Wetting & Capillarity: From Thin Film Mediated Processes to Droplet-Surface Interactions

Abstract

Wetting and capillarity remain ubiquitous in both daily lives and industrial applications. The present thesis explores several fundamentally interesting problems of practical relevance in wetting and capillarity. Two primary thematic directions are adopted in developing this thesis, namely, thin film-mediated wetting and direct quantification of droplet-substrate interfacial interaction.

Under thin film-mediated wetting, we first demonstrate the development of a robust liquid-liquid encapsulation framework where a liquid core analyte is stably wrapped by a thin layer of another shell-forming liquid. Two approaches to achieve encapsulation are discussed - impact-driven and magnet-assisted. The underlying mechanism leading to encapsulation are explored in detail for both approaches. We show that successful encapsulation by either approach provides efficient protection to the core analyte even in aggressive surroundings. Multiple practical use cases, including ultrafast encapsulation, shell-hardening, and subsequent extraction/handling of the wrapped cargo, the formation of encapsulated Janus droplets with similar/dissimilar core compositions, are reported. Further, impact-driven encapsulation with a magnetoresponsive (ferrofluid) shell layer is also illustrated, which allows magnet-assisted efficient, non-contact manipulation of the encapsulated cargo, including translation, controlled coalescence, and the release of the inner core. Although we can confirm successful wrapping via indirect evidence (e.g., alteration in wetting signature/physical appearance & efficient protection in aggressive surrounding upon encapsulation) in our liquid-liquid encapsulation framework, direct visualization of the thin wrapping film remains extremely challenging via standard optical means. To this effect, a complete interferometric framework is developed to detect and reconstruct the spatiotemporal dynamics of such ultrathin (~ nm - µm) liquid films. The framework is tested by investigating the dynamics of dropwise condensation of volatile, low-surface-tension test liquids. In doing so, a previously unknown spontaneous motion of the condensed microdroplets on high-surface energy test substrates is unraveled. The nucleated smaller microdroplets spontaneously migrate towards a bigger microdroplet in the vicinity. With rational experimentation and theoretical arguments, we attribute this motion to the combined effect of the formation of an ultrathin precursor film underneath the nucleated microdroplets and thermocapillary action. Finally, a direct practical application of thin film-mediated wetting in functional materials research, the development of a robust lubricant-infused surface, is reported. The fundamental concept of capillarity transport is leveraged to develop a lubricant-depletion tolerant, long-term stable, large-scale slippery surface with exceptional outdoor durability. If depleted, the thin surface lubricant layer, responsible for the slippery functionality of the material, can be self-replenished via unassisted capillary transport even after multiple lubricant loss-recovery cycles.

A key performance indicator of the liquid-liquid encapsulation protocol is the practical stability of the encapsulated cargo, which is primarily dictated by its interaction with the solid surfaces it encounters. The second thematic direction of the present thesis stems from the requirement of quantifying this droplet-substrate interfacial interaction. Contact angle goniometry was the standard method for characterizing such liquid-solid wetting interactions. However, as already established in the literature, optical goniometry suffers from significant imaging challenges due to optical noise caused by scattering and diffraction near the triple contact line. We show that the cantilever deflection approach, where a microdroplet attached to the tip of a flexible polymeric cantilever is used to probe the adhesion and frictional characteristics of test substrates, is a simple and more accurate alternative in this regard. However, in the conventional approach, the characteristic adhesion between a probe droplet and a target substrate is calculated at the instance of maximum deflection of the cantilever, which is assumed to coincide with the detachment of the probe droplet from the test substrate. This restricts the approach to low-energy (super)repellent surfaces only as the probe droplet cannot completely detach from the test substrate and instead gets split in two if the substrate has a higher surface energy. We critically revisit the conventional framework and establish that complete detachment of the probe droplet is not a strict prerequisite for the applicability of the framework as the sole physical criterion that has a direct correspondence with characteristic adhesion is the depinning of the triple contact line. It allows us to generalize the cantilever-based framework to higher energy surfaces. Further, with a detailed mechanistic analysis of the cantilever's motion, we establish that the instance of zero acceleration of the cantilever corresponds to the depinning of the triple contact line and, thus, to the characteristic adhesion. The developed methodology is general and straightforward. Simple tracking of the motion of the cantilever and subsequent computation of its acceleration, which can be automated, enable the user to characterize the ensuing adhesion interaction between the probe droplet and the test substrate even on higher energy surfaces which was a significant bottleneck in literature. Finally, we use the cantilever deflection method to study how bacteria-laden droplets interact with (super)repellent surfaces and, in the process, uncover an anomalous adhesion behavior when live bacteria are used as dopants inside the probe droplet where increasing the concentration of live bacteria in the probe droplet leads to a reduction in adhesion force. The anomalous behavior is attributed to the motility of the live bacteria.