The mechanism and universal scaling law of the contact line friction for the Cassie-state droplets on nanostructured ultrahydrophobic surfaces

Turning Non-Sticking Surface into Sticky Surface: Correlation between Surface Topography and Contact Angle Hysteresis

We present a surface modification technique that turns CuNi foam films with a high contact angle and non-sticking property into a sticky surface. By decorating with mesh-like biaxially oriented polypropylene (BOPP) and adjusting the surface parameters, the surface exhibits water-retaining capability even when being held upside down. The wetting transition process of droplets falling on its surface were systematically studied using the finite element simulation method. It is found that the liquid filled the surface microstructure and curvy three-phase contact line. Moreover, we experimentally demonstrated that this surface can be further applied to capture underwater air bubbles.

Dynamic Anti-Icing Performance of Flexible Hybrid Superhydropohobic Surfaces

Normal superhydrophobic surfaces with a rough topography provide pocketed air at the solid-liquid interface, which guides the droplet to easily detach from the surface at room temperature. However, at low temperatures, this function attenuates obviously. In this research, a flexible hybrid topography with submillimeter (sub-mm) and microcone arrays is designed to adjust the impacting behavior of the droplet. The sub-mm cone could provide rigid support to limit deformation, leading to reduced energy consumption during impact processes. However, the microcone could maintain surface superhydrophobicity under different conditions, preventing droplet breakage and the change of the droplet contact state during impact processes by providing multiple contact points. Under the synergistic effect, such a hybrid structure could provide much more pocket air at the solid-liquid interface to limit the spreading of liquid droplets and reduce the energy loss during the impact process. At a low temperature (-5 °C), even if the impact height is reduced to 1 cm, the droplets still could be bound off, and the hybrid superhydrophobic surface presents excellent dynamic anti-icing ability. The special flexible hybrid superhydropohobic surface has potential application in fast self-cleaning and anti-icing fields.

Contact Line Friction and Dynamic Contact Angles of a Capillary Bridge between Superhydrophobic Nanostructured Surfaces

The Cassie-Baxter state of wetting explains a large equilibrium contact angle and the slippery dynamics of a water droplet on a superhydrophobic rough surface. It also causes a contact angle hysteresis (CAH) which cannot be fully described by dynamic wetting theories including the molecular kinetic theory (MKT). We analyze the contact line dynamics on a superhydrophobic surface in the framework of the MKT. Multi-body dissipative particle dynamics simulations of a capillary bridge confined between two rough surfaces under steady shear are performed. We find that, in addition to the contact line friction force from the MKT, an additional friction force contribution is needed on rough surface, which is almost constant at all contact line velocities. Thus, it is directly related to the CAH. The CAH originates not only from contact line pinning but also from the shear flow due to the strong friction in the central region of the liquid-solid interface away from the contact line. The analysis of the particle flow inside the capillary bridge shows that liquid particles trapped in the grooves of the surface texture actually move with the same velocity as the surface, and exert strong additional friction to other liquid particles. This work extends the MKT to rough surfaces, as well as to elucidate the origin of the CAH of a capillary bridge. The finding would help to better understand also other situations of dynamic wetting on superhydrophobic surfaces.

Evaporation of squeezed water droplets between two parallel hydrophobic/superhydrophobic surfaces

HYPOTHESIS

A liquid droplet is apt to be deformed within a compact space in various applications. The morphological change of a droplet and vapor accumulation in the confined space between two parallel surfaces with different gaps and surface wettability are expected to significantly affect the evaporation dynamics of the squeezed droplet therein.

EXPERIMENTS

Here the evaporation dynamics of a squeezed droplet between two parallel hydrophobic/superhydrophobic surfaces are experimentally explored. By reducing the surface gap from 1000 μm to 400 μm, the evolution of contact angle, contact radius and volume of the evaporating droplet are measured. A diffusion-driven model based on a two-parameter ellipsoidal segment geometry is developed to predict the morphology and volume evolution of a squeezed droplet during evaporation.

FINDINGS

Evaporation dynamics of a squeezed water droplet via the constant contact radius (CCR) mode, the constant contact angle (CCA) mode, or the mixed mode are experimentally observed. Confirmed by our ellipsoidal segment model, the evaporation of the squeezed droplet is significantly depressed with the decreasing surface gap, which is primarily attributed to vapor enrichment in a more confined geometry. A linear scaling law between droplet volume and evaporation time is unveiled, which is verified by a simplified cylindrical model.

Partial Leidenfrost Evaporation-Assisted Ultrasensitive Surface-Enhanced Raman Spectroscopy in a Janus Water Droplet on Hierarchical Plasmonic Micro-/Nanostructures

The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro-/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. In this paper, we report a partial Leidenfrost evaporation-assisted approach for ultrasensitive SERS detection of low-concentration analytes in water droplets on hierarchical plasmonic micro-/nanostructures, which are fabricated by integrating nanolaminated metal nanoantennas on carbon nanotube (CNT)-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation on the hierarchical surfaces can provide a levitating force to maintain the water-based analyte droplet in the Cassie-Wenzel hybrid state, i.e., a Janus droplet. By overcoming the diffusion limit in SERS measurements, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, toward the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Here, we demonstrate that a partial Leidenfrost droplet on the hierarchical plasmonic surfaces can reduce the final deposition footprint of analytes by 3-4 orders of magnitude and enable SERS detection of nanomolar analytes (10^-9 M) in an aqueous solution. In particular, this type of hierarchical plasmonic surface has densely packed plasmonic hot spots with SERS enhancement factors (EFs) exceeding 10⁷. Partial Leidenfrost evaporation-assisted SERS sensing on hierarchical plasmonic micro-/nanostructures provides a fast and ultrasensitive biochemical detection strategy without the need for additional surface modifications and chemical treatments.

