# Liquid Patterning Using Droplet Impact on Textured Nonwetting Surfaces

## TL;DR

This paper introduces a method to control droplet shapes on nonwetting surfaces using textured silicon micropillars, enabling precise patterning for various industrial applications.

## Contribution

A novel approach to manipulate droplet contact shapes on nonwetting surfaces using structured micropillars and a unified analytical model for wetting morphology.

## Key findings

- Droplet contact shapes can be polygonal (square, hexagon, etc.) based on micropillar arrangement and density.
- Inline and staggered micropillar arrangements produce distinct polygonal wetting patterns.
- A unified analytical model accurately predicts droplet wetting and entrapped bubble retraction during impact.

## Abstract

Controlling the shape and contact area
that an impacting droplet
makes with a solid substrate has significant implications in numerous
industrial processes, including inkjet printing and spray cooling.
Here, we report a unique approach that offers an extraordinary ability
to precisely control and manipulate the contact shape of a droplet
impinging on nonwetting well-structured silicon micropillars. Our
experiments show that the wetted Wenzel-type contact area can take
on various polygonal shapes, including square, rectangle, hexagon,
octagon, and dodecagon, depending on the pillar density (diameter-to-spacing
ratio), arrangement (inline versus staggered), and/or the droplet
contact angle. Experiments show that inline pillars give rise to a
square, rectangle, or octagon shape while staggered pillars give rise
to a hexagon, dodecagon, or extended hexagon shape. Rooted in the
fundamentals of contact line physics, we develope a closed form unified
analytical model that accurately captures the steady-state and transient
wetting morphology of the impinging droplet. Furthermore, we show
that the model is applicable for analyzing entrapped bubble retraction
mechanism during high-velocity droplet impact. Lastly, the outcomes
of this study demonstrate the similarity of the shape of the wetted
area induced by droplet impact on nonwetting surfaces with that obtained
via sessile droplet evaporation on wetting surfaces. The shape selection
strategy reported in this study has promising applications in facile
microfabrication of lab-on-a-chip devices, polymer-based printed electronics,
biomicroarrays, and droplet-based electronics thermal management.