Figure 1

(a) Schematic diagram of the experimental system. (b) The measured (circles) and theoretical (line) [11] stress-strain relation of polyacrylamide gels in the range of strains over which our experiments were performed yield a shear modulus of $28\mathrm{kPa}\pm 1$.Reuse & Permissions

Figure 2

(a) A typical measurement of the crack velocity, $v$, using both visualization (circles) and the potential drop method, averaged over 0.4 ms (line). (inset) A photograph of a $1.5\times 0.5{\mathrm{mm}}^2$ section of the corresponding fracture surface at the onset of the microbranching instability (dashed line) at $v_c=0.34V_R$. (b) Photographs of branch lines (chains of successive microbranches) on fracture surfaces of a 2 mm thick gel and (inset) soda-lime glass. (c) Comparison of the branch-line scaling in 23% acrylamide and 10% bisacrylamide gels (squares) and glass (circles). In both materials the ratios of $\Delta X$, the distance between successive branches with their thickness, $\Delta Z$, are nearly identically distributed. All cracks shown propagate from left to right.Reuse & Permissions

Figure 3

The front wave velocity [5], $V_{\mathrm{FW}}=v/\cos (\alpha )$, as calculated from measured values of $v$ and $\alpha$ (see inset), normalized by $V_R$. As predicted in 16, $V_{\mathrm{FW}}=0.95V_R$ with a 95% confidence interval of $\pm 0.03$. The inset (of height 0.45 mm) shows typical front waves in gels which, as in glass, are generated by microbranch formation.Reuse & Permissions

Figure 4

(a) The critical velocity, $v_c$, for the onset of microbranches increases systematically with the mean acceleration, $a$, over the range $0.34V_R<v_c<0.75V_R$ (where $0.34V_R$ is the minimum observed value of $v_c$). The slope of a linear fit (line) yields a characteristic “activation” time of $\tau =0.5\mathrm{ms}$. (b) Histogram of the time intervals between $v=0.34V_R$ and $v_c$. The mean value of these activation times, 0.6 ms, is consistent with the value of $\tau$ derived from (a).Reuse & Permissions

Figure 5

The microbranching instability is hysteretic with $v$. Shown is a typical plot of $v$ as a function of the crack length, $L$, as the imposed strain driving the crack is decreased with $L$. Although the onset from a single crack to microbranching occurs at $v_c>1.7\mathrm{m}/\mathrm{s}=0.34V_R$ (dotted line), the reverse transition back to a single-crack state occurred only for $v<0.15v_c$. Despite the fact that $v$ in the shaded region is significantly below $v_c$, the corresponding fracture surface (inset photograph of the 0.5 mm thick surface between $22.1<L<27.2\mathrm{mm}$) shows pronounced microbranching. Note that the velocity fluctuations are caused by the microbranches and are far larger than our measurement error.Reuse & Permissions