Variation Of Contact Angle Research Articles

Size dependent influence of contact line pinning on wetting of nano-textured/patterned silica surfaces.

Wetting behavior on a heterogeneous surface undergoes contact angle hysteresis as the droplet stabilized at a metastable state with a contact angle significantly different from its equilibrium value due to contact line pinning. However, there is a lack of consensus on how to calculate the influence of pinning forces. In general, the pinning effect can be characterized as (i) microscopic behavior when a droplet is pinned and the contact angle increases/decreases as the droplet volume increases/decreases and (ii) macroscopic behavior as the pinning effects decrease and ultimately, disappear with the increase of the droplet size. The current work studied both behaviors using molecular dynamics (MD) simulation with more than 300 different size water droplets on silica surfaces with three different patterns across two different wetting conditions. Results showed that the contact angle increases linearly with increasing droplet volume through the microscopic behavior, while the droplet is pinned on top of a certain number of patterns. When we normalized the droplet size with the corresponding pattern size, we observed a "wetting similarity" that linear microscopic contact angle variations over different size heterogeneities continuously line up. This shows that the pinning force remains constant and the resulting pinning effects are scalable by the size ratio between the droplet and pattern, independent of the size-scale. The slope of these microscopic linear variations decreases with an increase in the droplet size as observed through the macroscopic behavior. We further found a universal behavior in the variation of the corresponding pinning forces, independent of the wetting condition. In macroscopic behavior, pinning effects become negligible and the contact angle reaches the equilibrium value of the corresponding surface when the diameter of the free-standing droplet is approximately equal to 24 times the size of the surface structure. We found that the pinning effect is scalable with the droplet volume, not the size of the droplet base.

The effect of uniform magnetic field on spatial-temporal evolution of thermocapillary convection with the silicon oil based ferrofluid

A uniform axial or transverse magnetic field is applied on the silicon oil based ferrofluid of high Prandtl number fluid (Pr ? 111.67), and the effect of magnetic field on the thermocapillary convection is investigated. It is shown that the location of vortex core of thermocapillary convection is mainly near the free surface of liquid bridge due to the inhibition of the axial magnetic field. A velocity stagnation region is formed inside the liquid bridge under the axial magnetic field (B = 0.3-0.5 T). The disturbance of bulk reflux and surface flow is suppressed by the increasing axial magnetic field. There is a dynamic response of free surface deformation to the axial magnetic field, and then the contact angle variation of the free surface at the hot corner is as following, ?hot, B = 0.5 T = 83.34? > ?hot, B = 0.3 T = 72.16? > > ?hot,B = 0.1 T = 54.21? > ?hot, B = 0 T = 43.33?. The results show that temperature distribution near the free surface is less and less affected by thermocapillary convection with the increasing magnetic field, and it presents a characteristic of heat-conduction. In addition, the transverse magnetic field does not realize the fundamental inhibition for thermocapillary convection, but it transfers the influence of thermocapillary convection to the free surface.

Transport dynamics of droplet impact on the wedge-patterned biphilic surface

Droplet impact on biphilic surfaces with a wettability contrast has been intensively studied in recent years. In this work the effects of tilting and apex angles on droplet transport dynamics after impacting on a wedge-patterned biphilic surface at low Weber numbers were investigated experimentally. The biphilic surface was fabricated by applying a hydrophobic polymer coating on a bare silicon surface. According to the experimental results, a larger apex angle below 67.4° can accelerate the droplet effectively at first. Then the friction force controls the droplet movement and reduces the speed. The tilting angle along the hydrophilic direction activates the droplet. If the gravity component is opposite to the hydrophilic direction and the tilting angle is over 15°, the droplet can hardly move toward the hydrophilic area. By modeling the hydrodynamics of the droplet movement after impact on a biphilic surface with assumptions of no evaporation, no Marangoni effect, negligible dynamic contact angle variation and negligible rotation effect, the surface tension values versus the position at different apex angles are derived. The predicted position versus time trends agree well with the experimental data. This study aims to provide a better understanding of the mechanisms of droplet hydrodynamics on wedge-patterned biphilic surfaces at low Weber numbers.

