Showing papers on "Wetting transition published in 2021"
TL;DR: In this article, a simple method, using combustion of asphalt, was reported to fabricate superhydrophobic surface that can change its wetting state from Cassie-Baxter state to Wenzel state.
24 citations
TL;DR: In this paper, the authors synthesized high quality Hf-ZnO thin films with various Hf contents (0, 3, 6, 9, 12, and 15 at. %), which showed both superhydrophilic (6% HfZnOs) and ultrahydrophobic (15% hfZnosO) wetting behavior.
Abstract: Herein, we successfully synthesized high-quality Hf-ZnO thin films with various Hf contents (0, 3, 6, 9, 12, and 15 at. %), which showed both superhydrophilic (6% Hf-ZnO) and ultrahydrophobic (15% Hf-ZnO) wetting behavior. Different characterization methods were opted to recognize the structural (XRD, SEM, AFM) and defect properties (XPS) of the pristine and doped materials, to understand the mechanisms underlying the tuning of wetting behavior (contact angle). Hafnium doping plays a noteworthy role in tuning the morphology of the ZnO nanostructures, roughness of the material surface, generation of defects, Lewis acid-base interactions, and wettability properties. We achieved a superhydrophilic surface with 6% Hf-ZnO owing to a smooth surface, less basicity, and maximum concentration of oxygen vacancies, and also an ultrahydrophobic surface with 15% Hf-ZnO because of the rough surface, high basicity, and minimum concentration of oxygen vacancies. The as prepared Hf-ZnO samples showed stable performance (stability, wearability, weatherability, and antifouling) under real-life conditions marking them multifunctional and biosafe material to be effectively used in solar and building's window. A wetting mechanism was established to relate the wetting behavior of the samples to oxygen vacancies (active sites for water dissociation: resulted due to charge mismatch of host cation (Zn2+) by the doped cation (Hf4+)), roughness (smooth surface (Wenzel) with minimum Rrms (0.588) portraying hydrophilic property and rough caltropic surface (Cassie-Baxter) with maximum Rrms (2.522) portraying hydrophobic property), basicity (H2O: Lewis Base; ZnO: Lewis acid; HfO2: Lewis base) and morphology (tube-like structure (0-6% Hf-ZnO) and caltrop-like structure (12-15% Hf-ZnO)).
19 citations
TL;DR: In this article, the bouncing behavior of a water droplet on super-hydrophobic surfaces (SHSs) was analyzed under the condition of large temperature difference between the SHS and the impinging droplet.
18 citations
TL;DR: In this paper, the influence of the Weber number (We), ridge height (H), and deviation distance (r) between the impacting point and the center of curvature on the lateral offset distance (ΔL) of bouncing drops was explored.
Abstract: In the first part of this research, we reported the experimental study of the drop impact on the superhydrophobic circular groove arrays, which resulted in a directional droplet transport In the second part, we further explored the influence of the Weber number (We), ridge height (H), and the deviation distance (r) between the impacting point and the center of curvature on the lateral offset distance (ΔL) of bouncing drops The suggested theoretical analysis is in reasonable agreement with the experimental observations We demonstrate that a Cassie-Wenzel wetting transition occurred within the microstructures of the relief under the threshold Weber number, for example, We ≅ 19-25, which switched the nature of drop bouncing The dynamic pressure plays a decisive role in the directional droplet transport The reported investigation may shed light on the solid-liquid interactions occurring on the patterned hierarchical surfaces and open up new opportunities for directional droplet transportation
16 citations
TL;DR: In this paper, the authors studied the wetting transition of a nanodroplet on pillar-arrayed surfaces induced by an external electric field via an energy-minimization method in conjunction with molecular dynamics simulations.
16 citations
TL;DR: In this article, the roughness of a solid surface can be divided into three regions: Region I, as inherent hydrophobicity increases, roughness can enhance the apparent hydrophilicity of a hydrophilic surface.
14 citations
TL;DR: In this article, a universal description of wetting on multiscale surfaces can be developed by using integral geometry coupled to thermodynamic laws, which separates the different hierarchy levels of physical description from the thermodynamic description.
13 citations
TL;DR: In this article, an approach that involves electronic screening while capturing molecular aspects of interfacial fluids is proposed, which describes electrostatic interactions within the metal through a virtual Thomas-Fermi fluid of charged particles, whose Debye length sets the screening length.
