From Contact Line Structures to Wetting Dynamics

Water Film Structure and Wettability of Different Quartz Surfaces: Hydrogen Bonding Across Various Cutting Planes

Quartz is ubiquitous in subsurface formations. The crystal faces have different atomic arrangements. Knowledge of the molecular structures on the surface of quartz is key in many processes. Molecular dynamics simulations are conducted to investigate the atomic arrangement effect on the water film structure, ion adsorption, and wettability at three different α-quartz surfaces. The interfacial structures depend on the quartz surface. Intrasurface hydrogen bonding between surface silanols differs in the α-quartz surface. At the (0001) surface, the OH density is 9.58 nm-2, consisting of Q2 units with two hydroxyl groups per silicone atom. At the (101̅0)-β surface, the OH density is 7.54 nm-2, consisting of Q3 units with one hydroxyl group per silicone atom; there is significant intrasurface hydrogen bonding. At the (101̅0)-α surface, the OH density is 7.54 nm-2, consisting of Q2 units; however, there is little intrasurface hydrogen bonding. The intrasurface hydrogen bonding results in the exposure of hydrogen-bond acceptors to the aqueous phase, causing water molecules to have an H-up (hydrogen toward surface) orientation. This orientation can be found at the (0001) and (101̅0)-β surfaces; it is related to the degree of ordering at the surface. The ordering at the (0001) and (101̅0)-β surfaces is higher than that at the (101̅0)-α surface. In aqueous systems with ions, cation adsorption is the most dominant at the (0001) surface due to the largest surface density of the intrasurface hydrogen bonding, providing interaction sites for cations to be adsorbed. We observe a pronounced decrease in water film thickness from the ions at the (0001) surface only, likely due to significant cation adsorption. In this work, we demonstrate that the hydrogen-bond network, which varies from the plane cut, affects the water film structure and ion adsorption. The contact is nearly zero despite the changes in the film thickness and molecular structure at the temperature of 318 K.

Real-time three-dimensional micro-imaging of liquid front in spreading sessile droplet under transient spreading states

Dynamic wetting behaviors of sessile droplets on substrates play crucial roles for various industrial chemical processes. In the case of complete wetting, it has been proposed that a precursor film which is nanometer-order thickness and micrometer-order length further expands outside the macroscopic contact line of the sessile droplet. While the time evolution of the precursor film is believed to strikingly affect the macroscopic wetting behavior (e.g., spreading velocity), it had been hard to visualize the three-dimensional shape of the precursor film at the early stage of wetting. Further, although the spreading velocity rapidly decreases upon time at the early stage of the wetting (transient state) and then converged to be a constant at later time (steady state), conventional fluid mechanics theories generally describe only the wetting behavior at the steady state. Therefore, experimental observation of time evolution of the precursor film shape in the transient state is essential to proceed the theories to the next step. Here, a monochromatic laser interference microscope was developed to visualize three-dimensional shape of the wetting front of sessile droplets in real time. By detecting an interference of the laser reflected at the liquid and substrate surfaces, the precursor film was successfully visualized with a time resolution of 20 ms and a thickness resolution of about 3.5 nm at worst. For 10 cSt silicone oil on Si substrate, a 60-nm-thick and 70-μm-long precursor film was observed for 2-10 min after dropping and its spreading velocity which decreased with time was quantitatively analyzed.

Spreading dynamics of microdroplets on nanostructured surfaces

HYPOTHESIS

Droplet spreading governs various daily phenomena and industrial processes. Insights about microdroplet spreading are limited due to experimental difficulties arising from microdroplet manipulation and substrate wettability control. For droplet sizes approaching the capillary length scale, the gravitational force plays an important role in spreading. In contrast, capillary and viscous forces dominate as the droplet size reduces to smaller length scales. We hypothesize that the dynamic spreading behavior of microdroplets whose radius is far lower than the capillary length differs substantially from established and well understood dynamics.

EXPERIMENTS

To systematically investigate the spreading dynamics of microdroplets, we develop contact-initiated wetting techniques combined with structuring-independent wettability control to achieve microdroplet (<500 μm) spreading on arbitrary surfaces while eliminating parasitic pinning effects (pining force ∼ 0) and initial impact momentum effects (Weber number ∼ 0).

FINDINGS

Our experiments reveal that the capillary-driven initial spreading of microdroplets is shorter, with significantly reduced oscillation dampening, when compared to millimeter-scale droplets. Furthermore, spreading along with capillary wave propagation results in coupling between the spreading velocity and dynamic contact angle at the contact line. These findings, along with our proposed microdroplet manipulation platform, may find application in microscale heat transfer, advanced manufacturing, and aerosol transmission studies.

