Microdroplet oscillations during optical pulling

TL;DR

This paper investigates the oscillations of water microdroplets subjected to optical pulling forces, revealing that shape deformations are jet-like and have microsecond lifetimes, which is crucial for microfluidic manipulation.

Contribution

It provides a linearized model of microdroplet surface oscillations under optical pulling, highlighting the deformation dynamics and force distribution.

Findings

Shape oscillations last microseconds for micrometer-sized droplets.

Force density peaks near the poles, causing jet-like protrusions.

Deformations are described to linear order in the model.

Abstract

It was recently shown theoretically that it is possible to pull a spherical dielectric body towards the source of a laser beam [Nature Photonics {\bf 5}, 531 (2011)], a result with immediate consequences to optical manipulation of small droplets. Optical pulling can be realised e.g.\ using a diffraction free Bessel beam, and is expected to be of great importance in manipulation of microscopic droplets in micro- and nanofluidics. Compared to conventional optical pushing, however, the radio of optical net force to stress acting on a droplet is much smaller, increasing the importance of oscillations. We describe the time-dependent surface deformations of a water microdroplet under optical pulling to linear order in the deformation. Shape oscillations have a lifetime in the order of microseconds for droplet radii of a few micrometers. The force density acting on the initially spherical droplet is strongly peaked near the poles on the beam axis, causing the deformations to take the form of jet-like protrusions.