Abstract: Of relevance to energy storage, electrochemistry and catalysis, ionic and dipolar liquids display unexpected behaviours—especially in confinement. Beyond adsorption, over-screening and crowding effects, experiments have highlighted novel phenomena, such as unconventional screening and the impact of the electronic nature—metallic versus insulating—of the confining surface. Such behaviours, which challenge existing frameworks, highlight the need for tools to fully embrace the properties of confined liquids. Here we introduce a novel approach that involves electronic screening while capturing molecular aspects of interfacial fluids. Although available strategies consider perfect metal or insulator surfaces, we build on the Thomas–Fermi formalism to develop an effective approach that deals with any imperfect metal between these asymptotes. Our approach describes electrostatic interactions within the metal through a ‘virtual’ Thomas–Fermi fluid of charged particles, whose Debye length sets the screening length λ. We show that this method captures the electrostatic interaction decay and electrochemical behaviour on varying λ. By applying this strategy to an ionic liquid, we unveil a wetting transition on switching from insulating to metallic conditions. Ionic and dipolar liquids display unexpected behaviours, especially in confinement, that are relevant to energy storage, electrochemistry and catalysis. An approach that involves electronic screening while capturing molecular aspects of interfacial fluids is now proposed.
13 citations
TL;DR: In this paper, the authors proposed a method to sweep microparticles deposited on superhydrophobic surfaces, which is essential in order to maintain the excellent functionality of super hydrophobic surface.
Abstract: Sweeping deposited particles is absolutely essential in order to maintain the excellent functionality of superhydrophobic surfaces. Many methods have been proposed to sweep microparticles deposited...
12 citations
TL;DR: In this article, the voltage polarity dependent electrowetting results with an equivalent electrical circuit model at the solid-liquid interface, and considering the hierarchical dual surface roughness (micro-nano scale), were analyzed.
12 citations
TL;DR: The present findings provide an alternative interpretation for complete wetting and are expected to be exploited for designing more effectively and efficiently superhydrophilic structures.
Abstract: In contrast to the wetting phenomenon of pure substance phase, we here concentrate on the wetting behavior of immiscible fluids with two components via numerical simulations. We investigate the energetic contribution, the wall energy and the surface excess energy, to the wetting behavior of liquid solutions varying with temperature. This investigation is in accordance with Cahn's wetting transition theory, where the surface composition plays a vital role. By analyzing the energetic contributions, we reveal two different physical mechanisms of complete wetting: (i) surface tension driven complete wetting, where the wetting microstructure is achieved via the outward spreading of the triple junction, and (ii) diffusion induced complete wetting, where the wetting film is achieved through a direct deposition of the solute on the substrate. The former one is consistent with the classic theory of Young's law, and the latter one is an alternative mechanism. To indicate the broad multiplicity of the microstructural arrangements, we take porous structures to exemplarily elucidate the formation of alternative perfect wetting structures. Differing from the wetting on a flat substrate, we show that the surface composition varying with temperature leads to a distinct wetting phenomenon in porous structures. The present findings provide an alternative interpretation for complete wetting and are expected to be exploited for designing more effectively and efficiently superhydrophilic structures.
TL;DR: In this article, the authors investigated the relationship between air cavity collapse and air film rupture for water droplets impacting soft, hydrophobic surfaces and provided a detailed characterisation of the collapsing dynamics of the air cavity and subsequent jetting.
Abstract: Soft surfaces impacted by liquid droplets trap more air underneath than their rigid counterparts. The extended lifetime of the air film not only facilitates bouncing behaviours of the impacting droplets but also increases the possibility of interactions between the air film itself and the air cavity formed inside the droplets by capillary waves. Such interactions may cause rupture of the trapped air film by a so-called dimple inversion phenomenon and suppress bouncing. In this work, we systematically investigate the relationship between air cavity collapse and air film rupture for water droplets impacting soft, hydrophobic surfaces. By constructing a bouncing-to-wetting phase diagram based on the rupturing dynamics of the trapped air film, we observe that the regime in which air film rupture is induced by dimple inversion consistently separates the bouncing regime and the one in which wetting is caused by random rupture. We also found that air film rupture by dimple inversion in-turn affects both the collapsing dynamics of the air cavity and the resulting high-speed jet. We then provide a detailed characterisation of the collapsing dynamics of the air cavity and subsequent jetting.
