Figure 1

Images of a spreading drop after impact onto dry glass at atmospheric pressure. (a) Side view showing thin-sheet ejection and splashing for a drop with $\mu =13\mathrm{mPa}·\mathrm{s}$, and $u_0=3.5\mathrm{m}/\mathrm{s}$. (b) Bottom view and intensity profile of a dyed $\mu =8\mathrm{mPa}·\mathrm{s}$, $u_0=3.3\mathrm{m}/\mathrm{s}$ drop used to measure liquid thickness: the ejected sheet is approximately $1/10$ the thickness of the lamella. Arrows indicate measured regions. (c) Schematic for reflected-light interference: a half-silvered mirror ($M1$) directs light from a monochromatic LED onto the impacting drop from below; a standard mirror ($M2$) redirects reflected light into a high-speed camera. (d) Example time-series interference image of an impacting drop with $\mu =18\mathrm{mPa}·\mathrm{s}$ and $u_0=3.5\mathrm{m}/\mathrm{s}$. The lamella is uniform in intensity, indicating an optically flat region. The thin sheet is ejected in the third panel. (e) Magnified images of initial air cavity. This initial air cavity closes completely well before the thin sheet is ejected. (f) Radius of curvature of cavity normalized by drop radius $r_c/r_0$ vs time showing the cavity is slender ($r_c\sim r_0$), and gently flattens in time. Complete cavity closure occurs $\sim 150\mu \mathrm{s}$ after impact, well before splashing occurs.Reuse & Permissions

Figure 2

The radius of curvature of the entrapped air cavity normalized to drop radius $r_c/r_0$ vs ambient pressure $P$. Inset schematic defines cavity height $h_c$, radius of curvature $r_c$, and lateral cavity radius $A$. As $P$ decreases, $r_c/r_0$ increases and the cavity becomes flatter. The line shows the best fit to $r_c/r_0\propto P^{-1}$. $r_c$ is measured $\sim 50\mu \mathrm{s}$ after impact.Reuse & Permissions

Figure 3

Interference signal $I_s$ used to determine air-gap thickness $h$ underneath the spreading lamella. (a) $I_s$ vs time, from the time the cavity collapses until just before sheet ejection. (b)–(d) show $I_s$ measured one frame ($\sim 50\mu \mathrm{s}$) before sheet ejection as a function of (b) impact velocity, (c) viscosity, and (d) pressure. These measurements place an upper bound on $h$ of 3.8 nm. The data are consistent with the lamella making direct contact with the substrate as it expands. Error bars represent drop-to-drop fluctuations.Reuse & Permissions