We study numerically the deformation of sessile dielectric drops immersed in a second fluid when submitted to the optical radiation pressure of a continuous Gaussian laser wave. Both drop stretching and drop squeezing are investigated at steady state where capillary effects balance the optical radiation pressure. A boundary integral method is implemented to solve the axisymmetric Stokes flow in the two fluids. In the stretching case, we find that the drop shape goes from prolate to near-conical for increasing optical radiation pressure whatever the drop to beam radius ratio and the refractive index contrast between the two fluids. The semi-angle of the cone at equilibrium decreases with the drop to beam radius ratio and is weakly influenced by the index contrast. Above a threshold value of the radiation pressure, these "optical cones" become unstable and a disruption is observed. Conversely, when optically squeezed, the drop shifts from an oblate to a concave shape leading to the formation of a stable "optical torus". These findings extend the electrohydrodynamics approach of drop deformation to the much less investigated "optical domain" and reveal the openings offered by laser waves to actively manipulate droplets at the micrometer scale.