Abstract
Capillary liquid bridges are ubiquitous in nature and are present in many industrial processes. In order to model their behavior, it is essential to develop suitable experimental tools that are able to characterize the bridges’ geometry and the associated capillary force they induce on the contacting surfaces. While many existing setups are capable of characterizing capillary bridges formed between conventional surfaces, quantitative measurements on smart surfaces such as liquid infused surfaces remain challenging. These surfaces typically exhibit weak contact line pinning and contact angle hysteresis, resulting in unusually small changes in the capillary force they exert upon extension or compression of the bridge. Although it is precisely these properties that drive the interest into liquid infused surfaces, they render experimental characterization challenging when compared to non-infused surfaces. Here, we tackle this issue by developing a relatively inexpensive setup capable of measuring capillary forces with sensitivity in the micronewton range while quantifying the bridge’s geometry. The setup is fully motorized and can vary the relative position of the contacting surfaces while maintaining synchronous force and geometry measurements. We also present a new analysis software developed to retrieve the relevant geometrical parameters of the bridge from optical observations while minimizing errors and noise. Using example surfaces, we demonstrate the setup’s capabilities, including for bridges between liquid infused surfaces.
Highlights
Capillary liquid bridges (CLBs) are created when a droplet of liquid forms a stable link between two surfaces, usually solids
CLB can be quantitatively described and modeled using the contact angles θ1 and θ2 formed by the liquid with each surface, the height h of the bridge, the radii of contact Rt and Rb of the bridge with the top and bottom surfaces, respectively, and the curvature of the CLB surface obtained by two radii of curvature R1 and R2 taken orthogonally from each other
The measurement setup developed in this paper is designed to overcome the difficulties associated with studying CLBs between liquid infused surfaces (LIS)
Summary
Capillary liquid bridges (CLBs) are created when a droplet of liquid forms a stable link between two surfaces, usually solids. While standard nanoscale and millimeter-sized “macroscale” measurements have enabled significant advances in the field, many phenomena involving CLBs fall in the in-between region, where capillary forces range between sub-micronewtons to hundreds of micronewtons. In this range, directly observing the CLB is usually still possible, but measuring the force it exerts on the contacting surfaces becomes challenging and requires bespoke experimental setups.. Such setups are usually expensive, highly specialized, and unsuitable for routine measurements This gap is all the more problematic with the advent of liquid infused surfaces (LIS), where CLBs are only expected to induce micronewton force changes when extended, even at the millimeter scale.
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