TL;DR: In this paper, the authors use both classical density functional theory (cDFT) and molecular dynamics (MD) simulations to study dilute and semi-dilute solutions of short polymer chains near a solid surface.
Abstract: In polymer nanoparticle composites (PNCs) with attractive interactions between nanoparticles (NPs) and polymers, a bound layer of the polymer forms on the NP surface, with significant effects on the macroscopic properties of the PNCs. The adsorption and wetting behaviors of polymer solutions in the presence of a solid surface are critical to the fabrication process of PNCs. In this study, we use both classical density functional theory (cDFT) and molecular dynamics (MD) simulations to study dilute and semi-dilute solutions of short polymer chains near a solid surface. Using cDFT, we calculate the equilibrium properties of polymer solutions near a flat surface while varying the solvent quality, surface–fluid interactions, and the polymer chain lengths to investigate their effects on the polymer adsorption and wetting transitions. Using MD simulations, we simulate polymer solutions near solid surfaces with three different curvatures (a flat surface and NPs with two radii) to study the static conformation of the polymer bound layer near the surface and the dynamic chain adsorption process. We find that the bulk polymer concentration at which the wetting transition in the poor solvent system occurs is not affected by the difference in surface–fluid interactions; however, a threshold value of surface–fluid interaction is needed to observe the wetting transition. We also find that with good solvent, increasing the chain length or the difference in the surface–polymer interaction relative to the surface–solvent interaction increases the surface coverage of polymer segments and independent chains for all surface curvatures. Finally, we demonstrate that the polymer segmental adsorption times are heavily influenced only by the surface–fluid interactions, although polymers desorb more quickly from highly curved surfaces.
TL;DR: In this article, the superhydrophobic ZnO@stearic acid nanoarrays have been prepared and synchronously introduced plentiful oxygen vacancies by using hydrothermal method on the Zn substrate.
TL;DR: In this article, a new modification method using a combination of atomic layer deposition (ALD) and atmospheric heating was proposed to alter the wettability of purely cellulosic chromatography paper.
Abstract: Cellulosic materials are widely used in daily life for paper products and clothing as well as for emerging applications in sustainable packaging and inexpensive medical diagnostics. Cellulose has a high density of hydroxyl groups that create strong intra- and interfiber hydrogen bonding. These abundant hydroxyl groups also make cellulose superhydrophilic. Schemes for hydrophobization and spatially selective hydrophobization of cellulosic materials can expand the application space for cellulose. Cellulose is often hydrophobized through wet chemistry surface modification methods. This work reports a new modification method using a combination of atomic layer deposition (ALD) and atmospheric heating to alter the wettability of purely cellulosic chromatography paper. We find that once the cellulosic paper is coated with a single ALD cycle (1cy-ALD) of Al2O3, it can be made sticky superhydrophobic after a 150 °C ambient post-ALD heating step. An X-ray photoelectron spectroscopy investigation reveals that the ALD-modified cellulosic surface becomes more susceptible to adsorption of adventitious carbon upon heating than an untreated cellulosic surface. This conclusion is further supported by the ability to use alternating air plasma and heat treatments to reversibly transition between the hydrophilic and hydrophobic states. We attribute the apparent abruptness of this wetting transition to a Cassie-Wenzel-like phenomenon, which is also consistent with the sticky hydrophobic wetting behavior. Using scanning probe methods, we show that the surfaces have roughness at multiple length scales. Using a Cassie-Wenzel model, we show how a small change in the surface's Young's contact angle-upon adsorption of adventitious carbon-can lead to an abrupt increase in hydrophobicity for surfaces with such roughnesses. Finally, we demonstrate the ability to spatially pattern the wettability on these 1cy-ALD-treated cellulosic papers via selective heating. This ALD-treated hydrophobic paper also shows promise for microliter droplet manipulation and patterned lab-on-paper devices.
TL;DR: In this paper, Li et al. found that low temperature could induce the wetting state transition from a Cassie-Baxter state to a Wenzel state on superhydrophobic surfaces with pillar heights of 250 and 300 μm.
