Figure 3. Ridge-growth dynamics and its effect on cusp formation.

(a) Representative sequential snapshots of a cusp during ridge growth for a EG 40% drop on a silicone gel (E≈3 kPa and h≈50 μm; Supplementary Movie 1). The cusp was instantly recovered (red arrow) right after depinning at 181 s (grey dotted line). Scale bar, 2 μm. (b) (Top) The ridge height u_z(0) increases at a constant rate of ~7 nm s⁻¹ until depinning at Δt (observing time)=181 s. The abrupt decrease right after depinning (red arrow) is attributed to an instantaneous elastic recovery. (Bottom) Δθ_S is unchanged during the ridge growth (θ_S=56.3±5.1°). X-shaped symbols in b is the values obtained after depinning. Black dashed lines are a guide to the eye. (c) A schematic illustration of ridge growth from t=t₁ to t=t₃. Invariant θ_S during the slow and linear ridge growth in b might be caused by a liquid-like viscous flow in the soft substrate.