Phase-change systems with self-propelled matter: Disks, droplets, and bubbles

Abstract

Phase-change phenomena such as boiling, condensation, and melting are ubiquitous in both natural and engineered systems. One example of a phase-change process producing motion is the Leidenfrost ratchet, where a superheated asymmetrically patterned surface rectifies the vapor flow to generate net viscous propulsion. Leidenfrost ratchets have been demonstrated for liquid–vapor and solid–vapor systems, yet solid–liquid systems remain largely uncharacterized. We first performed experiments with melting ice on a herringbone ratchet to characterize the governing physical mechanism. Two surface wettabilities were tested: hydrophilic and superhydrophobic. On the hydrophilic surface, viscous entrainment analogous to that of the Leidenfrost ratchet was observed. On the superhydrophobic surface, a distinct ``slingshot" mechanism emerged, in which a Laplace pressure mismatch across the melt puddle drives rapid re-centering of the ice disk, propelling it forward.

After characterizing the first solid–liquid ratchet, we extended the results using paraffin wax. The wax disks traveled ten times farther and twice as fast as the ice. The improved mobility of wax disks enables several practical applications for enhanced heat and mass transfer. The self-propelled motion reduces the melt thickness by sevenfold, enabling faster melting and enhanced thermal conduction. Wax disks also enable mass transfer applications, including the transport of payloads up to 16× their own weight, self-cleaning of contaminated surfaces, and deposition of embedded particles.

Another form of self-propulsion is jumping-droplet condensation, in which two quasi-spherical droplets on a superhydrophobic surface coalesce and spontaneously jump away from the surface. On superhydrophobic wheat leaves infected with pathogenic rust spores, coalescing droplets jump and carry spores away upon departure. Extending this study to infected barley leaves, a previously unreported ejection mechanism was observed, in which dry spores are launched during jumping-droplet condensation. This occurs when spores conglomerate into clusters, allowing droplets to nucleate and grow underneath the cluster of spores. Upon coalescence, the merged droplet jumps upward into the roof of the spore cluster, transferring its momentum and launching dry spores into the air. This previously unknown mode of spore dispersal carries implications for understanding disease transmission pathways in agricultural crops.

The final phase-change process addressed in this thesis is bubble departure during pool boiling. Traditionally, buoyancy governs bubble departure, and methods to reduce the departure diameter have relied on nano/micro-structured surfaces, surfactants, electric fields, or surface wettability modifications. It was discovered that the coalescence of two bubbles during pool boiling can produce departure diameters an order of magnitude smaller than conventional values. Here, we designed and fabricated a custom pool boiling chamber that can visualize the individual bubble coalescence events on micron-sized features. After preliminary testing, we discovered that the bare silicon wafers result in high surface adhesion and don't allow for the bubbles to depart the surface. Instead, we investigated the spatial control and nucleation density of bubbles on a micro-patterned silicon chip using a custom boiling chamber. This thesis primarily details the design and construction of the custom boiling chamber and the rationale for the micro-patterned feature parameters.

These collective works all leverage surface structures to impart dynamic heat and mass transfer phenomena to phase-change processes. Potential applications include: self-cleaning, accelerated energy transfer with phase-change materials, particle sensing, and enhanced heat transfer coefficients for boiling or condensation. Future work will include finishing ongoing projects related to the spatial control and coalescence-induced jumping of nucleating micro-bubbles, furthering our understanding of pathogenic spore dispersal, and extending the practicality of phase-change materials.