Coalescence-Induced Swift Jumping of Nanodroplets on Curved Surfaces

In this work, we use molecular dynamics simulations to investigate coalescence-induced jumping of nanodroplets on curved surfaces with different wettabilities. On a curved surface, a liquid bridge will first form between two coalescing droplets as on a flat surface. However, contrary to symmetry-breaking-induced jumping on a flat surface, coalescing droplets would jump earlier than the liquid bridge gets into contact with the curved surface. Such an early symmetry breaking is induced by the opposite motion of coalescing droplets along the lateral direction on the curved surface. We find that surface curvature can effectively facilitate the jumping of coalescing nanodroplets via enhanced symmetry breaking. The energy conversion efficiency is improved from ∼0.15% on a flat surface to ∼2.9% on a curved surface, which is about 20 times enhancement. In addition, we conducted an energy scaling analysis by considering the lumped effects of both viscous dissipation and contact line friction on the jumping behaviors. We conclude that curvature-enhanced jumping in the nanoscale can be ascribed to the mitigated contact line dissipation E_cl, whereas viscous dissipation E_vis is maintained almost at the same level. Therefore, we unveil a scaling law between the energy conversion efficiency η on surfaces with different curvatures and the product of contact line length and contact time. Interestingly, the increasing surface curvature could allow the occurrence of coalescence-induced jumping on a less superhydrophobic surface. Hence, a phase map of coalescence-induced jumping in terms of surface curvature ratio and surface wettability is presented. Essentially, the paradigm of curved surfaces in the nanoscale used in this study is characteristic of the topography of micro/nanostructured surfaces, on which coalescence-induced droplet jumping has been experimentally observed. This work justifies the critical role of nanoroughness in boosting coalescence-induced jumping of nanodroplets and sheds light on the passive control of nanodroplets jumping on functional surfaces.

A Tunable Optical Bragg Grating Filter Based on the Droplet Sagging Effect on a Superhydrophobic Nanopillar Array

Nanostructures have been widely applied on superhydrophobic surfaces for controlling the wetting states of liquid microdroplets. Many modern optic devices including sensors are also integrated with micro- or nanostructures for function enhancement. However, it is rarely reported that both microfluidics and optics are compatibly integrated in the same nanostructures. In this paper, a novel microfluidic-controlled tunable filter composed of an array of periodic micro/nanopillars on top of a planar waveguide is proposed and numerically simulated, in which the periodic pillars endow both the Bragg grating and the superhydrophobic functions. The tunability of grating is achieved by controlling the sagging depth of a liquid droplet into the periodic pillars. Simulation results show that a narrow bandwidth of 0.4 nm and a wide wavelength tuning range over 25 nm can be achieved by such a microfluidic-based tunable optofluidic waveguide Bragg grating filter. Moreover, this proposed scheme can be easily modified as a refractive index sensor with a sensitivity of 103 nm per refractive index unit.

Resistant energy analysis of self-pulling process during dropwise condensation on superhydrophobic surfaces

Recently the development of superhydrophobic surfaces with one-tier or hierarchical textures has drawn increasing attention because enhanced condensation heat transfer has been observed on such biomimetic surfaces in well-tailored supersaturation or subcooling conditions. However, the physical mechanisms underlying condensation enhancement are still less understood. Here we report an energy-based analysis on the formation and growth of condensate droplets on two-tier superhydrophobic surfaces, which are fabricated by decorating carbon nanotubes (CNTs) onto microscale fluorinated pillars. Thus-formed hierarchical surfaces with two tier micro/nanoscale roughness are proved to be superior to smooth surfaces in the spatial control of condensate droplets. In particular, we focus on the self-pulling process of condensates in the partially wetting morphology (PW) from surface cavities due to intrinsic Laplace pressure gradient. In this analysis, the self-pulling process of condensate tails is resisted by adhesion energy, viscous dissipation, contact line dissipation and line tension in a combined manner. This process can be facilitated by adjusting the configuration and length scale of the first-tier texture. The optimum design can not only lower the total resistant energy but also favor the out-of-plane motion of condensate droplets anchored in the first-tier cavity. It is also shown that engineered surface with hierarchical roughness is beneficial to remarkably mitigating contact line dissipation from the perspective of molecular kinetic theory (MKT). Our study suggests that scaling down surface roughness to submicron scale can facilitate the self-propelled removal of condensate droplets.

Characterizing the bifurcating configuration of hydrogen bonding network in interfacial liquid water and its adhesion on solid surfaces

The interfacial structures of liquid water molecules adjacent to a solid surface contribute significantly to the interfacial properties of aqueous solutions, and are of prime importance in a wide spectrum of applications. In this work, we use molecular dynamics (MD) simulations to explore the interfacial structures, mainly in term of hydrogen bonding network, of a liquid water film interacting intimately with solid surfaces, which are composed of [100] face centered cubic (FCC) lattices. We disclose the formation of a bifurcating configuration of hydrogen bonds in interfacial liquid water and ascribe its occurrence to the collective effects of water density depletion, hydrogen bonds and local polarization. Such bifurcating configuration of interfacial water molecules consists of repetitive layer by layer water sheets with intra-layer hydrogen bonding network being formed in each layer, and inter-layer defects, i.e., hydrogen bonds formed between two neighboring layers of interfacial water. A lower bound of 2.475 for the average number of hydrogen bonds per interfacial water molecule is expected. Our MD study on the interfacial configuration of water on solid surfaces reveals a quadratic dependence of adhesion on the solid–liquid affinity, bridging the gap between the macroscopic interfacial property W_adh and the microscopic parameter ε_SL of the depth of the Lennard-Jones solid–liquid potential.

Bifurcating configuration of hydrogen bonding network in interfacial liquid water influences its adhesion on solid surfaces.