Wetting Phase Diagram Research Articles

Predicting the Morphology of Multiphase Biomolecular Condensates from Protein Interaction Networks

Phase-separated biomolecular condensates containing proteins and RNAs can assemble into higher-order structures by forming thermodynamically stable interfaces between immiscible phases. Using a minimal model of a protein/RNA interaction network, we demonstrate how a “shared” protein species that partitions into both phases of a multiphase condensate can function as a tunable surfactant that modulates the interfacial properties. We use Monte Carlo simulations and free-energy calculations to identify conditions under which a low concentration of this shared species is sufficient to trigger a wetting transition. We also describe a numerical approach based on classical density functional theory to predict concentration profiles and surface tensions directly from the model protein/RNA interaction network. Finally, we show that the wetting phase diagrams that emerge from our calculations can be understood in terms of a simple model of selective adsorption to a fluctuating interface. Our work shows how a low-concentration protein species might function as a biological switch for regulating multiphase condensate morphologies. Published by the American Physical Society 2024

Wetting of nanoscale water films on hierarchically structured surfaces

Surfaces with hierarchical structures significantly enhance the hydrophobic properties of solids, proving crucial for diverse applications including self-cleaning, anti-icing, and contamination prevention. In this study, we directly observe the dynamic wetting transitions of nanoscale water films on desirable textured surfaces decorated with dual-scale roughness between various wetting states encompassing Cassie–Cassie, Wenzel–Cassie, Cassie–Wenzel, and Wenzel–Wenzel states. Additionally, detailed information on the wetting of the water film on desirable textured surfaces decorated with dual-scale roughness is obtained using atomistic simulations in conjunction with sampling techniques. Through observation of the dynamic wetting transition, two common types of wetting pathways are directly captured, dubbed the preferential primary intrusion and secondary intrusions. The wetting follows which pathway is dependent on Hs/Ss of the small-scale roughness. The mechanisms behind the wetting transitions are revealed based on corresponding free-energy pathways. Moreover, the effect of aspect ratio and intrinsic contact angle on the wetting behavior has been studied. Subsequently, we construct a wetting phase diagram to exhibit all the possible outcomes and identify different wetting regimes. This work paves the way to understanding the wetting mechanisms on nanoscale textured surfaces with two-tier roughness, which can help to design a hydrophobic surface with superior robustness.

Density functional approach to wetting behavior of water on solid surfaces modified by grafting of chains and their mixtures

We apply a classical density functional theory for associating fluids to study wetting of a simple model of water and graphite-like solid surface chemically modified by the grafting of chains and their mixtures. The grafting points are randomly distributed on the surface. The chain molecules consist of tangentially bonded spherical segments. We explore adsorption of water on such complex substrates and evaluate different projections of the wetting phase diagrams dependent on grafting density, chains length and composition of grafted chains mixture. Besides, the contact angle dependence on temperature is obtained and discussed. If the total grafting density is high enough, the prewetting line splits into two lines that join at a triple point. Each of the lines is associated with changes in the predominant configuration of segments of grafted chains. The role of attractive interaction of water with segments of chains is explored. Despite the qualitative similarity of the phase diagrams of one- and two-component grafted chains, the behavior of chain molecules during phase transitions may be different. The chains of one kind can stretch, while those of the second - can collapse. Such behavior corresponds to the “vertical separation” of the chains. Trends of this phenomenon are studied in detail.

A novel prewetting behavior of water adsorbed on solid surfaces modified with tethered chains resulting from a density functional theory

The density functional approach for classical associating fluids is used to explore wetting phase diagrams for model systems consisting of water and graphite-like solid surfaces chemically modified by a small amount of grafted chain molecules. The water-like fluid model is adopted from the work of Clark et al. [Molec. Phys. 104, 3561 (2006)]. It describes the bulk water vapor–liquid coexistence very well. Each chain molecule consists of tangentially bonded hard spheres. We focus on the investigation of the growth of water film on such a complex substrate and the construction of the wetting phase diagram. We have found that the topology of the phase diagram depends on the amount of grafted chains and on the length of chains. At low grafting densities and for short chains, the prewetting phase diagrams are similar to the diagrams for water on a non-modified solid surface. For higher grafting densities and for longer chains, the prewetting line can split into two lines that join at a triple point. Each of the lines is associated with changes in the predominant configuration of segments of grafted chains. They can either stretch or coil upon the growth of an adsorbed film. Trends of the behavior of wetting temperature and the critical temperatures of the branches of the prewetting transition are discussed.

