Surface Capillary Forces Research Articles

Patterned Hybrid Surfaces for Efficient Dew Harvesting.

Dew harvesting, minimally influenced by climate and geographical locations, is an ideal method for addressing water shortage problems. Superhydrophilic surfaces, characterized by their highest affinity for water, are particularly attractive for this purpose as they can attract more water molecules via condensation. However, a significant challenge arises from the high surface capillary force that impedes water from sliding down and being effectively collected. The resulting water film on the superhydrophilic surface tends to stay around the edge of the water collection surface, leading to evaporation loss and reduced collection efficacy. To overcome this problem, triangular patterns with low surface adhesion to water were introduced at the edge of superhydrophilic surfaces. This modification, achieved through a wet chemical method and masked oxygen plasma treatment, has significantly improved the efficiency of water collection. Results indicate that the hybrid surface reduced the time for the first water droplet to slide down by half and increased water collection efficiency by 78% compared to uniform superhydrophilic surfaces and by 536% compared to uniform superhydrophobic surfaces under a relative humidity of 55% with a temperature difference of 15 °C. The underlying principles were elucidated through computational simulations, and the mechanisms driving the enhancement in collection efficiency were explained.

Directional and Adaptive Oil Self‐Transport on a Multi‐Bioinspired Grooved Conical Spine

AbstractManipulating oil droplets in an aqueous solution offers great opportunities to develop advanced devices in the fields of water remediation, chemical micro‐reactor, etc. Although conical structures can achieve a directional oil droplet motion, the continuous and adaptive fluid self‐transport in complicated environments is still a challenge. Inspired by the distinctive oil transport capability of fishbone and the anisotropic grooved structure of rice leaves, this work presents a multi‐bioinspired grooved conical spine (BGCS) for improved oil manipulation. Benefiting from the cooperative effect of the asymmetric Laplace pressure and the surface capillary force, the BGCS possesses the reliable functions of ultrafast and continuous oil transport under water, in air, and even across the air–water interface. Compared with the original cone, the BGCS demonstrates a nine times faster transporting velocity and a five times larger capacity. It is envisioned that this updated oil‐collecting cone with its cooperative structure should find real‐world applications.

Structure evolution and stress transition in diamond-like carbon films by glancing angle deposition

In this article the diamond-like carbon films have been prepared by the strategy of the glancing angle deposition using unbalanced magnetron sputtering technique. It is found that the structure and the residual stress of the films are strongly dependent on the deposition angle. Of particular interest is that the stress of the films significantly transforms from compressive to tensile state as a function of the deposition angle, which is attributed to the result of the competition and mediation between compressive stress due to surface capillary forces of the island surface and tensile stress due to the coalescence of the islands during Volmer-Weber growth of the films. Based on this mechanism, the nearly stress-free film with desirable performance can be prepared at the proper deposition angle. The correlation between the residual stress and the atomic-bond structure also reveals that the stress transition is not only related to the sp2-bonded carbon content, but also closely to the changes of the heptatomic and pentatonic rings in the carbon network. This work demonstrated herein will provide a promising route to prepare the stress-free diamond-like carbon films with desirable performance.

A theoretical analysis of the electromigration-induced void morphological evolution under high current density

In this work, analysis of electromigration-induced void morphological evolution in solder interconnects is performed based on mass diffusion theory. The analysis is conducted for three typical experimentally observed void shapes: circular, ellipse, and cardioid. Void morphological evolution is governed by the competition between the electric field and surface capillary force. In the developed model, both the electric field and capillary force on the void’s surface are solved analytically. Based on the mass conversation principle, the normal velocity on the void surface during diffusion is obtained. The void morphological evolution behavior is investigated, and a physical model is developed to predict void collapse to a crack or to split into sub-voids under electric current. It is noted that when the electric current is being applied from the horizontal direction, a circular void may either move stably along the electric current direction or collapse to a finger shape, depending on the relative magnitude of the electric current and surface capillary force. However, the elliptical-shaped void will elongate along the electric current direction and finally collapse to the finger shape. On the other hand, the cardioid-shaped void could bifurcate into two sub-voids when the electric current reaches a critical value. The theoretical predictions agree well with the experimental observations.