Abstract: Superhydrophobic materials are significant for engineering applications in the anti-icing field because of their non-wetting property. The interface physical mechanisms of non-wetting properties are important to promote real applications of superhydrophobic surfaces, especially under low-temperature conditions. Here, we found that low temperature could induce the wetting state transition from a Cassie–Baxter state to a Wenzel state. This transition occurred at 14 °C (and 2 °C) on superhydrophobic surfaces with pillar heights of 250 μm (and 300 μm). As a consequence, the driving-force of the Cassie-Wenzel (C-W) wetting transition was induced by the contraction of air pockets on cooling, and the pressure of air pockets supporting the droplet decreased with the contraction degree. Decreasing the pressure of air pockets broke the mechanical equilibrium at the solid–liquid contact interface, and the continuous contraction overcame the resistance in the C-W wetting transition. Based on the analysis of work against resistance in the C-W wetting transition, lower C-W wetting transition temperature was mainly attributed to a higher pillar, which produced more work against resistance to require more energy. This energy was directly reflected by the energy required for continuous contraction of air pockets. Superhydrophobic surfaces with higher pillar structure remain stable non-wetting property at low-temperature conditions. This work provides theoretical support for the application of superhydrophobic materials in low-temperature environments.
TL;DR: In this paper, the effect of atomic O incorporation on the improvement of Ag wetting was investigated by monitoring its participation in and contribution to the Ag clustering and layering processes, and the size and shape of the O-incorporated Ag nanoparticles were precisely controlled to accelerate the wetting transition of Ag on ZnO substrates by circumventing the transmutation effect of O.
TL;DR: In this article, a non-monotonic dependency of surface wettability on the hybridization states, which is just inverse for polar and non-polar liquids, was found on the homogeneous DLC films with nanoscale surface roughness.
TL;DR: In this article, laser induced periodic surface structures were generated in a single step by applying picosecond laser irradiation at different repetition rates on a surface area of 15×15mm2 of a stainless steel-304 probe immersed in a confinement liquid medium and on ambient air.
Abstract: Laser induced periodic surface structures were generated in a single step by applying picosecond laser irradiation at different repetition rates on a surface area of 15 × 15 mm2 of a stainless steel-304 probe immersed in a confinement liquid medium and on ambient air. Periodicity modification from high to low spatial surface frequency LIPSS was observed by modifying the pulse repetition rate from 1.3 to 402 kHz at a constant fluence (7 J/cm2) under water confinement. Additional experiments at lower fluences (1.78 and 5.33 J/cm2) were performed to evaluate the periodicity under water confinement. Wettability analysis of the processed area yielded significative changes on the drop contact angle showing a wetting transition from hydrophilicity to hydrophobicity of the samples surface treated by multiple impact pulses under water confinement.
TL;DR: In this article, the authors investigated the wettability of a surface patterned with trapezoidal nanopillars and found that the dewetting and wetting of the gap between pillars are related to the Cassie-Baxter (CB) and Wenzel (WZ) states of macroscopic water deposited on the pillared surface.
TL;DR: In this paper, the authors describe the role of nanostructure patterning in regulating phase transition at the liquid/solid interface, and show that it plays a significant role in regulating various phenomena such as phase transition.
Abstract: Surfaces with nanostructure patterning are broadly encountered in nature, and they play a significant role in regulating various phenomena such as phase transition at the liquid/solid interface. He...
TL;DR: This work seeks to investigate the dependence between the contact-line velocity and the slip length in a generalized Navier boundary condition (GNBC), by confronting numerical simulations to experimental data.
TL;DR: In this article, the thermally activated process of the dewetting of the trenches by a macroscopic liquid water was studied by using all-atom molecular dynamics simulation, and molecular insights for designing the texture of a surface with an improved hydrophobicity.
TL;DR: In this paper, the authors study non-equilibrium analogues of surface phase transitions in a minimal model of active particles in contact with a purely repulsive potential barrier that mimics a thin porous membrane.
Abstract: We study non-equilibrium analogues of surface phase transitions in a minimal model of active particles in contact with a purely repulsive potential barrier that mimics a thin porous membrane. Under conditions of bulk motility-induced phase separation, the interaction strength $\varepsilon_w$ of the barrier controls the affinity of the dense phase for the barrier region. We uncover clear signatures of a wetting phase transition as $\varepsilon_w$ is varied. In common with its equilibrium counterpart, the character of this transition depends on the system dimensionality: a continuous transition with large density fluctuations and gas bubbles is uncovered in 2d while 3d systems exhibit a sharp transition absent of large correlations.
