Focal Plane Shift Imaging for the Analysis of Dynamic Wetting Processes

Abstract

Droplet-surface interactions are common to a plethora of natural and industrial processes due to their ability to rapidly exchange energy, mass, and momentum. Droplets are particularly of interest due to their large surface-to-volume ratios and hence enhanced transport properties. For example, coalescence-induced droplet jumping on superhydrophobic surfaces has recently received much attention for its potential to enhance heat transfer, anti-icing, and self-cleaning performance by passively shedding microscale water droplets. To study droplet jumping, researchers typically use a two-camera setup to observe the out-of-plane droplet motion, with limited success due to the inability to resolve the depth dimension using two orthogonal cameras. Here we develop a single-camera technique capable of providing three-dimensional (3D) information through the use of focal plane manipulation, termed "focal plane shift imaging" (FPSI). We used FPSI to study the jumping process on superhydrophobic surfaces having a wide range of structure length scales (10 nm < l < 1 μm) and droplet radii (3 μm < R < 160 μm). We benchmarked the FPSI technique and studied the effects of droplet mismatch, multidroplet coalescence, and multihop coalescence on droplet jumping speed. Furthermore, we were able to resolve the full 3D trajectory of multiple jumping events, to show that, unlike previously theorized, the departure angle during droplet jumping is not a function of droplet mismatch or number of droplets coalescing prior to jumping. Rather, angular deviation arises due to in-plane motion postcoalescence governed by droplet pinning. The outcomes of this work both elucidate key fundamental aspects governing droplet jumping and provide a powerful imaging platform for the study of dynamic droplet processes that result in out-of-plane motion such as sliding, coalescence, or impact.

Cloaking Dynamics on Lubricant‐Infused Surfaces

“…Briefly, samples were placed adjacent to a HTMS‐toluene (CAS no. 108‐88‐3, Sigma‐Aldrich) solution (5% v/v) in a sealed glass container inside an atmospheric pressure furnace (Blue M, Lindberg) at 80 °C for 3 h. [ 46a,47 ] For simplicity, the delineation was herein dropped between SLIPS and LIS and l the samples were called LIS. The LIS samples were fabricated by dip coating the functionalized CuO and boehmite surfaces in the lubricant of choice (Table 1) for a total of 10 min.…”

Liquid-Vapor Phase-Change Heat Transfer on Functionalized Nanowired Surfaces and Beyond

“…Advanced visualization techniques with high temporal and high spatial resolutions are crucial to the understanding of the dynamic processes. Recent techniques such as focal plane shift imaging, 306 laser scanning confocal microscopy, 307 environmental SEM, 46,70,152 infrared thermometry, 90,[308][309][310] liquid crystal thermography, 311,312 synchrotron X-ray imaging, 81 and sometimes a combination of these can potentially achieve simultaneously high spatial and high temporal resolutions in the dynamic phase-change processes. In addition to surface structures and wettability, it is also important to explore the influence of working fluid properties on the phase-change heat transfer processes due to the wide variety of working fluids used in industrial systems such as FC fluids, 144,212,313 pentane, 314 ethanol, [315][316][317] R-134a, 289 isopropanol, hexane, and octane.…”

Droplet Jumping: Effects of Droplet Size, Surface Structure, Pinning, and Liquid Properties

“…Coalescence-induced droplet jumping has received increased attention in the past few years due to its capacity to passively shed micro- and nanoscale droplets, which can enhance anti-icing, defrosting, self-cleaning, − and condensation heat transfer performance. − During droplet coalescence, the rigid substrate breaks symmetry, resulting in momentum transfer away from the surface, driving the jumping process. , Extensive studies have been conducted investigating the hydrodynamics of jumping droplet coalescence, , jumping velocity, ,− and directionality , on various superhydrophobic surfaces. In pursuit of increased jumping velocity, − decreased jumping droplet size, ,, and controlled direction, ,− surfaces have been carefully designed as a passive method to exert a delicate influence on coalescing droplets via surface interactions.…”