Impact dynamics of flexible hydrogels on solid substrates of different wettabilities

TL;DR

This study investigates how flexible hydrogel drops impact solid substrates with different wettabilities, revealing elastic number-dependent behaviors, force scaling laws, and the role of polymer chains in rebound suppression.

Contribution

It provides a comprehensive experimental analysis of hydrogel impact dynamics across a wide elastic number range and substrate wettabilities, with new force scaling laws and insights into rebound mechanisms.

Findings

Impact force scales as El^{0.38} at high elastic numbers.

Maximum spreading is independent of substrate wettability at high El.

Rebound is suppressed by polymer chain anchoring, occurring only when elastic forces overcome adhesion.

Abstract

In this work, we perform experiments with spherical polyacrylamide (PAAm) hydrogel drops/spheres, spanning a broad range of shear moduli and impact velocities on hydrophilic (plasma-treated glass) and hydrophobic (silane-coated) substrates, yielding an elastic number El variation of five orders of magnitude. Transient spreading morphology and impact force were simultaneously resolved using synchronized high-speed imaging and piezoelectric force sensing. At low elastic numbers ($El < 1$), impacting hydrogels exhibit a hybrid poroelastic response: a liquid-rich contact foot is expelled from the polymer network and spreads independently, while the bulk drop undergoes viscoelastic contact-line pinning into a pancake geometry at maximum deformation. At high elastic numbers ($El > 1$), contact foot spreading is suppressed, and deformation is accurately described by a neo-Hookean energy balance, yielding a maximum spreading factor independent of substrate wettability. Further, we show that the normalized peak impact force $F^*$ collapses to a constant value consistent with the Wagner limit for $El < 1$ and follows a power-law scaling $F^* \sim El^{0.38}$ for $El > 1$, in close agreement with both Hertzian and neo-Hookean predictions, and independent of substrate wettability. Furthermore, we highlight that post-impact retraction is suppressed across nearly the entire parameter space due to adsorbed polymer chains anchoring the receding gel network to the substrate, producing circumferential ridge instabilities; rebound occurs only when elastic restoring forces overcome the work of adhesion.