Scanning Drop Friction Force Microscopy

Abstract

Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine the wetting dynamics. Still, existing techniques (e.g., contact angle goniometry) are not sufficient to localize inhomogeneities and image wetting variations. We overcome these limitations through scanning drop friction force microscopy (sDoFFI). In sDoFFI, a 15 μL drop of Milli-Q water is raster-scanned over a surface. The friction force (lateral adhesion force) acting on the moving contact line is plotted against the drop position. Using sDoFFI, we obtained 2D wetting maps of the samples having sizes in the order of several square centimeters. We mapped areas with distinct wetting properties such as those present on a natural surface (e.g., a rose petal), a technically relevant superhydrophobic surface (e.g., Glaco paint), and an in-house prepared model of inhomogeneous surfaces featuring defined areas with low and high contact angle hysteresis. sDoFFI detects features that are smaller than 0.5 mm in size. Furthermore, we quantified the sliding behavior of drops across the boundary separating areas with different contact angles on the model sample. The sliding of a drop across this transition line follows a characteristic stick−slip motion. We use the variation in force signals, advancing and receding contact line velocities, and advancing and receding contact angles to identify zones of stick and slip. When scanning the drop from low to high contact angle hysteresis, the drop undergoes a stick−slip−stick−slip motion at the interline. Sliding from high to low contact angle hysteresis is characterized by the slip−stick−slip motion. The sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces.

Chemically robust superhydrophobic surfaces with a self‐replenishing nanoscale liquid coating

“…The surface could remain superhydrophobic after this process. After 10 freezing/melting cycles, we measured the friction force required to move a 10 µL water drop on the surface using a drop adhesion force instrument 66,67 . The result (Supporting Information: Section ) shows that compared to that on PDMS‐SH surface, the maximum force for a water drop on L‐PDMS‐SH (7.5%) to start moving did not change a lot after the 10 cycles of test.…”

“…After 10 freezing/melting cycles, we measured the friction force required to move a 10 µL water drop on the surface using a drop adhesion force instrument. 66,67 The result (Supporting Information: Section 6)…”

Molecular transport and water condensation inside mesopores with wettability step gradients

“…S2 †), which was consistent with the ellipsometry and CV results. To understand drop uid transport, the friction force of water drops 29,41,42 was measured on a mesoporous surface with adjustable wettability (Fig. 4a).…”

Deep Learning to Analyze Sliding Drops

“…Chemically heterogeneous samples have been prepared by the process of double chemical vapor deposition with the use of a glass mask. 37 Standard microscopic glass slides were cleaned with Milli-Q water, then with acetone, ethanol, and 5 min oxygen plasma treatment at 300 W and 0.3 bar (Diener Electronic Femto). Then, they were treated by chemical vapor deposition of PFOTS in a desiccator.…”