Comparison of spontaneous wetting and drop impact dynamics of aqueous surfactant solutions on hydrophobic polypropylene surfaces: scaling of the contact radius

Abstract

In this paper, we comparatively investigated the spontaneous wetting and the impact dynamics of surfactantladen drops on hydrophobic polypropylene surfaces. We used the organic anionic surfactant sodium dodecyl sulfate (SDS) and two silicone-based nonionic surfactants, one of which was a so-called superspreader. The wetting dynamics during spontaneous spreading could be divided into an early inertiadominated stage and a later viscosity-dominated stage. Both could be described by power laws with different exponents. Further, wetting dynamics during the faster inertial stage was independent of surfactant properties, while surfactants played a role in the slower viscous stage. During drop impact, regardless of the impact speed the early wetting stage was controlled by inertial and capillary forces only. But, different from spontaneous spreading, surfactants influenced the wetting dynamics of this early stage. After the drops reached the maximum spreading radius r max they started recoiling and reducing again their contact radius. We observed, however, that despite reducing the static surface tension of the water drops, not all surfactants promoted them to spread out to a larger radius than the pure water drops during the early impact stage. We thus introduced a new time scale, τ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ρr 3 max =γ p , that uses the maximum spreading radius of the drops upon impact and that allows rescaling the durations of the inertial wetting stages to a universal master curve. Finally, we observed that only those drops containing superspreader restarted wetting the surface after recoiling, but the dynamics is not yet completely clear.

3D-Printed Surface Architecture Enhancing Superhydrophobicity and Viscous Droplet Repellency

“…This first criterion sets the lower limit in Weber number for pancake bouncing, which we observe on our surface architecture to be at 7 We  . The second necessary condition for pancake bouncing is that the time for the liquid to penetrate and 9 empty the macro-texture, t  , matches the time the droplet takes to spread out to its maximum diameter, max t , which means that the ratio of these two times 20 We  we observe partial rebound on the macro-textured surfaces and the S-PTFE surface (see Figure S3). two protruding macro features in the square lattice, 0.4…”

“…The indentations were included in the surface design to allow for a mechanically stable architecture with short protrusions while still providing a large available surface area to store surface energy during droplet impact. Here we found that for 20 We  these indentations simultaneously increase the risk of pinning during the droplet retraction stage, which slows down the receding liquid within the macro-texture and can result in droplet breakup in the macro-texture (see Figure S3 for side-view image sequences at 20 We  on the S-HFS and the T-HFS surface). To facilitate dewetting and limit pinning events, it is important to reduce the presence of sharp edges, thus preventing liquid from remaining in the texture.…”

“…Furthermore, due to its definition based on the time for the liquid to penetrate and empty the macro-texture, t  , k is only applicable to macro-textured surfaces. For 20 We  we observe incomplete rebound for both the S-PTFE and the T-PTFE surfaces. We conclude that the impalement resistance of the PTFE coating is not sufficient for 20 We  (see Figure S3 for side-view image sequences at 20 We  on the S-PTFE and the T-PTFE surface).…”

“…In contrast, for 20 We  the droplets still fully rebound from the S-HFS surfaces, as the HFS coating is optimized for best impalement resistance. Interestingly, impacting droplets on T-HFS surfaces do not completely rebound, but liquid remains stuck in the macro-texture of the surface.…”

Forced dynamic dewetting of structured surfaces: Influence of surfactants

“…However, the dynamic wetting of multicomponent liquids, like surfactant solutions, is less understood. In recent years, an increasing amount of research has been done on this topic [3,[20][21][22][23][24][25][26][27]. Surfactant molecules absorb at the interface and change the interfacial tension [28].…”