Statistical mechanics study on wetting behaviors of Ne on Mg surface

Wetting behavior of Ne adsorbed on a Mg surface, first investigated by means of a grand canonical Monte Carlo method in a previous publication (M. J. Bojan, G. Stan, S. Curtarolo, W. A. Steele, and M. W. Cole, Phys. Rev. E, 1999, 59, 864), is again studied by means of classical density functional theory. The Ne-Ne interaction is taken to be of the Lennard-Jones form, while the Ne-surface interaction is derived from an electronic density functional theory. The wetting phase diagram is calculated, and the isotherm shapes, energy and structural properties of the adsorbed films are examined. The present calculations indicate that the system exhibits first-order pre-wetting transition at temperatures above a wetting temperature of Tw≈24K, and below a critical pre-wetting temperature of Tpwc≈25.09K. The present findings include (i) in the pre-wetting temperature region, the pre-wetting transition is mixed with many layering transitions; after pre-wetting, the film thickness discontinuously increases (due to frequent occurrences of the layering transitions) and eventually diverges as the chemical potential approaches and eventually equals the saturation value. (ii) Occurrence of the layering transition remains above Tpwc, and the increase of the film thickness with the chemical potential is discontinuous. (iii) Below the wetting temperature, the layering transitions frequently occur and tend to gather together more closely as the saturation is approached.

Wetting phase transition of two segregated Bose–Einstein condensates restricted by a hard wall

The wetting phase transition in the system of two segregated Bose–Einstein condensates (BECs) restricted by a hard wall is studied by means of the double-parabola approximation (DPA) applied to the Gross–Pitaevskii (GP) theory. We found the interfacial tension and the wetting phase diagram which depend weakly on the spatial restriction.

Effects of Hierarchical Surface Roughness on Droplet Contact Angle.

Superhydrophobic surfaces often incorporate roughness on both micron and nanometer length scales, although a satisfactory understanding of the role of this hierarchical roughness in causing superhydrophobicity remains elusive. We present a two-dimensional thermodynamic model to describe wetting on hierarchically grooved surfaces by droplets for which the influence of gravity is negligible. By creating wetting phase diagrams for droplets on surfaces with both single-scale and hierarchical roughness, we find that hierarchical roughness leads to greatly expanded superhydrophobic domains in phase space over those for a single scale of roughness. Our results indicate that an important role of the nanoscale roughness is to increase the effective Young's angle of the microscale features, leading to smaller required aspect ratios (height to width) for the surface structures. We then show how this idea may be used to design a hierarchically rough surface with optimally high contact angles.

Isothermal solid–liquid transitions in the (Ni,B)/ZrB2 system as revealed by sessile drop experiments

In the framework of joining processes of ultrahigh temperature ceramics (UHTCs), sessile drop experiments were performed in the Ni–B/ZrB2 system in the range 1110° ≤ T ≤ 1200 °C. They show that, at temperatures between 1110 and 1150 °C, isothermal solid–liquid transitions occur in a sequence; while in fact at T ≥ 1200 °C, the drop melts without any further phase transition, at lower temperatures, complete melting is followed by a solidification stage and final remelting. This complex behavior, which can be very relevant when utilizing Ni–B alloys for brazing processes (e.g., by the transient liquid phase bonding technique), is successfully interpreted on the basis of the complete B–Ni–Zr phase diagram newly computed by CALPHAD: Isothermal sections, calculated between 1110 and 1150 °C, show that the composition of the drop enters, crosses, and leaves the primary solidification region of the Zr2Ni21B6 ternary compound. The use of thermodynamic modeling for the explanation of experimental data sets a link between wetting and joining experiments and phase diagrams assessment.

Cassie–Baxter to Wenzel state wetting transition: a 2D numerical simulation

We simulate in two dimensions the wetting behavior of a liquid droplet placed on a solid surface structured by a regular distribution of pillars. For this purpose, as in a recent study, we used the Potts Hamiltonian formalism. We focused our investigations on the transition between the wetting regimes characteristic of the super-repellent property of our surfaces. We determined several physical variables influencing such a property and we also built the wetting phase diagrams of which two different states and a transition zone were identified. The transition happens between a Cassie–Baxter state (CB) and the Wenzel state. We also show that the wetting transition is affected by fluctuations and liquid compressibility.

