Self-peeling of impacting droplets

Abstract

Whether an impacting droplet 1 sticks or not to a solid surface has been conventionally controlled by functionalizing the target surface 2-8 or by using additives in the drop 9,10 . Here we report on an unexpected self-peeling phenomenon that can happen even on smooth untreated surfaces by taking advantage of the solidification of the impacting drop and the thermal properties of the substrate. We control this phenomenon by tuning the coupling of the shorttimescale fluid dynamics-leading to interfacial defects upon local freezing-and the longer-timescale thermo-mechanical stresses-leading to global deformation. We establish a regime map that predicts whether a molten metal drop impacting onto a colder substrate 11-14 will bounce, stick or self-peel. In many applications, avoiding adhesion of impacting droplets around designated target surfaces can be as crucial as bonding onto them to minimize waste or cleaning. These insights have broad applicability in processes ranging from thermal spraying 12,15 and additive manufacturing [16][17][18][19] to extreme ultraviolet lithography 20 .We release molten tin droplets of millimetric size (R = 0.95 ± 0.03 mm) from a nozzle, such that they hit a target surface with moderate velocity (v = 1.9 ± 0.1 m s −1 , see Methods). Figure 1a shows a comparison between the deposition and subsequent solidification of a droplet (initial temperature T d,0 = 240 • C) onto a silicon wafer (top) and a glass slide with ∼200 times lower thermal conductivity (bottom). While the solidified droplet (or splat) sticks to the glass surface, it unexpectedly detaches from the silicon surface: we call this phenomenon 'self-peeling' (see Fig. 1a(top) at 3 and 100 ms, the increasing air gap between droplet and surface being a manifestation of this self-peeling). Two main differences can be observed at this stage. First, interferometric profilometry (see Fig. 1b) confirms that the bottom of the droplet impacting on glass is perfectly flat, while it has a nearly spherical bending curvature κ = 16 m −1 when deposited on silicon.Second, both bottom interfaces show a texture 21 formed by annular regions spaced by air ridges (insets of Fig. 1b) that act as interfacial defects. Their number is much higher for silicon than for glass. The striking contrasts in bending and defect density suggest that a balance between thermally induced stress and adhesion determines whether or not a droplet can self-peel upon solidification.To unravel the competing phenomena, we study the dynamics of the impact using high-speed photography through a transparent surface (see Supplementary Movie 2). Immediately upon contact, micrometre-sized annular air ridges are being trapped. Along the splat radius, their height increases up to a few micrometres, and their width up to tens of micrometres (as measured by profilometry). We also extract the distance d between ridges and the contact line velocity v CL from Supplementary Movie 2, and find that the characteristic timescale τ = d/v CL to form these ridges increases radially. Figure 2a ...

Inhibiting Random Droplet Motion on Hot Surfaces by Engineering Symmetry‐Breaking Janus‐Mushroom Structure

“…Furthermore, as a result of the random motion, the temperature of the hot surface tends to be non‐uniform, which in turn causes an unwanted stress distribution. [ 3 ] Thus, it is essential to concentrate or confine all the impacting droplets onto a localized hotspot region for efficient thermal management.…”

Steerable Droplet Bouncing for Precise Materials Transportation

“…Droplet manipulation is a ubiquitous phenomenon in various natural processes and biological processes such as selfcleaning, [1][2][3][4] drag reduction, [3,[5][6][7][8] and microfluidics. [9][10][11][12][13] As one of the key aspects for droplet manipulation, [7,[14][15][16][17][18] the directional transportation of liquid droplets has aroused great interest due to its importance in applications including water collecting, [19][20][21] anti-icing [16,22,23] and materials transportation.…”

Sticking behavior and transformation of tin droplets on silicon wafers and multilayer-coated mirrors

“…During freezing after impact release stresses and effects of surface tension may built up, and flattened splats may detach from the substrate [2]. Very recently, for liquid tin droplets impinging at fairly low velocity, de Ruiter et al reported so-called self-peeling when examining smooth, flat, horizontally oriented surfaces of various materials with different thermal conductivity and effusivity [9]. We have also examined such conditions where tin drops do not stick to the substrate due to self-peeling, and observed even a further step in some cases, namely receding and self-contraction of already spread-out pancake-like tin splats.…”

“…Self-peeling of droplets is expected to be less pronounced on oxidic materials [9]. To analyze the dependence of the adhesion of tin drops on the layer material, we study bare and strongly oxidized silicon wafers as well as Mo/Si coated mirrors without cap layer and with protective zirconium nitride (ZrN) or zirconium oxide (ZrO 2 ) cap layers.…”