Lubricant‐infused surfaces (SLIPSs/LISs) enable omniphobicity by reducing droplet pinning through creation of an atomically smooth liquid–liquid interface. Although SLIPSs/LISs provide efficient omniphobicity, the need for lubricant adds additional barriers to heat and mass transport and affects three‐phase contact line dynamics. Here, evaporation dynamics of microscale water droplets on SLIPSs/LISs are investigated using steady and transient methods. Although steady results demonstrate that evaporation on SLIPSs/LISs is identical to solid functional surfaces having equivalent apparent contact angle, transient measurements show significant increases in evaporation timescale. To understand the inconsistency, high‐speed optical imaging is used to study the evaporating droplet free interface. Focal plane shift imaging enables the study of cloaking dynamics by tracking satellite microdroplet motion on the cloaked oil layer to characterize critical timescales. By decoupling the effect of substrate material and working fluid via experiments on both microstructured copper oxide and nanostructured boehmite with water and ethanol, it is demonstrated that lubricant cloaking cannot be predicted purely by thermodynamic considerations. Rather, coalescence dynamics, droplet formation, and surface interactions play important roles on establishing cloaking. The outcomes of this work shed light onto the physics of lubricant cloaking, and provide a powerful experimental platform to characterize droplet interfacial phenomena.
more » « less- PAR ID:
- 10455852
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials Interfaces
- Volume:
- 7
- Issue:
- 19
- ISSN:
- 2196-7350
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
The evaporation of droplets on surfaces is a ubiquitous phenomenon essential in nature and industrial applications ranging from thermal management of electronics to self-assembly-based fabrication. In this study, water droplet evaporation on a thin quartz substrate is analyzed using an unsteady two-step arbitrary Lagrangian-Eulerian (ALE) moving mesh model, wherein the evaporation process is simulated during the constant contact radius (CCR) and contact angle (CCA) modes. The numerical model considers mass transfer in the gas domain, flow in the liquid and gas domains, and heat transfer in the solid, liquid, and gas domains. Besides, the model also accounts for interfacial force balance, including thermocapillary stresses, to obtain the instantaneous droplet shape. Experiments involving droplet evaporation on unheated quartz substrates agree with model predictions of contact radius, contact angle, and droplet volume. Model results indicating temperature and velocity distribution across an evaporating water droplet show that the lowest temperatures are at the liquid-gas interface, and a single vortex exists for the predominant duration of the droplet's lifetime. The temperature of the unheated substrate is also significantly reduced due to evaporative cooling. The interfacial evaporation flux distribution, which depends on heat transfer across the droplet and advection in the surrounding medium, shows the highest values near the three-phase contact line. In addition, the model also predicts evaporation dynamics when the substrate is heated and exposed to different advection conditions. Generally, higher evaporation rates result from higher substrate heating and advection rates. However, substrate heating and advection in the surrounding gas have minimal effects on the relative durations of CCR and CCA modes for a given receding contact angle. Specifically, in this case, a 40× increase in substrate heating rate or 7.5× increase in gas velocity can only change these relative durations by 3%. This study also highlights the importance of surface wettability, which affects evaporation dynamics for all the conditions explored by the numerical model.more » « less
-
Markides, C. N. (Ed.)Designing air-water systems for industrial applications requires a fundamental understanding of mass accommodation at the liquid-vapor interface, which depends on many factors, including temperature, vapor concentration, and impurities that vary with time. Hence, understanding how mass accommodation changes over a droplet’s lifespan is critical for predicting the performance of applications leveraging evaporation. In this study, experimental data of water droplets on a gold-coated surface evaporating into dry nitrogen is coupled with a computational model to measure the accommodation coefficient at the liquid-vapor interface. We conduct this measurement by combining macroscopic observations with the microscopic kinetic theory of gasses. The experiments utilize a sensitive piezoelectric device to determine the droplet radius with high accuracy