(19f) Visualizing Drying Stresses in Liquid Coatings on Soft Substrates

Coatings are a key component of product design and essential to various manufacturing processes. During the coating process, liquids are deposited onto solid substrates in the form of films or droplets and subsequently dried, causing them to solidify and develop a microstructure. During this solidification, the coating develops a strong adhesion to the substrate and undergoes shrinkage due to capillary forces. This constrained shrinkage due to adhesion builds up stress at the solid-liquid interface leading to a variety of defects such as curling, delamination, and cracking, which are undesirable and influence the quality of the coatings.

In the past decade we have observed rapid advancement in soft electronics for emerging needs in flexible wearable sensors. The fabrication of these soft electronics relies on the deposition of inks on soft materials. Recently, 3D printing has been used to fabricate biomedical devices and sensors directly on the skin and inside the body on soft, compliant organics for patient monitoring and wound treatment. Understanding the stress development on deformable substrates where coated liquids undergo unconstrained shrinkage is vital to the design and optimization of such applications.

In this work we use traction force microscopy (TFM) to visualize and quantify the stress development during drying of aqueous polyvinyl alcohol (PVA) droplets on PDMS elastic substrates. We examine the effect of substrate stiffness on the stress development via the temporal evolution of three-dimensional stress profile during drying. Our findings improve the understanding of stress development during drying and allow us to provide guidelines for the design and optimization of coatings on soft substrates.