# Optical manipulation for studies of collisional dynamics of micron-sized droplets under gravity

## TL;DR

This paper introduces a novel optical trapping and imaging method to study the collisional dynamics of micron-sized droplets under gravity, enabling precise control and high-speed visualization of droplet collisions.

## Contribution

The authors develop a new experimental technique combining optical trapping, electro-optical release, and high-speed imaging to study droplet collisions without optical confinement effects.

## Key findings

- Precise control of droplet impact parameters.
- High-speed imaging at up to 63 kHz.
- Potential for detailed microphysics studies.

## Abstract

A new experimental technique for creating and imaging collisions of micron-sized droplets settling under gravity is presented. A pair of glycerol droplets is suspended in air by means of two optical traps. The droplet relative velocities are determined by the droplet sizes. The impact parameter is precisely controlled by positioning the droplets using the two optical traps. The droplets are released by turning off the trapping light using electro-optical modulators. The motion of the sedimenting droplets is then captured by two synchronized high-speed cameras, at a frame rate of up to 63 kHz. The method allows the direct imaging of the collision of droplets without the influence of the optical confinement imposed by the trapping force. The method will facilitate efficient studies of the microphysics of neutral, as well as charged, liquid droplets and their interactions with light, electric field and thermodynamic environment, such as temperature or vapor concentration.