Understanding contact angle hysteresis on an ambient solid surface

Geometry Dependence of the Receding Angle of a Droplet on a Solid Cylindrical Surface

Droplets that partially wet solid surfaces exhibit hysteresis in their contact angle. The values of the minimum (receding) and maximum (advancing) angles, θR and θA, are empirically well-defined and thought to be unique for a given set of materials. We measured the contact angles of water droplets hanging from hydrophobic, PDMS-functionalized glass and found that the value of θR varies with the curvature of the glass. The effect is substantial: θR changes from 86.0 ± 1.9° on a flat plate to 95.6 ± 1.9° on a 2 mm diameter rod of the same material. The measured values of θA were independent of geometry (θA = 103.2 ± 0.9°). We found a consistent trend among PDMS-functionalized glass cylinders with diameters ranging from 2 to 12.7 mm. We also measured the speed at which the contact line moved just after receding; these results showed a receding speed ∝ cos(θR) - cos(θE) and a consistent equilibrium contact angle, θE = 103.4 ± 2.3°. Finally, we measured the sliding of water droplets as rods were tilted. The larger θR (and thus smaller hysteresis) for a 2 mm-diameter rod led to droplets sliding at a tilt angle of just 21° from horizontal, compared to the 48° minimum tilt for a 7 mm rod. The results show that hysteresis arises from an energy barrier that depends on the shape of the droplet and contact line, both of which change with substrate curvature. The results may lead to designing surfaces that better trap water droplets or shed them for self-cleaning or water-harvesting applications.

Avalanches and Extreme Value Statistics of a Mesoscale Moving Contact Line

We report direct atomic force microscopy measurements of pinning-depinning dynamics of a circular moving contact line (CL) over the rough surface of a micron-sized vertical hanging glass fiber, which intersects a liquid-air interface. The measured capillary force acting on the CL exhibits sawtoothlike fluctuations, with a linear accumulation of force of slope k (stick) followed by a sharp release of force δf, which is proportional to the CL slip length. From a thorough analysis of a large volume of the stick-slip events, we find that the local maximal force F_{c} needed for CL depinning follows the extreme value statistics and the measured δf follows the avalanche dynamics with a power law distribution in good agreement with the Alessandro-Beatrice-Bertotti-Montorsi (ABBM) model. The experiment provides an accurate statistical description of the CL dynamics at mesoscale, which has important implications to a common class of problems involving stick-slip motion in a random defect or roughness landscape.

Evaluation and assessment of the wettabilty and water contact angle of modified poly methyl methacrylate denture base materials against PEEK in cast partial denture framework: an in vitro study

INTRODUCTION

The prevalence of adults with partially dental arches is expected to be more than imagined and patients requiring replacement of missing teeth are slowly increasing in number too. Removable partial dentures are known to provide for substantial replacement for the missing teeth with also added advantages when compared to fixed or implant prosthesis, mainly in elderly patients. Denture base material performance and durability are greatly influenced by wettability and water contact angle. In the case of dentures; adequate moisture distribution is necessary to ensure excellent wettability which has an influence on comfort and oral health. The purpose of conducting this study was to find out whether the advancements made using PEEK (Polyether ether ketone) would prove to be more beneficial than the current upgrades in the current material spectrum.

MATERIALS AND METHODS

This study was performed under in vitro conditions. All the fabrication and processing was done only by one operator. The materials used were divided into three groups each comprising 20 samples. Group A was modified polymethylmethacrylate (Bredent Polyan), Group B was polyoxymethylene acetal resin (Biodentaplast) and Group C was PEEK. An Ossila Goniometer was used to measure the contact angle. The three types of liquids used for the testing included distilled water, natural saliva and mouth wetting solution (Wet Mouth Liquid, ICPA India). Human saliva was collected from an individual with no medical conditions and normal salivary secretion.

RESULTS

The data was analyzed using One-way ANOVA test and a pairwise comparison using the Post Hoc Tukey's Honest Significant Difference. Table 1 consists of the mean water contact angles of the denture base materials and mean contact angles of various denture base materials. In saliva, mouth wetting solution and distilled water, the highest mean and least mean contact angle was seen in Polyan and Biodentaplast respectively. A signicant difference was seen between PEEK and Polyan and Biodentaplast and Polyan on further comparison.

CONCLUSION

From the resources and the materials at our disposal, it could be concluded that Polyan, Biodentaplast and PEEK and could be used as viable options in cast partial denture framework.

Manipulation of Contact Angle Hysteresis at Electrified Ionic Liquid-Solid Interfaces

Room-temperature ionic liquids (RTILs) are intriguing fluids that have drawn much attention in applications ranging from tribology and catalysis to energy storage. With strong electrostatic interaction between ions, their interfacial behaviors can be modulated by controlling energetics of the electrified interface. In this work, we report atomic-force-microscope measurements of contact angle hysteresis (CAH) of a circular contact line formed on a micron-sized fiber, which is coated with a thin layer of conductive film and intersects an RTIL-air interface. The measured CAH shows a distinct change by increasing the voltage U applied on the fiber surface. Molecular dynamics simulations were performed to illustrate variations of the solidlike layer in the RTIL adsorbed at the electrified interface. The integrated experiments and computations demonstrate a new mechanism to manipulate the CAH by rearrangement of interfacial layers of RTILs induced by the surface energetics.