Transition of Marangoni convection instability patterns during evaporation of sessile droplet at constant contact line mode

Due to continuous variation of contact angle of a sessile droplet during evaporation at constant contact line mode, the behaviors of Marangoni convection inside the droplet would inevitably vary with evaporation. In order to understand these behaviors, the Marangoni convection instability patterns and their transition phenomena in a sessile droplet during evaporation are investigated through a series of experiments. For easily volatile droplet of methanol the hydrothermal waves occur firstly near the three-phase contact line and then they are transformed to Bénard-Marangoni cells when the contact angle is lower than a critical value. The hydrothermal waves finally disappear with only Bénard-Marangoni cells remaining in the droplet when the contact angle is below the second threshold. For highly volatile droplet of R-113 the stationary longitudinal rolls appear firstly and then they are replaced by the Bénard-Marangoni cells. The basic linear flow in R-113 droplet caused by fast evaporation is a crucial reason to form the stationary longitudinal rolls in it. The critical conditions for the transitions between these kinds of the Marangoni convection instability patterns are determined and the underlying mechanisms of these transition phenomena are analyzed carefully.

The simulation of transport processes in cementitious materials with embedded healing systems

SummaryA new model for simulating the transport of healing agents in self‐healing (SH) cementitious materials is presented. The model is applicable to autonomic SH material systems in which embedded channels, or vascular networks, are used to supply healing agents to damaged zones. The essential numerical components of the model are a crack flow model, based on the Navier‐Stokes equations, which is coupled to the mass balance equation for simulating unsaturated matrix flow. The driving forces for the crack flow are the capillary meniscus force and the force derived from an external (or internal) pressure applied to the liquid healing agent. The crack flow model component applies to non‐uniform cracks and allows for the dynamic variation of the meniscus contact angle, as well as accounting for inertial terms. Particular attention is paid to the effects of curing on the flow characteristics. In this regard, a kinetic reaction model is presented for simulating the curing of the healing agent and a set of relationships established for representing the variation of rheological properties with the degree of cure. Data obtained in a linked experimental programme of work are employed to justify the choice and form of the constitutive relationships, as well as to calibrate the model's evolution functions. Finally, a series of validation examples are presented that include the analysis of a series of concrete beam specimens with an embedded vascular network. These examples demonstrate the ability of the model to capture the transport behaviour of this type of SH cementitious material system.

A Stable Anti-Fouling Coating on PVDF Membrane Constructed of Polyphenol Tannic Acid, Polyethyleneimine and Metal Ion.

A hydrophilic and anti-fouling coating layer was constructed on a polyvinylidene fluoride (PVDF) microfiltration membrane by a novel surface modification method. The pristine membrane was firstly coated by (3-chloropropyl) trimethoxysilane/polyethyleneimine and tannic acid. Then, the metal ion was induced on the coating layer to coordinate with tannic acid and polyethyleneimine, forming a more stable and hydrophilic coating on the surface. The membrane’s surface morphology and chemical element analysis showed that the Tannic acid/ polyethyleneimine (TA/PEI) coating layer was denser and had more stability after the addition of metal ions, and this may be due to the coordination bond formed between the TA/PEI coating and metal ions. The results of the water contact angle and pure water flux measurements showed that the hydrophilicity and wettability of the modified membranes were improved obviously after introducing the metal ion layers. The anti-fouling performance and stability of the modified membrane were also characterized by the underwater oil contact angle (OCA), the separation efficiency, and the contact angle variation value for before and after the rinsing experiment. The modified membrane showed obvious stability and antifouling. Moreover, the retention rate of some composite membranes could reach 99.6%.

The Role of Wettability on the Response of a Quartz Crystal Microbalance Loaded with a Sessile Droplet

In this work, the interaction between a sessile droplet’s contact angle and a quartz crystal microbalance (QCM) is elucidated. We differentiate the QCM’s frequency response to changes in the droplet contact area from variations in the dynamic contact angle. This is done by developing a computational model that couples the electrical and mechanical analysis of the quartz substrate with the visco-acoustic behavior of the sessile droplet. From our analysis, we conclude that changes in the contact angle have an effect on the frequency response of the QCM when the droplet height is on the order of the viscous decay length or smaller. On the other hand, changes in the interfacial contact area of the sessile droplets have a significant impact on the frequency response of the QCM regardless of the droplet size.