and imaging to measure the droplet contact angle. This setup also quantifies the trace amounts of non-volatile impurities in the droplet. For water droplets evaporating in a pure nitrogen stream, the accommodation coefficient directly relates to vapor flux over the droplet’s surface and is affected by the presence of impurities. We obtained a surface-averaged accommodation coefficient close to 0.001 across multiple water droplets evaporating close to room temperature. This quantification can aid in conducting a more accurate analysis of evaporation, which can assist in the improved design of evaporation-based applications. We believe the modeling approach presented in this work, which integrates the kinetic theory of gases to the macroscale flow behavior, can provide a basis for predicting evaporation kinetics in the presence of extremely dry non-condensable gas streams.more » « less
-
Altering soil wettability by inclusion of hydrophobicity could be an effective way to restrict evaporation from soil, thereby conserving water resources. In this study, 4-μL sessile water droplets were evaporated from an artificial soil millipore comprised of three glass (i.e. hydrophilic) and Teflon (i.e. hydrophobic) 2.38-mm-diameter beads. The distance between the beads were kept constant (i.e. center-to-center spacing of 3.1 mm). Experiments were conducted in an environmental chamber at an air temperature of 20°C and 30% and 75% relative humidity (RH). Evaporation rates were faster (i.e. ∼19 minutes and ∼49 minutes at 30% and 75% RH) from hydrophilic pores than the Teflon one (i.e. ∼24 minutes and ∼52 minutes at 30% and 75% RH) due in part to greater air-water contact area. Rupture of liquid droplets during evaporation was analyzed and predictions were made on rupture based on contact line pinning and depinning, projected surface area just before rupture, and pressure difference across liquid-vapor interface. It was observed that, in hydrophilic pore, the liquid droplet was pinned on one bead and the contact line on the other beads continuously decreased by deforming the liquid-vapor interface, though all three gas-liquid-solid contact lines decreased at a marginal rate in hydrophobic pore. For hydrophilic and hydrophobic pores, approximately 1.7 mm2 and 1.8–2 mm2 projected area of the droplet was predicted at 30% and 75% RH just before rupture occurs. Associated pressure difference responsible for rupture was estimated based on the deformation of curvature of liquid-vapor interface.more » « less
-
We numerically study drop impact on slippery lubricated surfaces at varied impact speeds to comprehend the cloaking of the water drop by the lubricant. We employ a multi-material and multi-phase interface reconstruction method to capture the interaction between the drop and the lubricants of varying interfacial tensions. We demonstrate that cloaking occurs when lubricant water interfacial tensions are low and impact speeds are low. Our research demonstrates that the thickness of the encapsulating lubricant layer varies over time. At moderate impact speeds of 0.25 and 0.5 m/s, the drop displaces a large amount of lubricant, generating a lubricant–water jet, as we also demonstrate. At high impact speeds of 5 and 30 m/s, a secondary impingement forms, which displaces a significant amount of lubricant to reveal the underneath substrate that was not visible at lower impact speeds. Finally, we investigate the drop impact on lubricant infused micro-wells with varying spacing. We find that small spacing between the micro-well walls can limit lubricant drainage and displacement. The substrates with micro-wells exhibit far less splashing than those without. Furthermore, we demonstrate that micro-wells are better at preserving lubricants than substrates without micro-wells.
-
We introduce an accurate and efficient method for characterizing surface wetting and interfacial properties, such as the contact angle made by a liquid droplet on a solid surface, and the vapor–liquid surface tension of a fluid. The method makes use of molecular simulations in conjunction with the indirect umbrella sampling technique to systematically wet the surface and estimate the corresponding free energy. To illustrate the method, we study the wetting of a family of Lennard-Jones surfaces by water. For surfaces with a wide range of attractions for water, we estimate contact angles using our method, and compare them with contact angles obtained using droplet shapes. Notably, our method is able to capture the transition from partial to complete wetting as surface–water attractions are increased. Moreover, the method is straightforward to implement and is computationally efficient, providing accurate contact angle estimates in roughly 5 nanoseconds of simulation time.more » « less