Photoelastic probing of shear and pressure forces on drop impact

Liquid drops can damage and erode hard surfaces by successive impacts on the same location. Impact after impact, each drop exchanges energy with the surface and removes an unnoticeable amount of material through an unknown mechanism. However, recent drop-on-solid-impact experiments have revealed that the physics inside the solid during the shock is as essential as the fluid dynamics of the impinging drop due to impulsive interaction among the materials.

To better understand the dynamics between drops and solid materials, we analyzed drop impact on photoelastic-material substrates, enabling visualization of the solid's Rayleigh (surface) and shear (volume) waves. The technique reveals oscillating stress patterns traveling outward the target, reminiscent of axisymmetric explosion in semi-infinite media. We also present a theoretical approximation focusing on early times when the force rapidly increases to its maximum and when the largest damage occurs. We argue that erosion is rooted in the combined effect of normal and shear loads. Crater evolution due to drop impact on gypsum substrate is shown for comparison.

Our research opens avenues for probing drop impact via non-direct methods by measuring shear and pressure and for better characterizing the erosion mechanism. The findings hold significance in various fields, ranging from material science to impact engineering, with potential applications in erosion-resistant coatings and impact mitigation strategies.