Understanding the dielectric and mechanical properties of self‐passivated Al–epoxy nanocomposites

Epoxy nano passivated aluminium composites with optimised size and filler contents were fabricated. Variation in contact angle and surface roughness is insignificant with increasing filler into nanocomposites but it showed a drastic reduction on corona ageing. Water droplet initiated corona inception voltage (CIV) is high under the negative DC voltage followed with positive DC and AC voltages. The bandwidth of ultra-high frequency signal generated due to water droplet initiated corona discharge lies in the range of 0.5–1.2 GHz. Surface potential measurements have shown that the decay in the potential was fast initially, and it became slower and sluggish subsequently. The trap energy density versus trap depth plot exhibits shallow traps and deep traps at around 0.8 and 0.87 eV, respectively. Permittivity, conductivity and loss factor have increased with an increase in the filler content in nanocomposites. The bulk resistance and capacitance of samples were determined for obtaining the equivalent parallel RC circuit model. Incorporation of nanofillers increases the glass transition temperature and reduces the tan δ with increasing frequencies as evident from dynamic mechanical analysis studies. A direct correlation is observed between the plasma temperature measured through laser-induced breakdown spectroscopy spectra and hardness of the material.

Retention and infiltration of bacteria on a plant leaf driven by surface water evaporation

Evaporation-driven internal flows within a sessile droplet can transport microorganisms close to the leaf surface and facilitate their infiltration into the available openings, such as stomata. Here, using microfabricated surfaces out of polydimethylsiloxane, the sole effects of evaporation of sessile droplets in contamination of plant leaves was studied. These surfaces were patterned with stomata, trichomes, and grooves that are common surface microstructures on plant leaves. Evaporation of sessile droplets, containing bacterial suspensions, on real leaves and fabricated surfaces was studied using confocal microscopy. To provide insight about the effects of leaf hydrophobicity and surface roughness on the bacterial retention and infiltration, variations of contact angle of sessile droplets at these surfaces were measured during evaporation. The results showed that evaporation-driven flow transported bacteria close to the surface of spinach leaves and fabricated surfaces, leading to distinct infiltration into the stomata. Larger size and wider spacing of the micropores, and a more hydrophilic surface, led bacteria to spread more at the droplet base area and infiltrate into more stomata. Evaporation-driven movement of contact line, which can sweep bacteria over the leaf surface, was shown to lead to bacterial infiltration into the stomatal pores. Findings should help improve microbial safety of leafy greens.

Capillary Trapping Following Imbibition in Porous Media: Microfluidic Quantification of the Impact of Pore‐Scale Surface Roughness

AbstractDue to diagenesis, pores in subsurface rocks such as sandstones exhibit varying degrees of surface roughness in the forms of authigenic cement coatings and mineral dissolution. Previous work describing capillary trapping in porous media has primarily focused on pore‐space geometry, wettability, and fluid viscosity contrast, while acknowledging, but not quantifying, the potential impact of surface roughness. We introduce a method to implement surface roughness with controlled variation of hillock density and heights into glass microfluidic chips and investigate surface roughness impacts on gas trapping following imbibition of water into air. We demonstrate that surface roughness with hillock height‐to‐pore‐depth ratios (herein called Ω) less than a media‐dependent threshold (Ω = 6%–10% in the micromodels) does not promote nonwetting phase (gas) trapping. By contrast, rougher micromodels with Ω values larger than the aforementioned roughness threshold show a dramatic increase in the saturation of trapped gas (gas saturation values up to 64%) due to an observed change in imbibition dynamics from binary filling to pendular‐ring formation within pore throats as well as capillary pinning within pore bodies. Furthermore, when the micromodel intermediate capillary number results are compared to Land's model, only the roughest microfluidics chips (Ω > 10%) fall within the literature‐described values of the characteristic trapping constant, C, implying that surface roughness is also a key gas trapping control, independent of or in addition to pore‐space geometry and wettability. An a priori menisci stability criterion and a heuristic explanation based on local contact angle variations are proposed to explain surface roughness‐induced trapping.