How droplets pin on solid surfaces

HYPOTHESIS

When a droplet starts sliding on a solid surface, the droplet-solid friction force develops in a manner comparable to the solid-solid friction force, showing a static regime and a kinetic regime. Today, the kinetic friction force that acts on a sliding droplet is well-characterized. But the mechanism underlying the static friction force is still less understood. Here we hypothesize that we can further draw an analogy between the detailed droplet-solid and solid-solid friction law, i.e., the static friction force is contact area dependent.

METHODS

We deconstruct a complex surface defect into three primary surface defects (atomic structure, topographical defect, and chemical heterogeneity). Using large-scale Molecular Dynamics simulations, we study the mechanisms of droplet-solid static friction forces induced by primary surface defects.

FINDINGS

Three element-wise static friction forces related to primary surface defects are revealed and the corresponding mechanisms for the static friction force are disclosed. We find that the static friction force induced by chemical heterogeneity is contact line length dependent, while the static friction force induced by atomic structure and topographical defect is contact area dependent. Moreover, the latter causes energy dissipation and leads to a wiggle movement of the droplet during the static-kinetic friction transition.

Dynamic wetting of various liquids: Theoretical models, experiments, simulations and applications

Dynamic wetting is a ubiquitous phenomenon and frequently observed in our daily life, as exemplified by the famous lotus effect. It is also an interfacial process of upmost importance involving many cutting-edge applications and has hence received significantly increasing academic and industrial attention for several decades. However, we are still far away to completely understand and predict wetting dynamics for a given system due to the complexity of this dynamic process. The physics of moving contact lines is mainly ascribed to the full coupling with the solid surface on which the liquids contact, the atmosphere surrounding the liquids, and the physico-chemical characteristics of the liquids involved (small-molecule liquids, metal liquids, polymer liquids, and simulated liquids). Therefore, to deepen the understanding and efficiently harness wetting dynamics, we propose to review the major advances in the available literature. After an introduction providing a concise and general background on dynamic wetting, the main theories are presented and critically compared. Next, the dynamic wetting of various liquids ranging from small-molecule liquids to simulated liquids are systematically summarized, in which the new physical concepts (such as surface segregation, contact line fluctuations, etc.) are particularly highlighted. Subsequently, the related emerging applications are briefly presented in this review. Finally, some tentative suggestions and challenges are proposed with the aim to guide future developments.

Energies Associated with a Meniscus along a Flat Vertical Wall

In this theoretical work, existing equations for the height and shape of a liquid meniscus along a vertical flat wall are used to estimate the volume and energies associated with its formation. A mechanism and an associated energy balance are proposed. Equations for the work of wetting, surface energy, gravitational energy, and dissipation are derived. This analysis shows that the energy spontaneously released during wetting is either stored in the air-liquid interface of the newly formed meniscus, stored in its bulk liquid as gravitational energy, or dissipated as heat. The absolute magnitude of these energies depends on the surface tension and density of the liquid as well as the wettability of the wall.

Numerical Investigation on the Symmetric Breakup of Bubble within a Heated Microfluidic Y-Junction

This study numerically investigated the symmetric breakup of bubble within a heated microfluidic Y-junction. The established three-dimensional model was verified with the results in the literature. Two crucial variables, Reynolds number (Re) and heat flux (q), were considered. Numerical results demonstrated that the bubble breakup was significantly affected by phase change under the heated environment. The “breakup with tunnel” and “breakup with obstruction” modes respectively occurred at the low and high q. The breakup rate in pinch-off stage was much larger than that in squeezing stage. As Re increased, the bubble broke more rapidly, and the critical neck thickness tended to decrease. The bubble annihilated the vortices existing in the divergence region and made the fluid flow more uniform. The heat transfer was enhanced more drastically as Re was decreased or q was increased, where the maximum Nusselt number under two-phase case was 6.53 times larger than single-phase case. The present study not only helps understanding of the physical mechanisms of bubble behaviors and heat transfer within microfluidic Y-junction, but also informs design of microfluidic devices.

