Water droplets impinging on micro-grooved polydimethylsiloxane surfaces were studied. Depending on the impact velocity and surface roughness, different phenomena such as no bouncing, complete rebound (CR), bouncing occurring with droplet breakup (BDB), partial rebound, and sticky state were observed. The lower limit of impact velocity for bouncing droplets can be determined by balancing the kinetic energy of the droplet with energy barrier due to contact angle hysteresis. To predict the upper limit of impact velocity for bouncing droplets, a high-speed camera was used to record droplet impact at an ultrahigh speed and it was found that the transition from CR to BDB was attributed to a local wetting transition from the Cassie–Baxter state to the Wenzel state. Based on the experimental observation, a theoretical model was developed to predict the upper limit of impact velocity taking into account the penetration of the liquid into the micro-grooves. In addition, there was a shorter contact time of bouncing droplets with the decrease in the Weber number and surface roughness has a small influence on the contact time in our experiments.

Impinging dilute sodium dodecyl sulfate (SDS) droplets on micropillar-arrayed polydimethylsiloxane surfaces were experimentally investigated. It was found that the behaviors of impinging droplets greatly depend on surface roughness and SDS concentration. Similar to pure water droplets, there exists a narrow range of dimensionless Weber number, We, for the complete rebound of impacting SDS droplets. The lower and upper limits of impact velocity were theoretically analyzed and compared with experimental data. The addition of SDS could greatly shorten the contact time of bouncing droplets. Besides, surface roughness has little influence on the maximum spreading factor while SDS concentration has an obvious influence and the maximum spreading factor nearly follows a scaling law of ${\text{We}}^{1/4}$ .

Morphologies of evaporative deposition, which has been widely applied in potential fields, were induced by the competition between internal flows inside evaporating droplets. Controlling the pattern of deposition and suppressing the coffee-ring effect are essential issues of intense interest in the aspects of industrial technologies and scientific applications. Here, evaporative deposition of surfactant-laden nanofluid droplets over silicon was experimentally investigated. A ring-like deposition was formed after complete evaporation of sodium dodecyl sulfate (SDS)-laden nanofluid droplets with an initial SDS concentration ranging from 0 to 1.5 CMC. In the case of initial SDS concentrations above 1.3 CMC, no cracks were observed in the ring-like deposition, indicating that the deposition patterns of nanofluid droplets could be completely changed and cracks could be eliminated by sufficient addition of SDS. With the increase of the initial concentration of hexadecyl trimethylammonium bromide (CTAB), the width of the deposition ring gradually decreased until no ring-like structure was formed. On the contrary, with the increase of the initial Triton X-100 (TX-100) concentration, the width of the deposition ring gradually increased until a uniform deposition was generated. Moreover, when the initial TX-100 concentration was high, a "tree-ring-like" pattern was discovered. Besides, morphologies of evaporative pattern due to the addition of surfacants were qualitatively analyzed.

Thin films are widely-used functional materials that have attracted much interest in academic and industrial applications. With thin films becoming micro/nanoscale, developing a simple and nondestructive peeling method for transferring and reusing the films remains a major challenge. Here, we develop an electro-capillary peeling strategy that achieves thin film detachment by driving liquid to percolate and spread into the bonding layer under electric fields, immensely reducing the deformation and strain of the film compared with traditional methods (reaching 86%). Our approach is evaluated via various applied voltages and films, showing active control characterizations and being appropriate for a broad range of films. Theoretically, electro-capillary peeling is achieved by utilizing the Maxwell stress to compete with the film’s adhesion stress and tension stress. This work shows the great potential of the electro-capillary peeling method to provide a simple way to transfer films and facilitates valid avenues for reusing soft materials.