Wetting, spreading, and adsorption on randomly rough surfaces

The wetting properties of solid substrates with customary (i.e., macroscopic) random roughness are considered as a function of the microscopic contact angle of the wetting liquid and its partial pressure in the surrounding gas phase. Analytic expressions are derived which allow for any given lateral correlation function and height distribution of the roughness to calculate the wetting phase diagram, the adsorption isotherms, and to locate the percolation transition in the adsorbed liquid film. Most features turn out to depend only on a few key parameters of the roughness, which can be clearly identified. It is shown that a first-order transition in the adsorbed film thickness, which we term "Wenzel prewetting", occurs generically on typical roughness topographies, but is absent on purely Gaussian roughness. It is thereby shown that even subtle deviations from Gaussian roughness characteristics may be essential for correctly predicting even qualitative aspects of wetting.

Wetting transition in the two-dimensional Blume-Capel model: A Monte Carlo study

The wetting transition of the Blume-Capel model is studied by a finite-size scaling analysis of L×M lattices where competing boundary fields ±H_{1} act on the first or last row of the L rows in the strip, respectively. We show that using the appropriate anisotropic version of finite-size scaling, critical wetting in d=2 is equivalent to a "bulk" critical phenomenon with exponents α=-1, β=0, and γ=3. These concepts are also verified for the Ising model. For the Blume-Capel model, it is found that the field strength H_{1c}(T) where critical wetting occurs goes to zero when the bulk second-order transition is approached, while H_{1c}(T) stays nonzero in the region where in the bulk a first-order transition from the ordered phase, with nonzero spontaneous magnetization, to the disordered phase occurs. Interfaces between coexisting phases then show interfacial enrichment of a layer of the disordered phase which exhibits in the second-order case a finite thickness only. A tentative discussion of the scaling behavior of the wetting phase diagram near the tricritical point is also given.

Discussion notes on “Universality in the dependence of the drop contact angle on liquid-solid interactions and temperature obtained by the density functional theory”, by G.O. Berim & E. Ruckenstein

Discussion notes on “Universality in the dependence of the drop contact angle on liquid-solid interactions and temperature obtained by the density functional theory”, by G.O. Berim & E. Ruckenstein

Wetting transitions of continuously varying or infinite order from a mean-field density-functional theory

We consider a mean-field density-functional model for three-phase equilibria and wetting. The model features two densities and two control parameters, one of which is related to order parameter asymmetry or spatial anisotropy. The global wetting phase diagram in the space of these two parameters is rich. It features first-order, second-order, continuously-varying-order and infinite-order wetting transitions. The divergence of the wetting layer thickness is usually logarithmic as a function of the distance to the transition, but, in contrast, algebraic upon approach of an infinite-order transition. Further, an approximate interface potential approach is proposed, which allows us to derive analytic predictions for the singular behaviour of thermodynamic functions near wetting, in accordance with accurate numerical computations. It is conjectured that previously developed mean-field models, such as, for example, one for ferromagnets with cubic anisotropy also contain a segment of infinite-order transitions. We speculate that the possibility of varying the spatial anisotropy of the magnetic interaction in these systems might well lead the way towards the first experimental realization of infinite-order wetting.

Wetting reversal in colloid-polymer systems

The wetting of a phase-separated colloid-polymer mixture in contact with a hard wall is analyzed using free volume theory in a Nakanishi-Fisher-type approach. We present results for the wetting phase diagram for several model approximations. Our analysis is compared with a previous analysis by Aarts [J. Chem. Phys. 120, 1973 (2004)]. We find that there is a crossover from wetting to drying at a threshold value for the colloid-polymer size ratio and that the transitions are close to the critical point and of second order in nature.

A Microscopic Model of Interface‐Reaction‐Controlled Sintering of Spherical Particles of Different Phases

A microscopic model is presented, which describes the sintering of equal‐sized spherical particles that differ in phase. A wetting phase diagram of sintering was developed that classifies the equilibrium shapes that can arise during sintering of two spheres with different surface energies. The formation and growth of the interface between the dissimilar phases was studied from the point of view of solid‐state wetting. The three‐dimensional computer simulation of interface‐reaction‐controlled sintering was conducted using the Surface Evolver program. The deviation of dynamic contact angle from its equilibrium value was proportional to the contact line velocity and inversely proportional to the triple junction mobility. The stability of particle cluster configuration in equilibrium was also examined using Cahn's concept on grain‐boundary energy ratios in two‐phase materials.