Wetting of surfaces decorated by gas-phase synthesized silver nanoparticles: Effects of Ag adatoms, nanoparticle aging, and surface mobility

The wetting state of surfaces can be rendered to a highly hydrophobic state by the deposition of hydrophilic gas phase synthesized Ag nanoparticles (NPs). The aging of Ag NPs leads to an increase in their size, which is also associated with the presence of Ag adatoms on the surface between the NPs that have a strong effect on the wetting processes. Furthermore, surface airborne hydrocarbons were removed by UV-ozone treatment, providing deeper insight into the apparent mobility of the NPs on different surfaces and their subsequent ripening and aging. In addition, the UV-ozone treatment revealed the presence of adatoms during the magnetron sputtering process. This surface treatment lowers the initial contact angle of the substrates and facilitates the mobility of Ag NPs and adatoms on the surface of substrates. Adatoms co-deposited on clean high surface energy substrates will nucleate on Ag NPs that will remain closely spherical and preserve the pinning effect due to the water nanomeniscus. If the adatoms are co-deposited on a UV-ozone cleaned low surface energy substrate, their mobility is restricted, and they will nucleate in two-dimensional islands and/or nanoclusters on the surface instead of connecting to existing Ag NPs. This growth results in a rough surface without overhangs, where the wetting state is reversed from hydrophobic to hydrophilic. Finally, different material surfaces of transmission electron microscopy grids revealed strong differences in the sticking coefficient for the Ag NPs, suggesting another factor that can strongly affect their wetting properties.

Direct Writing Large-Area Multi-Layer Ultrasmooth Films by an All-Solution Process: Toward High-Performance QLEDs

With increasing the film area/layer, deteriorating in both smoothness and uniformity of thin-films frequently happen, which remains a barrier for making large-area quantum dot light-emitting diodes (QLEDs) by solution processes. Here, we demonstrated a facile all-solution process guided by the conical fiber array to write multi-layer ultrasmooth thin-films directly in centimeter scale. The side-by-side fibrous array helps to align surface tensions at the tri-phase contact line to facilitate large-area homogeneous deposition, which was verified by theoretical simulation. The Laplace pressure along individual conical fiber contributes to the steady liquid transfer. Thin-films with small roughness (<2.03 nm) and large-area (2×2 cm² ) uniformity were prepared sequentially on the target substrate, leading to large-area high-performance QLEDs. The result offers new insights for fabricating large-area high-performance thin-film devices.

Spreading and receding of oil droplets on silanized glass surfaces in water: Role of three-phase contact line flow direction in spontaneous displacement

HYPOTHESIS

The spontaneous displacement of both spreading and receding droplets on surfaces are extensively involved in numerous technical applications. We hypothesize that the spreading and receding displacement behaviors could be interpreted differently due to opposite flow directions at the three-phase contact line.

EXPERIMENTS

We performed two groups of displacement experiments using different initial setups of oil droplets on silanized glass surfaces in aqueous surroundings.

FINDINGS

The different initial configurations mostly resulted in oil displacement in opposite directions: either spreading or receding of the oil droplet. Different static states were observed at the end of the spreading and receding processes on surfaces with the same wettability due to the contact angle hysteresis. The dynamic displacement was analyzed using the hydrodynamic and molecular kinetic models, which showed distinct applicabilities for the data description of the spreading and receding possesses. The model analysis further indicated the different nature of these possesses, in particular, the resistance to displacement dynamics, which was illustrated by the interpretation of the microscopic slip length and contact line friction in the respective models. This study can shed light on the fundamental role of the displacement direction in the spontaneous liquid-liquid displacement.

Dynamic wetting of solid-liquid-liquid system by molecular kinetic theory

HYPOTHESIS

The mechanisms of dynamic wetting of solid-liquid-liquid (SLL) system, especially the viscosity effects of two liquids, can be investigated by the molecular kinetic theory (MKT).

METHODS

The molecular kinetic theory combined with published data was used to study the roles of a fluid viscosity and a solid surface in dynamic wetting.

FINDINGS

First, the MKT on dynamic wetting was introduced and its limitation was analyzed. Second, a viscosity effect and a solid surface effect were considered. The viscosity effect was divided into three parts for the first time, including two pure liquid zones and a mixing zone. Third, a coefficient activation free energy model was proposed, considering the effects of mixing liquids and a solid surface. Finally, the key parameters in the MKT and the application and validation of the coefficient activation free energy model were discussed in detail. This model can explain the energy dissipation in a vicinity of a three-phase contact-line successfully in a SLL wetting system. This work sheds light on the physical mechanisms of fluid and solid surface properties on the dynamic wetting in a SLL system.

