An official website of the United States government
The freezing process is significantly influenced by environmental factors and surface morphologies. At atmospheric pressure, a surface below the dew and freezing point temperature for a given relative humidity nucleates water droplets heterogeneously on the surface and then freezes. This paper examines the effect of nanostructured surfaces on the nucleation, growth, and subsequent freezing processes. Microsphere Photolithography (MPL) is used to pattern arrays of silica nanopillars. This technique uses a self-assembled lattice of microspheres to focus UV radiation to an array of photonic jets in photoresist. Silica is deposited using e-beam evaporation and lift-off. The samples were placed on a freezing stage at an atmospheric temperature of 22±0.5°C and relative humidities of 40% or 60%. The nanopillar surfaces had a significant effect on droplet dynamics and freezing behavior with freezing accelerated by an order of magnitude compared to a plain hydrophilic surface at 60% RH where the ice bridges need to cover a larger void for the propagation of the freezing front within the growing droplets. By pinning droplets, coalescence is suppressed for the nanopillared surface, altering the size distribution of droplets and accelerating the freezing process. The main mechanism affecting freezing characteristics was the pinning behavior of the nanopillared surface.
more »
« less
Multifunctional Microcavity Surfaces for Robust Capture and Direct Rapid Sampling of Concentrated Analytes
Evaporation patterns of liquid droplets containing nanoparticles or colloids have extensive applications in diagnostics and printing. Controlling these patterns by studying the evaporation behavior of colloidal droplets on surfaces is important for enhancing sensing platforms. In this study, A liquid‐repellent microcavity surface is introduced to robustly capture deposited analytic particles. The proposed microcavity surface maintains stable air pockets for liquid repellency and strong pinning for the spatial stabilization of the evaporating droplet, thereby resulting in a coffee‐ring concentration. This microcavity surface also acts as a “microcontainer” for the deposited particles, thereby protecting them against external damage. To demonstrate the multifaceted capabilities of microcavity surfaces, further comparison is done of three different surface structures, planar, micropillared, and that with microcavities in a hexagonal arrangement, by analyzing their evaporation dynamics and dried deposit patterns. The microcavity surface exhibits superior particle capture, thereby revealing its applicability in on‐site testing. Using the direct rapid sampling of analytical materials, the potential of the fabricated microcavity surface for point‐of‐care testing is demonstrated. The proposed microcavity surfaces suggest new avenues for the development of more robust and sensitive sensing platforms.
more »
« less
- Award ID(s):
- 2227527
- PAR ID:
- 10477589
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Small Structures
- Volume:
- 5
- Issue:
- 3
- ISSN:
- 2688-4062
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
A comprehensive understanding of heat transfer mechanisms and hydrodynamics during droplet impingement on a heated surface and subsequent evaporation is crucial for improving heat transfer models, optimizing surface engineering, and maximizing overall effectiveness. This work showcases findings related to heat transfer mechanisms and simultaneous tracking of the moving contact line (MCL) for subcooled impinging droplets across a range of surface temperatures, utilizing a custom MEMS device, at multiple impact velocities. Experimental results show that heat flux caused by droplet impingement has a weaker dependence on surface temperature than receding MCL heat transfer due to evaporation, which is significantly surface temperature dependent. The measurements also demonstrate that when a droplet impacts a heated surface and evaporates, the process can be divided into two segments based on the effective heat transfer rate: an initial conduction-dominated segment followed by another segment dominated by surface evaporation. For subcooled impinging droplets, the effect of oscillatory motion is found to be negligible, unlike in a superheated regime; hence, heat conduction into the droplet entirely governs the first segment. Results also show that heat flux at the solid-liquid interface of an impinging droplet increases with the rise of either impact velocity or surface temperature. In the subcooled regime, droplets impacting a heated surface have approximately 1.6 times higher vertical heat flux values than gently deposited droplets. Furthermore, this study quantifies the contributions of buoyancy and thermocapillary convection within the droplet to the overall heat transfer.more » « less
-
Abstract Achieving mobile liquid droplets on solid surfaces is crucial for various practical applications, such as self‐cleaning and anti‐fouling coatings. The last two decades have witnessed remarkable progress in designing functional surfaces, including super‐repellent surfaces and lubricant‐infused surfaces, which allow droplets to roll/slide on the surfaces. However, it remains a challenge to enable droplet motion on hydrophilic solid surfaces. In this work, we demonstrate mobile droplets containing ionic surfactants on smooth hydrophilic surfaces that are charged similarly to surfactant molecules. The ionic surfactant‐laden droplets display ultra‐low contact angle and ultra‐low sliding angle simultaneously on the hydrophilic surfaces. The sliding of the droplet is enabled by the adsorbed surfactant ahead of three‐phase contact line, which is regulated by the electrostatic interaction between ionic surfactant and charged solid surface. The droplet can maintain its motion even when the hydrophilic surface has defects. Furthermore, we demonstrate controlled manipulation of ionic surfactant‐laden droplets on hydrophilic surfaces with different patterns. We envision that our simple technique for achieving mobile droplets on hydrophilic surfaces can pave the way to novel slippery surfaces for different applications.more » « less
-
null (Ed.)Condensation figure (CF) is a simple and cost-effective method to inspect patterns and defects on product surfaces. This inspection method is based on energy differentials on surfaces. Due to wettability contrast, water droplets are preferentially nucleated and grown on hydrophilic regions. The formed CF can further be segmented for the recognition and measurement of the patterns on the surfaces. The state-of-the-art CF methods are closeenvironmental, while controlled open-environmental CF has broader applications in manufacturing and quality inspection. The lack of open-environmental CF for such applications is mostly because of the unavailable droplet size control methods. In this paper, we designed a high-resolution optical surface inspection system based on open environment droplet-size-controlled CFs. This is done by real-time imaging and recognizing the condensed droplet sizes and densities on surfaces, and accordingly tuning the vaporization and evaporation of droplets on the surface by the vapor flow rate. Our experimental results show that the average diameter of droplets can be controlled below 3.5 µm in a laboratory setup for different metal substrates. We also test the system for inspecting self-assembled monolayer patterns with linewidth of 5 µm on a gold surface; this can be promisingly used for online quality monitoring and in-process control of printed patterns in flexible devices manufacturing.more » « less
-
null (Ed.)Evaporation and breakup of liquid droplets are common in many applications of the shock-driven multiphase instability (SDMI), such as in liquid-fuelled detonation engines, multiphase ejector pumps and turbines and explosive dispersal of liquid particles (i.e. chemical or biological agents). In this paper, the effects of evaporation and breakup of droplets on the mixing induced by the SDMI are considered through simulations and compared with experimental results. The evaporation model is validated against previous experimental data. The capabilities of the simulations and particle models are then demonstrated through a qualitative comparison with experimental results where breakup effects are negligible (i.e. small droplets). The simulation results are explored further to quantify the effects of evaporation (i.e. mixing enhancement) in the SDMI, providing further insight into the experimental results. A new breakup model, derived from previous works, is then presented for low Reynolds number (below 500), low Weber number (below 100) droplets in a shock-driven multiphase instability. The breakup model capabilities are then demonstrated through a comparison with experimental results where breakup effects are significant (larger droplet sizes). Finally, the simulation results are used to highlight the importance of breakup parameters on the evaporation rate and large-scale mixing in the SDMI. Overall, it is shown that evaporation is enhanced by the large-scale hydrodynamics instability, the SDMI, and that breakup of the droplets significantly increases the strength of the instability, and rate of droplet evaporation.more » « less