Wetting Dynamics of a Lipid Monolayer

Direct measurement and control of the dynamic wetting properties of a lipid-coated water-air interface over a wide range of surface tension variations have many important applications. However, the wetting dynamics of the interface near its partial-to-complete wetting transition has not been fully understood. Here, we report a systematic study of the wetting dynamics of a lipid-coated water-air interface around a thin glass fiber of diameter 1-5 μm and length 100-300 μm. The glass fiber is glued onto the front end of a rectangular cantilever to form a "long-needle" atomic-force-microscope probe. Three surface modifications are applied to the glass fiber to change its wetting properties from hydrophilic to hydrophobic. A monolayer of phospholipid dipalmitoylphosphatidylcholine (DPPC) is deposited on the water-air interface in a homemade Langmuir-Blodgett trough, and the surface tension γ_L of the DPPC-coated water-air interface is varied in the range of 2.5 ≲ γ_L ≲ 72 mN/m. From the measured hysteresis loop of the capillary force for the three coated fiber surfaces with varying γ_L, we observe a sharp transition from partial to complete wetting when γ_L is reduced to a critical value (γ_L)_c. The obtained values of (γ_L)_c are 27 ± 1 mN/m for a DPPC-coated fiber surface and 23 ± 1 mN/m for an trichloro(1H,1H,2H,2H-perfluorooctyl) silane (FTS)-coated surface. Below (γ_L)_c, the contact angle θ₀ of the liquid interface is found to be zero for both hydrophobic fiber surfaces and the corresponding spreading parameter S becomes positive. For the FTS-coated fiber surface, the height of capillary rise exhibits a jump when γ_L is reduced to (γ_L)_c, which indicates that a rapidly advancing liquid film is formed on the fiber surface when the partial-to-complete wetting transition takes place. Our experiment thus establishes a quantitative method by which many other liquid interfaces coated with polymers, surfactants, and biomolecules (such as proteins and lipids) may be characterized dynamically.

State and Rate Dependent Contact Line Dynamics over an Aging Soft Surface

We report direct atomic-force-microscope measurements of capillary force hysteresis (CFH) of a circular contact line (CL) formed on a long glass fiber, which is coated with a thin layer of soft polymer film and intersects a water-air interface. The measured CFH shows a distinct overshoot for the depinning of a static CL, and the overshoot amplitude grows logarithmically with both the hold time τ and fiber speed V. A unified model based on the slow growth of a wetting ridge and force-assisted barrier crossing is developed to explain the observed time (or state) and speed (or rate) dependent CL depinning dynamics over an aging soft surface. The experimental findings have important implications to a common class of problems involving depinning dynamics in a defect or roughness landscape, such as friction of solid interfaces.

Capture and re-entrainment of microdroplets on fibers

The capture of liquid microdroplets on fibers, webs, and surfaces is important in a range of natural and industrial processes. One such application is the fibrous filtration of aerosols. Contact angle and wetting dynamics have a significant influence on capture and re-entrainment, yet there is no comprehensive model that accounts for these properties and their influence on capture efficiency. In this study, a series of computational simulations using liquid droplets and air are carried out to investigate the influence of equilibrium and dynamic contact angles on the capture and re-entrainment of mist droplets. A range of operating conditions for droplet-fiber diameter ratios, flow velocities, and contact angles, encapsulating both super-oleophilic and super-oleophobic media, are considered. All simulations are carried out using the volume of fluid (VOF) interface capturing approach in the finite volume solver interFoam within OpenFOAM. The physics of microdroplet impacting on a fiber is discussed and three distinct regimes for the spreading of the droplet around the fiber-inertia, capillary, and stagnation pressure controlled-are identified. It was found that the classification of filtration media for any fluid system, rather broadly as philic or phobic, based on the equilibrium contact angle alone may be insufficient for two reasons: (i) the characteristics of droplet-fiber interaction, including capture or re-entrainment, differs significantly over the range of contact angles for both philic and phobic media; and more importantly (ii) equilibrium contact angle plays little role in the initial stages of the droplet-fiber interaction that predominantly dictates the fate of the droplet. On the contrary, it is the contact angle dynamics that influences the initial stages of droplet impact on fibers, while commercial filters are seldom characterized based on this property. The isolated influence of equilibrium, advancing and receding contact angles on the potential mechanisms that can result in full or partial capture or re-entrainment are highlighted. The influence of equilibrium and advancing and receding hystereses are summarized in the form of a capture-regime map that shows four distinct regimes: (i) likely capture, (ii) likely re-entrainment with minimal or no capture, (iii) receding contact angle assisted partial or full capture, and (iv) advancing contact angle inhibited partial or full re-entrainment.