Actuation of a Nonconductive Droplet in an Aqueous Fluid by Reversed Electrowetting Effect

Manipulation of a conductive droplet by electrowetting has been a popular topic in microfluidics whereby wettability of the droplet on a solid surface is increased by applying a voltage between the conductive droplet and the insulated surface. However, the opposite phenomenon, e.g., decreasing the wettability of a nonconductive droplet and increasing its contact angle (CA) by the reversed electrowetting (REW) effect, has been scarcely reported. Such a process involves not only the transient dynamics of droplet dewetting but also a critical condition for droplet detachment from the adhesive surface. In this work, actuation of a nonconductive droplet in an aqueous surrounding fluid by REW is studied experimentally. Silicone oil is used for the actuated droplet, and filtered water is used as the surrounding fluid. The solid substrate is made of a glass substrate coated with an indium tin oxide (ITO) film and then deposited by a dielectric layer of Parylene C. Potential difference is applied between the substrate and the surrounding fluid, eliminating the disturbance from the top needle on the motion of the droplet. Three different regimes are identified in the full range of operation. An underactuated regime occurs at low applied voltages, in which the CA of the droplet shows a monotonic increase with the increase of voltage (V). The friction coefficient of the contact line decreases with V before the CA saturation (V_s) but shows little change when V > V_s. At high voltages, the contact line of the sessile droplet is contracted excessively by REW. The droplet shows oscillation, and it refers to the overactuated regime. A combined time scale is proposed, and it verifies that the viscous dissipation of the contact line and liquid inertia show comparable contributions in the droplet dynamics. At sufficiently high voltages, the droplet is rejected completely from the surface. A critical equation for the threshold voltage of droplet detachment is built, and its validity is confirmed by experimental results.

Spreading of biologically relevant liquids over the laser textured surfaces

HYPOTHESIS

The distribution of biological objects upon the spreading of biologically relevant dispersions over laser textured surfaces is affected by the dispersion composition and substrate chemistry and roughness.

EXPERIMENTS

To examine the role of the substrate texture in biologically relevant liquid spreading, the dynamic behavior of droplets of water and dispersions of bacterial cells and viruses and dynamic behavior of droplet/air surface tension were addressed. A new procedure to simultaneously distinguish three different spreading fronts was developed.

FINDINGS

The study of spreading of water and the biologically relevant liquids over the laser textured substrate indicate the development of three spreading fronts associated with the movement of bulk droplet base, the flow along the microchannels, and the nanotexture impregnation. The anisotropy of spreading for all types of liquid fronts was found. Despite the expected difference in the rheological behavior of water and the biologically relevant liquids, the spreading of all tested liquids was successfully described by power-law fits. The droplet base spreading for all tested liquids followed the Tanner law. The advancing of water and dispersions in the microchannels along both fast and slow axes was described by Washburn type behavior. The impregnation of the nanotexture by water and biologically relevant liquids demonstrated universality in power fit description with an exponent n = 0.23.

Dependences of Formation and Transition of the Surface Condensation Mode on Wettability and Temperature Difference

In this work, we use molecular dynamics (MD) simulations to investigate the dependences of formation and transition of surface condensation mode on wettability (β) and vapor-to-surface temperature difference (ΔT). We build a map of different surface condensation modes against β and ΔT based on plenty of MD simulation results and reveal five formation mechanisms and two transition mechanisms. At low β and ΔT, the high free energy barrier (ΔG*) prevents any surface clusters from surviving, therefore no-condensation (NC) is observed. The formation of dropwise condensation (DWC) could evolve from either nucleation or film rupture. Similarly, the formation of filmwise condensation (FWC) could evolve from either nucleation or the adsorption-induced film. The transition between NC and DWC is determined by ΔG* according to classical nucleation theory. The transition between DWC and FWC depends on the stability of condensate film; there emerges the competition between the trend that the uneven condensate film contracts and ruptures to droplets favored by lower β and the trend that the uneven condensate film continues growing promoted by higher ΔT. We finally present a schematic overview of all of the mechanisms revealed for a better understanding of the physical phenomenon of the surface condensation mode.

Optical and Superhydrophilic Characteristics of TiO2 Coating with Subwavelength Surface Structure Consisting of Spherical Nanoparticle Aggregates

It is expected that the applications of photocatalytic coatings will continue to extend into many areas, so it is important to explore their potential for enhanced functionality and design flexibility. In this study, we investigated the effect of a subwavelength surface structure in a TiO2 coating on its optical and superhydrophilic characteristics. Using submicron-scale spherical aggregates of TiO2 nanoparticles, we fabricated a TiO2 film with a subwavelength surface structure. Optical examination showed the enhanced transmittance of visible light compared to that of a plain surface. This was considered to be a result of a graded refractive index at the air–TiO2 interface. The effect of the subwavelength surface structure on optical transmittance was also demonstrated by the numerical simulation of visible light propagation in which Maxwell’s equations were solved using the finite-difference time-domain method. In addition, superhydrophilic behavior without ultraviolet light illumination was observed for the subwavelength-structure film via the measurement of the contact angle of a water drop. Furthermore, it was confirmed that the photocatalytic activity of the proposed film was comparable with that of a standard TiO2 film. It was suggested that the control of the subwavelength surface structure of a TiO2 film could be utilized to achieve novel properties of photocatalytic coatings.