TL;DR: In this paper, the Laplace pressure-driven spontaneous movement of condensed droplets on hierarchical superhydrophobic surfaces has been exploited to increase the condensation efficiency via the fast removal of the condensates from the surfaces.
24 Sep 2021
TL;DR: In this article, a theoretical model for the Cassie-Baxter-Wenzel wetting transition triggered by an external voltage applied to a droplet placed on a mirco-pillared surface or a porous substrate was proposed.
Abstract: Understanding the critical condition and mechanism of the droplet wetting transition between Cassie-Baxter state and Wenzel state triggered by an external electric field is of considerable importance because of its numerous applications in industry and engineering. However, such a wetting transition on a patterned surface is still not fully understood, e.g., the effects of electrowetting (EW) number, geometry of the patterned surfaces and droplet volume on the transition have not been systematically investigated. In this paper, we propose a theoretical model for the Cassie-Baxter-Wenzel wetting transition triggered by an external voltage applied to a droplet placed on a mirco-pillared surface or a porous substrate. It is found that the external field applied lowers the energy barrier for the droplet to cross over and complete the wetting transition. Our calculations also indicate that for a fixed droplet volume, the critical EW number (voltage) will increase (decrease) along with the surface roughness for a micropillar-patterned (porous) surface, and if the surface roughness is fixed, a small droplet tends to ease the critical EW condition for the transition. Besides, three-dimensional phase diagrams in terms of EW number, surface roughness, and droplet volume are constructed to illustrate the wetting transition. Our theoretical model can be used to explain the previous experimental results about the Cassie-Baxter-Wenzel wetting transition reported in the literature.
TL;DR: In this article, the authors investigated the wetting behavior of nanodroplets deposited on hydrophilic textured surfaces with discontinuous and continuous grooves, and showed that the partial-to-total wetting transition can occur as surface textures change from discontinuous to continuous.
TL;DR: Wang et al. as discussed by the authors applied the thermodynamic stable model on different organic liquid films by spreading parameters to predict a priori whether an arbitrary combination of solid roughness and organic liquid is suitable for designing lubricant-infused surfaces (LIS) used in gallium droplets.
Abstract: Wettability of liquid metal gallium is of vital significance in the field of modern industries, such as direct writing printing and microfluidics. A liquid interface is a recently developed and promising approach to regulate wettability but has not been well applied in liquid metals yet. This study focuses on the wetting performance of gallium droplets on organic liquid films. The results show that the organic liquid film could change the wetting state of the gallium droplet. Based on the solid substrate roughness and surface tension of the organic liquid, we could estimate whether the gallium droplet is in a slippery Wenzel or a Cassie state. Subsequently, we apply the thermodynamic stable model on different organic liquid films by spreading parameters to predict a priori whether an arbitrary combination of solid roughness and organic liquid is suitable for designing lubricant-infused surfaces (LIS) used in gallium droplets. More interestingly, we found that the "cloaking" could delay surface oxide formation, which will benefit the manipulation of liquid metal droplets. This paper would provide a better understanding of wettability of liquid metal on an organic liquid surface.
TL;DR: Theoretical and numerical studies were conducted to investigate the transitional interpillar spacing for dual-scale structures, where wetting transition between the Wenzel and Cassie-Baxter states occurs in the primary and secondary pillars as mentioned in this paper.
Abstract: Theoretical and numerical studies were conducted to investigate the transitional interpillar spacing for dual-scale structures, where wetting transition between the Wenzel and Cassie-Baxter states occurs in the primary and secondary pillars. A theoretical formula was derived for the transitional interpillar spacing based on the continuum picture of water. Molecular dynamics (MD) simulations were carried out by varying the interpillar spacing for the primary pillars for single- and dual-scale structures with various pillar heights. The results obtained from the theoretical formula agreed reasonably well with the results obtained from MD simulations, especially when the primary pillar height was relatively high. The transitional interpillar spacing increases as the pillar height and the number of secondary pillars increase. The effect of the secondary pillars on the transitional interpillar spacing was also evaluated using the difference in the grand potentials between the Wenzel and Cassie-Baxter states. These results show that the dual-scale structures increase the transitional interpillar spacing with an increase in the surface hydrophobicity.