Critical Conditions for Jumping Droplets

A droplet initially overstretched on a solid substrate pulls back to lower the contact area and may jump away from the substrate. The condition to realize such macroscopic behaviors is often dictated by microscopic characteristics, such as contact-line pinning, in nontrivial ways. Here we theoretically and experimentally reveal the hidden contribution of contact-line pinning in forming the critical condition for detachment of a droplet from a solid substrate, among other dominating hydrodynamical effects. Our results demonstrate the relation between classical theories on contact-line pinning and various droplet manipulating applications in microfluidics and bioprinting.

Contact angles: From past mistakes to new developments through liquid-solid adhesion measurements

A contact angle observed for a liquid-solid system is not necessarily a unique value and a few different contact angles need to be carefully considered in relation to liquid spreading, adhesion and phase separation. Despite the conceptual simplicity of the contact angle and over 200 years of investigation, interpretations of experimental contact angles remain controversial, and mistakes are quite common. Here, the physics behind equilibrium contact angles are restated and their misuse in modern literature is briefly discussed. Selected advances made in the 20th century that shaped current interpretations of experimental contact angles are also critically reviewed and evaluated. Understanding of contact angles for liquids on solids has improved in the last two decades and this progress is driven by advanced imaging techniques and improved methodologies in contact angle measurements, often in tandem with direct force measurements for liquid droplets in contact with solids. In our laboratory, a microelectronic balance system is employed to measure the force of liquid droplet spontaneous spreading and the water-solid adhesion forces at different stages of droplet retraction and separation. A microbalance equipped with a camera and data acquisition software measures these forces directly, monitors droplet-surface separation including distances over which the droplet stretches, and collects optical images simultaneously. The images are used to analyze capillary and surface tension forces based on measured droplet dimensions, shapes of surfaces and values of contact angles. These force measurements have significantly furthered our fundamental understanding of advancing, receding and most stable contact angles, and their correlations with adhesion, and are summarized in this review.

Friction of Droplets Sliding on Microstructured Superhydrophobic Surfaces

Liquid transport is a fundamental process relevant to a wide range of applications, for example, heat transfer, anti-icing, self-cleaning, drag reduction, and microfluidic systems. For these applications, a deeper understanding of the sliding behavior of water droplets on solid surfaces is of particular importance. In this study, the frictional behavior of water droplets sliding on superhydrophobic surfaces decorated with micropillar arrays was studied using a nanotribometer. Our experiments show that surfaces with a higher solid area fraction generally exhibited larger friction, although friction might drop when the solid area fraction was close to unity. More interestingly, we found that the sliding friction of droplets was enhanced when the dimension of the microstructures increased, showing a distinct size effect. The nonmonotonic dependence of friction force on solid area fraction and the apparent size effect can be qualitatively explained by the evolution of two governing factors, that is, the true length of the contact line and the coordination degree of the depinning events. The mechanisms are expected to be generally applicable for other liquid transport processes involving the dynamic motion of a three-phase contact line, which may provide a new means of tuning liquid-transfer behavior through surface microstructures.

Capillary force on a tilted cylinder: Atomic Force Microscope (AFM) measurements

HYPOTHESIS

The capillary force in situations where the liquid meniscus is asymmetric, such as the one around a tilted object, has been hitherto barely investigated even though these situations are very common in practice. In particular, the capillary force exerted on a tilted object may depend on the dipping angle i.

EXPERIMENTS

We investigate experimentally the capillary force that applies on a tilted cylinder as a function of its dipping angle i, using a home-built tilting Atomic Force Microscope (AFM) with custom made probes. A micrometric-size rod is glued at the end of an AFM cantilever of known stiffness, whose deflection is measured when the cylindrical probe is dipped in and retracted from reference liquids.

FINDINGS

We show that a torque correction is necessary to understand the measured deflection. We give the explicit expression of this correction as a function of the probes' geometrical parameters, so that its magnitude can be readily evaluated. The results are compatible with a vertical capillary force varying as 1/cosi, in agreement with a recent theoretical prediction. Finally, we discuss the accuracy of the method for measuring the surface tension times the cosine of the contact angle of the liquid on the probe.

Simultaneous observation of asymmetric speed-dependent capillary force hysteresis and slow relaxation of a suddenly stopped moving contact line

We report direct atomic-force-microscope measurements of capillary force hysteresis (CFH) and relaxation of a circular moving contact line (CL) formed on a long micron-sized hydrophobic fiber intersecting a liquid-air interface. By using eight different liquid interfaces with varying solid-liquid molecular interactions, we find a universal behavior of the asymmetric speed dependence of CFH and CL relaxation. A unified model based on force-assisted barrier crossing is used to connect the mesoscopic measurements of CFH and CL relaxation with the energy barrier height E_{b} and size λ associated with the surface defects. The experiment demonstrates that the CL pinning (relaxation) and depinning dynamics are closely related and can be described by a common microscopic framework.