Windmill droplets. Optically induced rotation of biphase oil-in-water droplets

TL;DR

This paper demonstrates the optically induced rotation of biphasic oil-in-water droplets, revealing out-of-equilibrium phenomena and potential applications in microfluidic devices and fundamental optical manipulation principles.

Contribution

It reports the first observation of stable, laser power-dependent rotation of biphasic droplets and introduces a new indirect rotation mechanism via trap arrays.

Findings

Droplets rotate around the laser beam axis with frequency proportional to beam power.

Rotation can be induced indirectly using a circular trap array without direct beam interaction.

Out-of-equilibrium behavior observed in optically trapped biphasic droplets.

Abstract

In the field of microdroplet manipulation, optical tweezers have been used to form and grow droplets, to transport them, or to measure forces between droplet pairs. However, the exploration of out-of-equilibrium phenomena in optically trapped droplets remains largely uncharted. Here, we report the rotation of biphasic droplets fabricated by co-emulsifying two immiscible liquids (i.e., hydrocarbon and fluorocarbon oils) with a refractive index mismatch in water. When trapped, droplets of a specific geometry rotate around the axis of the laser beam, in what appears to be a dissipative, out-of-equilibrium phenomenon. The rotational frequency, obtained from image analysis is stable and proportional to the beam power. Remarkably, droplets that do not interact with the trapping beam can also be rotated indirectly. This is achieved by positioning the droplets at the center of a circular arrangement of multiple, sequentially activated traps. In this case, the droplet orients towards the location of the active trap by a mechanism yet to be understood. Altogether, our results demonstrate out-from-equilibrium phenomenology in optically trapped biphase droplets, which could inspire the development of devices based on them (e.g., optically induced mixing, etc.). In addition, they may shed light on fundamental principles of optical manipulation of asymmetric particles.