How droplets dry on stretched soft substrates

TL;DR

This study explores how stretching soft substrates influences droplet evaporation, revealing anisotropic drying patterns, contact line dynamics, and nanoparticle deposition structures that depend on substrate deformation.

Contribution

It provides new insights into the anisotropic evaporation and deposition behaviors of droplets on stretched soft substrates, linking substrate deformation to pattern formation.

Findings

Droplets form elongated contact lines on stretched substrates.

Contact line recedes faster along the stretching direction.

Nanoparticle deposits become elongated and patterned along specific directions.

Abstract

Liquid droplets usually wet smooth and homogeneous substrates isotropically. Recent research works have revealed that droplets sit, slide and spread anisotropically on uniaxially stretched soft substrates, showing an enhanced wettability and contact line mobility along the stretching direction. This phenomenon arises from the anisotropic deformation of the substrate below the contact line. Here, we investigate how the stretching of soft substrates affects droplets drying. We observe that water droplet evaporates with an elongated non-circular contact line on the stretched substrates and switches the elongation direction during evaporation. The contact line velocity and its temporal evolution depend on the orientation of the contact line relative to the stretching direction. On the substrate stretched by 250%, the contact line recedes about 20% of the droplet lifetime earlier along the stretching direction and faster than its perpendicular direction. When nanoparticles are added into the liquid, the circular deposition pattern, i.e., the so-called coffee-ring, becomes elongated along the direction perpendicular to the stretching direction. Particularly, such non-circular deposition pattern exhibits periodic height gradients along its rim. The finer structure of the pattern can be controlled by applying different stretching ratios to the soft substrate and thus are correlated to the anisotropic surface stresses near the contact line.