PH-Responsive mixed-thiol-modified surface of roughened GaN: A wettability and SERS study

Chemical etching of gallium nitride (GaN) crystals produces various morphological patterns such as ‘nanowhiskers’. Due to the microscopic roughness etched GaN surfaces may be converted via further chemical functionalization into superhydrophobic materials or substrates for surface-enhanced Raman scattering (SERS). Here, properties of Au-sputtered GaN nanowhiskers functionalized with two mixed thiols: aliphatic non-polar 1-butanethiol (C4SH) and aromatic ionizable 4-mercaptobenzoic acid (4-MBA) are characterized. Changing the ratio of two thiols results in large variations of water droplet contact angle. Coinciding sigmoidal pH-dependencies of the contact angle and relative spectral SERS intensities assigned to ionized 4-MBA species are observed for surfaces with approximately 1:2 M 4-MBA:C4SH ratio. Stability of 4-MBA + C4SH layers was probed through thiol-exchange with a non-ionizable aromatic thiol: 2-naphthalenethiol (2NT). While the SERS characteristics of (4-MBA + C4SH)-Au/GaN platform exposed to 2NT acquires new spectral features the layer maintains pH-responsive wettability indicating the presence of 4-MBA possibly conserved through the π-stacking interaction with 2NT. Our results demonstrate that deposition of multicomponent thiol layers on nanowhiskers could facilitate development of pH-responsive self-cleaning sensors based on GaN. The results are discussed in light of mechanisms underlying superhydrophobicity and SERS.

Three-dimension simulation of two-phase flows in a thin gas flow channel of PEM fuel cell using a volume of fluid method

This study investigates the two-phase flow in a thin gas flow channel of PEM fuel cells and wall contact angle's impact using the volume of fluid (VOF) method with tracked two-phase interface. The VOF results are compared with experimental data, theoretical solution and analytical data in terms of flow pattern, pressure drop and water fraction. Stable film flow is predicted, as observed experimentally, for the contact angle ranging from 5° to 40° including varying contact angles at different walls of a channel. The contact angle is found to have small impact on the gas pressure drop for the stratified flow regime, but it determines the meniscus of the two-phase interface, which affects the optical detection of the liquid thickness in experiment. The work is important to study of two-phase flow dynamics, multichannel design, experimental design and control of two-phase flows in thin gas flow channels for PEM fuel cells.

A novel combined method for fabrication of stable corrosion resistance superhydrophobic surface on Al alloy

In this study, a highly stable superhydrophobic surface with excellent corrosion resistance was fabricated on the 6020 aluminum alloy. In this regard mixed shot peening-etching treatment and silane modification was used. The wettability of surfaces was studied by contact angle measurement. Also, the influence of the shot peening pretreatment on the corrosion resistance of fabricated samples was investigated in 3.5 wt% NaCl aqueous solution, using weight loss, and electrochemical impedance spectroscopy (EIS) tests. The contact angle of 153°± 2 and the sliding angle of 8°± 2 was obtained on a shot peened sample after silane modification. The corrosion test results show high corrosion resistance of modified shot peened sample which was attributed to the fabricated superhydrophobic surface. Functionalization of the surface with silane has been confirmed by Fourier Transform Infrared Spectroscopy FTIR by showing the SiOSi (987 cm−1) and Al-Cl (3100 cm−1) bonds. The effect of shot peening and silane modification on the morphology of the surfaces were determined by scanning electron microscopy SEM. Furthermore, the chemical stability of the samples during immersion in 3.5 wt. % NaCl solution for 3 days was examined by EIS and SEM tests. Also, the variation of the contact angle with immersion time was recorded. The variation of water contact angle on the superhydrophobic shot peened sample after 72 -hs of immersion time was shown to be negligible (water contact angle of 152° after 3 days). In conclusion the extracted data from these tests confirmed the good chemical stability of the fabricated superhydrophobic surface on the shot peened sample.

