  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.
