Anisotropic Wettability of Bioinspired Surface Characterized by Friction Force

Abstract

Bioinspired surfaces with special wettabilities attract increasing attention due to their extensive applications in many fields. However, the characterizations of surface wettability by contact angle (CA) and sliding angle (SA) have clear drawbacks. Here, by using an array of triangular micropillars (ATM) prepared by soft lithography, the merits of measuring the friction force of a water droplet on ATM over measurements of CA and SA in characterizing the surface wettability are demonstrated. The CA and SA measurements show ignorable differences in the wettabilities of ATM in opposite directions (1.13%) and that with different periodic parameters under the elongation ranging from 0 to 70%. In contrast, the friction measurement reveals a difference of > 10% in opposite directions. Moreover, the friction force shows a strong dependence on the periodic parameters which is regulated by mechanical stretching. Increasing the elongation from 0 to 50% increases the static and kinetic friction force up to 23.0% and 22.9%, respectively. Moreover, the stick-slip pattern during kinetic friction can reveal the periodic features of the measured surface. The friction force measurement is a sensitive technique that could find applications in the characterization of surface wettabilities.

The Influence of Temperature on Anisotropic Wettability Revealed by Friction Force Measurement

“…Similar to the solid-solid friction, a liquid-solid friction curve also has three sections: static friction, kinetic friction, and the transition zone from static friction to kinetic friction ( Figure 4 a) [ 39 ]. The peak value during the static friction period is considered as the static friction force ( F S ), and the mean value during the kinetic friction is calculated to be the kinetic friction force ( F K ) [ 19 ]. As with the solid-solid friction, F S is larger than F K .…”

Adhesion behaviors of water droplets on bioinspired superhydrophobic surfaces

“…While this technique was carried out on a conventional CA goniometer setup, however, the deformation of the capillary is not able to couple with the measurement of CAs (figure 7(d)). To this end, we developed a new technique based on the monitoring of the position of the capillary's projection (MPCP) (figure 8(a)) [16,34,49]. Instead of imaging the capillary, the MPCP technique monitors the capillary's projection along with the advancing and receding angles (figure 8(a)).…”

“…Using the MPCP technique, a droplet with a volume of 3.0 μl generated a friction force of 1.3-10.4 μN on a fresh lotus leaf. Furthermore, the friction force increased following the sliding velocity and the volume of the water droplet (figure 8(d)) [16,49].…”

“…Magnetic field [24,83] and mechanical stress [25,33,34,49,[84][85][86] have also been used to reversibly change the surface structures and therefore switch the surface adhesion behaviors of droplets. Utilizing the remote control of a magnetic field, Chen et al [24] prepared a dense array of magnetic micropillars consisting of carbonyl iron powders and PDMS by the electrostatic air spray deposition method (figure 15(d)).…”

“…(f) Adhesion switching of polystyrene nanoparticle-embedded PDMS surfaces by mechanical strain. Reproduced with permission from [49].…”