Temperature-based adhesion tuning and superwettability switching on superhydrophobic aluminum surface for droplet manipulations

Superhydrophobic surfaces with tunable water adhesion and adjustable wettability have the capacity to manipulate water droplets under complicated conditions and, therefore, have great prospects in various domains. Hence, developing a facile method to prepare highly stable superhydrophobic surfaces with tunable water adhesion and adjustable wettability is of great importance. Herein, we prepare the superhydrophobic aluminum surface by combining laser etching and stearic acid modification. The variations of water contact angle, water sliding angle and water adhesive force with temperature of the prepared surface are characterized during the adhesion tuning process to investigate its temperature-responsive characteristics. A low/high adhesion switching of water on the prepared surface is subsequently achieved under alternating temperature. In addition, a facile approach is designed to achieve the superwettability switching process (from superhydrophobic state to superhydrophilic state) on the prepared surface. The prepared superhydrophobic surface displays excellent recoverability, stability and repeatability in the adhesion switching process and the superwettability switching process, even after being cycled for 10 times.

Silk fibroin coated TiO2 nanotubes for improved osteogenic property of Ti6Al4V bone implants.

Titanium and its alloys especially Ti6Al4V have long been used in biomedical implants. Although, Ti6Al4V is biocompatible, yet there has been consistent effort to improve its osteoconductive and osteogenic property to enhance the implant performance. In this regard, surface modification of Ti6Al4V implants with TiO2 nanotubes and subsequent application of biopolymeric coating has started emerging as a promising approach. Keeping this perspective in mind, here we have coated nano TiO2 modified Ti6Al4V surface with silk fibroin isolated from B. mori cocoons. The coating of silk fibroin was done on the implant using electrophoretic deposition technique at three different voltages. Topography analysis by AFM confirms the uniform coating of silk fibroin. A variation in contact angle from 89.7 ± 2° to 83.6 ± 2° was observed when tested for wettability using drop shape analyzer. The biocompatibility studies showed SF coated substrates support the adhesion of both MG63 bone cells and human mesenchymal stem cells (hMSCs). The formation of peripheral vinculin complexes on SF surface confirmed the adhesion through focal adhesion points. Consequently, SF coating improved the cellular expression of alkaline phosphatase by 1.1 times compared to the polished Ti6Al4V surface (PTi64). Increased expression of late osteogenic markers osterix and osteocalcin was also observed in hMSCs cultured on SF coated nanotubular surface compared to PTi64. These results together implied that coating of silk fibroin on TiO2 modified Ti6Al4V surface improve the osteogenic potential of the implant.

An Experimental Investigation of Properties of Nanofluid and Its Performance on Thermosyphon Cooled by Natural Convection

An attempt is made here to characterize thermal conductivity of water-based Al2O3 nanofluid and then use the same in a circular finned thermosyphon (TPCT) to measure its thermal performance. The concentration of Al2O3 nanofluid is varied within 0.05–0.25% by volume. The thermal conductivity of nanofluid is increased with concentration of Al2O3 nanoparticles as well as with temperature. A maximum of 26.7% enhancement of thermal conductivity is observed at 45 °C for 0.25% concentration by volume of nanofluid in comparison to that of de-ionized (DI) water. Variations of surface tension and contact angle of Al2O3 nanofluid are also compared with DI water. One of the smallest TPCT with different heat inputs (4 W, 8 W, and 12 W) and different inclinations (30 deg, 45 deg, 60 deg, and 90 deg) is tested for different concentration of Al2O3 nanofluid, which will find application in smaller electronic units. It is found that use of nanofluid decreases the wall temperature distribution of TPCT. Thermal resistance of TPCT decreases whenever TPCT is filled with nanofluid and a maximum of 36.4% reduction in thermal resistance is noted for 0.25% volume of nanoparticles at 4 W with an inclination of 60 deg. It is also found that performance of TPCT is higher at 60 deg inclination compared to other inclinations, especially for lower heat input.

Changes on the surface properties of foliated marbles at different cutting orientations

A single calcite crystal shows a pronounced anisotropy that can be transferred to the bulk stone, especially in the case of marbles. The aim of this paper is to analyse the effect of the preferential crystallographic orientation of the anisotropic calcite crystals on the surface properties of foliated marbles. Variations of microhardness, roughness, solubility and contact angle in rock surfaces are studied taking into account their relative position with respect the metamorphic foliation. Results show that more than 50% of the crystals have their c-axis oriented close to the normal of the axial foliation plane. This fact has a direct response on its surface properties: i) the microhardness in surface parallel to metamorphic foliation is a 14% higher than it is in the perpendicular surface; ii) contact angle of the water on the parallel faces is slightly lower than it is on the perpendicular one (42.75° and 53.25°, respectively); and iii) a lower chemical reactivity is estimated on parallel faces, which results in a lesser roughness of weathered surfaces. Results from this paper determine the optimal direction in which an ornamental piece should be cut in order to guarantee its maximum durability.