The transition from liquid to solid-like behaviour in ultrahigh viscosity aerosol particles

For the first time, a measurement of the viscosity of microparticles composed of Newtonian fluids has been made over a range of 12 orders of magnitude (10−3 to 109 Pa s), extending from dilute aqueous solutions to the solid-like behaviour expected on approaching a glass transition. Using holographic optical tweezers to induce coalescence between two aerosol particles (volume <500 femtolitres), we observe the composite particle relax to a sphere over a timescale from 10−7 to 105 s, dependent on viscosity. The damped oscillations in shape illustrate the interplay of surface capillary forces and bulk fluid flow as the relaxation progresses. Viscosity values estimated from the extrapolation of measurements from macroscopic binary aqueous solutions of sucrose are shown to diverge from the microparticle measurements by as much as five orders of magnitude in the limit of ultrahigh solute supersaturation and viscosity. This is shown to be a consequence of the sensitivity of the viscosity to the composition of the particles, specifically the water content, and the often incorrect compositional dependence on water activity that are assumed to characterise aerosols and amorphous phases under dry conditions. For ternary mixtures of sodium chloride, sucrose and water, the measured viscosities similarly diverge from model predictions by up to three orders of magnitude. The Stokes–Einstein treatment for relating the diffusivity of water in sucrose droplets to the particle viscosity is found to depart from the measured viscosities by more than one order of magnitude when the viscosity exceeds 10 Pa s and up to six orders of magnitude at the highest viscosities accessed. Coalescence is shown to proceed with unit efficiency even up to the highest accessible viscosity. These measurements provide the first comprehensive account of the change in a material property accompanying a transition from a dilute solution to an amorphous semi-solid state using aerosol particles to probe the change in rheological properties.

Tension of freely suspended fluid filaments

Stable fluid filaments with diameters of several micrometers and slenderness ratios well above 1000 are unique objects formed by some liquid crystalline phases of bent-core mesogens. We present a technique to determine filament tensions from their deflection under defined loads. A strong temperature dependence is observed, with a minimum near the clearing temperature. Both the nonlinear relation between filament tension and diameter and the substantial increase of the tension with lower temperatures indicate contributions of volume terms, in addition to surface capillary forces. We discuss a model that relates these bulk terms to elastic forces, originating from the undulated smectic layer structure. This model can explain the origin of the filament stability.

Nanometer-scale flow of molten polyethylene from a heated atomic force microscope tip

We investigate the nanometer-scale flow of molten polyethylene from a heated atomic force microscope (AFM) cantilever tip during thermal dip-pen nanolithography (tDPN). Polymer nanostructures were written for cantilever tip temperatures and substrate temperatures controlled over the range 100–260 °C and while the tip was either moving with speed 0.5–2.0 µm s−1 or stationary and heated for 0.1–100 s. We find that polymer flow depends on surface capillary forces and not on shear between tip and substrate. The polymer mass flow rate is sensitive to the temperature-dependent polymer viscosity. The polymer flow is governed by thermal Marangoni forces and non-equilibrium wetting dynamics caused by a solidification front within the feature.

Hydroxypropyl methylcellulose (HPMC) formulated films: Relevance to adhesion and friction surface properties

Cellulose derivatives constitute one of the most dedicated polymers used in the production of film coatings (tablets, capsules …). The effect of the composition of hydroxypropyl methylcellulose (HPMC)-stearic acid films on the surface physico-chemical properties was investigated. Incorporation of stearic acid into HPMC films induces strong changes on surface structure, hydrophobic/hydrophilic character, adhesion and friction characteristics. The variations observed suggest an accumulation of stearic acid on film surfaces, resulting a decrease of its roughness and surface free energy. The nano-adhesion and nano-friction experiments were evaluated by using AFM/FFM, while the macro-adhesion and macro tribological studies were performed on tack test and pin-on-disk tribometer respectively. In all the cases, it was found that the hydrophobic character of stearic acid leads to decrease adhesion and friction at the two scales. The surface capillary force plays a key role in these properties. In addition, the study indicates the interplay of adhesion in friction property of HPMC films and the role of stearic acid as lubricant.

Drift of a reacting droplet due to the chemoconcentration capillary effect

The motion of a droplet with a first-order chemical reaction taking place at its surface with the participation of a surfactant dissolved in the external medium is considered. Approximate expressions are obtained for the velocity and other characteristics of the autonomous motion of the droplet caused by the surface capillary forces due to the nonuniform distribution of the surfactant over the surface of the moving droplet.