Critical wetting transitions in two-dimensional systems subject to long-ranged boundary fields

Using the quasiexact density-matrix renormalization-group method and ground-state analysis we study interface delocalization transitions in wide two-dimensional Ising strips subject to long-ranged boundary fields with opposite signs at the two surfaces. Based on this approach, our explicit calculations demonstrate that critical wetting transitions do exist for semi-infinite two-dimensional systems even if the corresponding effective interface potentials decay asymptotically for large l as slow as l(-delta) with delta<2, where l is the distance of the mean interface position from the one-dimensional surface. This supersedes opposite claims by Kroll and Lipowsky [Phys. Rev. B 28, 5273 (1983)] and by Privman and Svrakić [Phys. Rev. B 37, 5974 (1988)] obtained within effective interface models. The corresponding wetting phase diagram is determined, including the cases delta=2 and delta=49 with the latter mimicking short-ranged surface fields. Our analysis highlights the limits of reliability of effective interface models.

Wetting Transitions in Polydisperse Fluids

The properties of the coexisting bulk gas and liquid phases of a polydisperse fluid depend not only on the prevailing temperature but also on the overall parent density. As a result, a polydisperse fluid near a wall will exhibit density-driven wetting transitions inside the coexistence region. We propose a likely topology for the wetting phase diagram, which we test using Monte Carlo simulations of a model polydisperse fluid at an attractive wall, tracing the wetting line inside the cloud curve and identifying the relationship to prewetting.

Wetting phase diagrams of a polyacid brush with a triple point

The (pre)wetting behavior of an annealed polyelectrolyte (PE) brush by an electrolyte solution that is strongly segregated from an apolar phase is analyzed. In this complex interface, there are interactions on various length scales. There are short-range interactions with the (uncharged) surface, and there are interactions on the length scale of the brush height. Using either the ionic strength or the water-surface interaction strength as the control parameters, it is possible to approach and induce a wetting transition in this system. The first-order wetting transition, promoted by favorable short-range substrate interactions with the surface, is in competition with the wetting transition controlled by the detachment of the fluid interface from the periphery of the PE brush. The electric double layer on top of the PE brush contributes with a repulsive forces to the disjoining pressure that tends to thicken the wetting film, and therefore, the transition in all cases is first order. Various phase portraits of the wetting phase diagram are envisioned. One of these features the crossing of two prewetting lines. At the crossing point three surface states coexist. This triple point is analyzed in some detail with the help of a molecular-level self-consistent field model.

Extraordinary Wetting Phase Diagram for Mixtures of Bose-Einstein Condensates

The possibility of wetting phase transitions in Bose-Einstein condensed gases is predicted on the basis of Gross-Pitaevskii theory. The surface of a binary mixture of Bose-Einstein condensates can undergo a first-order wetting phase transition upon varying the interparticle interactions, using, e.g., Feshbach resonances. Interesting ultra-low-temperature effects shape the wetting phase diagram. The prewetting transition is, contrary to general expectations, not of first order but critical, and the prewetting line does not meet the bulk phase coexistence line tangentially. Experimental verification of these extraordinary results is called for, especially now that it has become possible, using optical methods, to realize a planar "hard wall" boundary for the condensates.

Wetting of a spherical particle by a nematic liquid crystal.

We discuss how the curvature of a substrate influences wetting by a nematic liquid crystal concentrating on the surface of a spherical particle. Our investigation is based on Landau-de Gennes free energy formulated in terms of second-rank nematic order parameter Q(ij). We review the method to treat wetting transitions in curved geometries and calculate the wetting phase diagram in terms of the temperature and a surface coupling parameter. We find that the length of the prewetting line which corresponds to the boundary-layer transitions introduced by Sheng [Phys. Rev. A 26, 1610 (1982)] gradually decreases with a decrease in particle radius until it vanishes completely below a critical radius of about 100 nm. The prewetting line ends at a critical point which we study in detail. By interpreting the effect of curvature as an effective shift in temperature in Landau-de Gennes theory, we are able to formulate a good estimate for the critical temperature as a function of the inverse particle radius. It demonstrates that splay deformations around the particle significantly influence nematic wetting of curved surfaces.