Micron-scale droplet deposition on a hydrophobic surface using a retreating syringe

TL;DR

This study investigates droplet deposition on hydrophobic surfaces using a syringe, revealing three distinct regimes based on retraction speed, with experimental and numerical analysis identifying dominant mechanisms and droplet size outcomes.

Contribution

The paper introduces a combined experimental and numerical approach to classify droplet deposition regimes and explains the mechanisms governing droplet size variation.

Findings

Three deposition regimes identified based on retraction speed

Numerical simulations agree with experiments but are limited by contact angle models

Droplet size can be controlled by adjusting syringe retraction speed

Abstract

Droplet deposition onto a hydrophobic surface is studied experimentally and numerically. A wide range of droplet sizes can result from the same syringe, depending strongly on the needle retraction speed. Three regimes are identified according to the motion of the contact line. In Region I, at slow retraction speeds, the contact line expands and large droplets can be achieved. In Region II, at moderate needle speeds, a quasi-cylindrical liquid bridge forms resulting in drops approximately the size of the needle. Finally, at high speeds (Region III), the contact line retracts and droplets much smaller than the syringe diameter are observed. Scaling arguments are presented identifying the dominant mechanisms in each regime. Results from nonlinear numerical simulations agree well with the experiments, although the accuracy of the predictions is limited by inadequate models for the behavior of the dynamic contact angle.