Capillary fracture of ultrasoft gels: heterogeneity and delayed nucleation

TL;DR

This study investigates the formation of capillary fractures in ultrasoft gels caused by surfactant droplets, revealing variability in fracture timing and pattern, and introduces a new method to probe material toughness and energy landscape.

Contribution

It uncovers the stochastic nature of fracture nucleation in ultrasoft gels and links delay times to elastocapillary length rather than elastic modulus, offering novel insights into fracture dynamics.

Findings

Fracture delay times follow a Weibull-like distribution.

Single fractures are primarily governed by elastocapillary length.

Multiple fractures occur simultaneously, reducing delay variability.

Abstract

A droplet of surfactant spreading on an ultrasoft ($E \lesssim 100$ Pa) gel substrate will produce capillary fractures at the gel surface; these fractures originate at the contact-line and propagate outwards in a starburst pattern. There is an inherent variability in both the number of fractures formed and the time delay before fractures form. In the regime where single fractures form, we observe a Weibull-like distribution of delay times, consistent with a thermally-activated process. The shape parameter is close to 1 for softer gels (a Poisson process), and larger for stiffer gels (indicative of aging). For single fractures, the characteristic delay time is primarily set by the elastocapillary length of the system, calculated from the differential in surface tension between the droplet and the substrate, rather than the elastic modulus as for stiffer systems. For multiple fractures, all fractures appear simultaneously and long delay times are suppressed. The delay time distribution provides a new technique for probing the energy landscape and fracture toughness of ultrasoft materials.