Variations of the Static Contact Angle of Ferrofluid Droplets on Solid Horizontal Surfaces in External Uniform Magnetic Fields.

The application of external uniform magnetic fields to ferrofluid droplets affects their magnetic order at the nanoscale as well as their shape at the macroscale, thus changing their contact angle with the surface. In this work, the effects of external uniform magnetic fields on the contact angles between different oil-based ferrofluid droplets and a handful of horizontal surfaces of varying wettability were studied. The contact angle is no longer constant around the ferrofluid droplet; rather, it varies in a rich yet predictable way. Droplets dispensed in the presence of the magnetic field on oleophobic surfaces adjust such that the contact angle increases at the front and back ends and decreases at the two perpendicular positions. The opposite behavior is reported for ferrofluid droplets on oleophilic surfaces. These direction-dependent changes in the contact angle can have a significant impact on the behavior of ferrofluid droplets on gradient surfaces where they can either diminish or enhance the surface tension gradient. Our work is fundamentally relevant to potential applications involving the controlled movement of ferrofluid droplets on surfaces like the lab-on-a-chip under the combined effects of a magnetic field and either a surface tension gradient or an electric field (i.e., electrowetting). It is important to understand how the two effects interact for the optimal utilization of these effects in future applications.

Preparation and characterization of superhydrophobic surface based on polydimethylsiloxane (PDMS)

Biomimetic superhydrophobic surfaces exhibit excellent self-cleaning properties due to their special micro/nano-scale binary structures. In order to prepare the superhydrophobic surface of the polydimethylsiloxane (PDMS), a facile fabrication method for replicating micro/nano-scale binary aluminium structures into PDMS is presented. The microscopic morphology, composition, surface roughness (Ra) and wettability of the sample surface were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, roughness measurement equipment and contact angle meter respectively. Based on the measurements of the contact angles of deionized water (DI water) and ethanediol, surface free energies of the coatings were estimated according to the Owens two-liquid method. The superhydrophobic PDMS exhibited lower surface free energy than flat PDMS with a DI water contact angle (WCA) of 165°. The surface roughness (Ra) increased with the increasing of etching time in the range 0–80 min, and then decreased with the change of etching time, similar to the variation of contact angle with etching time. Moreover, the prepared surface had different micro-morphologies and its wettability was changed by regulating the chemical etching time. In addition, the superhydrophobic PDMS also showed good self-cleaning properties and the bouncing effect of the water droplets.

Synergistic effects of ethylene propylene diene copolymer and carbon nanofiber on the thermo-mechanical properties of polypropylene/high-density polyethylene composites

Ethylene–propylene–diene copolymer (EPDM) modified polypropylene (PP)/high-density polyethylene (HDPE) ternary blends and their composites with carbon nanofibers (CNF) were prepared. The PP/HDPE binary blends containing 80/20 concentration was used for the preparation of ternary blends and composites. The amount of EPDM in the ternary blend was 2, 4.8, 9.1, 13 and 16.6 wt%. The best mechanical properties were obtained for the ternary blends containing 13 wt% EPDM. These blends were further modified with CNF of varying concentrations from 0.043 to 1.7 wt%. The mechanical properties of the ternary composites especially with higher concentrations of CNF show a synergistic effect with good improvements in impact strength, tensile strength, tensile elongation, and tensile toughness. The DMA studies validate the homogeneity of the ternary composites. The DSC studies reveal better miscibility/compatibility of the PP/HDPE blends in the presence of EPDM and CNF. The thermal stability of the ternary composites was enhanced due to the better stability of EPDM and the uniform dispersion of the fillers in the polymer matrix. The contact angle values were improved with the addition of EPDM, however, for the ternary composites, only marginal variations in contact angle were observed.