Figure 1

(a)–(c) ${\theta }_{\mathrm{m}\mathrm{a}\mathrm{x}}$ and ${\theta }_r$ as a function of rotation rate for three different liquid contents. The error bars represent the standard deviation of the observed angle distributions. (d) The maximum angle ${\theta }_{\mathrm{m}\mathrm{a}\mathrm{x}}$, the average avalanche size $⟨\Delta \theta ⟩$, and the width of the avalanche size distribution $2{\sigma }_{\Delta \theta }$ as a function of liquid content. (The data were taken using small beads, $d=0.5\mathrm{m}\mathrm{m}$).Reuse & Permissions

Figure 2

Dynamics of avalanches of different types with grain size $d=0.5\mathrm{m}\mathrm{m}$. (a) Snapshots (at 0.1–0.2 s time intervals) of four single avalanches corresponding to different liquid contents. The third dimension is used for a brightness-coded representation of the local slope ($\alpha$). The green lines indicate approximate slip planes in the correlated regime, and the red line shows the traveling quasiperiodic surface features in the viscoplastic regime. (b) The local slope with the same brightness coding, as a function of space and time. A horizontal line corresponds to a surface profile at a given instant, and time propagates downwards (thus avalanches propagate leftwards and down). The stripes indicate lasting surface features. (c)–(e) The characteristic features of the avalanches averaged over several hundred independent avalanches. The displayed quantities are (c) local surface angle, (d) the rate of change of local height, and (e) the local grain flux (the white lines indicate the point of maximum current). Note that the surface patterns at the highest liquid contents are robust against averaging. The standard deviations of the averaged data sets were typically 5%–25% of the mean value. In some cases, however, the noise originating from taking numerical derivatives leads to standard deviations comparable to the mean. For this reason, averaging over several hundred independent events was absolutely necessary, resulting in standard errors ranging from 0.2%–6%.Reuse & Permissions

Figure 3

The surface velocity $v$ during continuous flow as a function of rotation rate $\omega$ in the granular regime, $\tau =0.005\%$, and in the viscoplastic regime, $\tau =0.25\%$ (large beads, $d=0.9\mathrm{m}\mathrm{m}$). The continuous lines represent power-law fits to the data. The linear behavior for the viscoplastic flow indicates that flow depth is independent of the rotation rate.Reuse & Permissions