Surfaces Of Different Wettability Research Articles

A new insight into dynamic contact angle measurements: Implementation of smooth splines method

The contact angle (CA) of a drop is a common quantitative measurement that describes a surface’s wettability. However, manual selection of the drop’s solid–liquid–vapor contact points (CPs) by the user introduces substantial human error to the final measured CA. Hence, in this article we introduce a novel method for CA measurement that uses a smoothing spline function to simultaneously cancel probable image noise, find the solid–liquid contact point, and calculate the CA at the contact point. The accuracy of this technique is tested through measurement of a series of synthetic drop images with known CAs ranging from 5°to 175°, which are generated using a new mathematical approach and include a reflection area to accurately model real experiments. In these tests, the smoothing splines approach is shown to be extremely accurate in measuring both the CP position and the CA, with negligible error (R2=0.9999). Then, for added validation, the smoothing splines approach is tested on four real surfaces of different wettability: glass, single crystal silicon, polished polytetrafluoroethylene, and patterned superhydrophobic polytetrafluoroethylene. When tested on real surfaces, the smoothing splines measurement approach provides reliable contact angle data for all four different surfaces tested, while also reducing human error. Overall, our proposed measurement approach for the CA is shown to be more accurate than traditional measurement approaches such as the commonly used polynomial fitting approach, and the more recently introduced mask approach, which also includes automatic CP detection.

Simulation of coalescence dynamics of droplets on surfaces with different wettabilities

The head-on-collision coalescence dynamics for droplets on surfaces with different wettabilities was numerically investigated by using the method of coupled level set and volume of fluid, and a high-speed video experiment was used to validate the simulation results. The simulated coalescence behavior of droplets is consistent with the experiment results. We compared droplets with different Weber numbers (3.67 ≤ We ≤ 50) coalescing on solid surfaces of different wettabilities and contact angles (80° ≤ α ≤ 160°). The result is a regional division diagram that relates Weber number and contact angle based on different coalescence phenomena. The factors causing droplet deposition, air entrapment, bounce, and partial bounce during collision coalescence are described based on an analysis of interactions involving inertial forces, surface tension, and wall adhesion forces. Furthermore, the effect of Weber number and contact angle on droplet coalescence behavior is elaborated by analyzing the relationship between coalescence time and wettability radius under different Weber numbers and contact angles. Finally, through an energy analysis, we explain the maximum spreading radius and oscillation of droplets with different Weber numbers on solid surfaces with different wettabilities and in the process of head-on-collision coalescence.

A diffusion-enhancing icing theory for the freezing transition of supercooled large water droplet in impact

The freezing behavior of supercooled large droplet(SLD) in impact causes severe threats to civil aircraft. However, the mechanism by which the impact dynamics of SLD accelerates the icing still remains unclear. In this paper the impact and freezing of water droplets with different supercooling on inclined surfaces of different wettability were investigated experimentally and theoretically. Compared to one/two freezing morphologies and linear supercooling-freezing time relation reported previously, four freezing morphologies were observed in different supercoolings, and the transition of freezing morphologies resulted in the piecewise nonlinear supercooling-freezing time relation. By comparing the characteristic times of droplet dynamics and horizontal icing, it was found that the dynamics-icing interaction could lead to higher heat conduction area in higher supercooling. Theoretical analysis shows that the unsteady heat conduction enhances the freezing rate in a short time initially. The coupled effect of large contact area and initial heat flux greatly enhances the diffusion of latent heat in freezing of droplet with high supercooling. Based on this machanism, a diffusion-enhancing icing theory is proposed. Compared with previous icing theories, this theory can give much better predictions about the freezing rate of impinging supercooled water droplets, especially in conditions of high supercooling and hydrophobic surface.

Successive droplet impingement onto heated surfaces of different wettabilities

New high-performance electronic components generate a massive amount of heat per unit of area, and spray cooling is a field of interest for this application. Spray cooling achieves higher heat transfer coefficients than commonly used forced air convection cooling systems. In this study, we performed multiple droplet impingement experiments over heated surfaces in the non-boiling regime, a scenario that is closer to the phenomena of spray cooling than individual droplet impact. This paper focuses on the effects of droplet interactions with surfaces exhibiting distinct wettabilities. A surface thermocouple was used concomitantly as the impact surface and the temperature sensor. The surface thermocouple was originally hydrophilic (uncoated), which later we treated with a low cost commercial superhydrophobic coating (coated). High-speed imaging showed the droplet motion during impact. We proposed a one-dimensional inverse heat conduction semi-analytical solution, adopting a time-dependent droplet heat flux as one of the boundary conditions to represent the droplet cooling effect. We compared the heat transfer performances of both uncoated and coated surfaces for different droplet velocities but same initial temperature. The results showed higher heat transfer rates on uncoated surface relative to the coated one. Also, higher droplet velocities showed higher heat transfer mainly due to the increase in the droplet spreading diameter in contact with the solid. Successive droplets showed lower heat flow for the uncoated surface because of the low thermal effusivity and thickness of the heated solid, where most of the heat is removed by the first droplet. The first droplet thermal resistance and damping effects for the subsequent droplets, reduced the overall cooling performance. On the other hand, for the coated surface we found that the consecutive droplets continued to transfer heat with a similar rate as the previous one, which is desirable for spray cooling. However, the reduced overall heat transfer performance for the coated surface can be explained by the air pockets within the coating structure.

Using Knock-Out Mutants to Investigate the Adhesion of Staphylococcus aureus to Abiotic Surfaces.

The adhesion of Staphylococcus aureus to abiotic surfaces is crucial for establishing device-related infections. With a high number of single-cell force spectroscopy measurements with genetically modified S. aureus cells, this study provides insights into the adhesion process of the pathogen to abiotic surfaces of different wettability. Our results show that S. aureus utilizes different cell wall molecules and interaction mechanisms when binding to hydrophobic and hydrophilic surfaces. We found that covalently bound cell wall proteins strongly interact with hydrophobic substrates, while their contribution to the overall adhesion force is smaller on hydrophilic substrates. Teichoic acids promote adhesion to hydrophobic surfaces as well as to hydrophilic surfaces. This, however, is to a lesser extent. An interplay of electrostatic effects of charges and protein composition on bacterial surfaces is predominant on hydrophilic surfaces, while it is overshadowed on hydrophobic surfaces by the influence of the high number of binding proteins. Our results can help to design new models of bacterial adhesion and may be used to interpret the adhesion of other microorganisms with similar surface properties.

Evolution and Shape of Two-Dimensional Stokesian Drops under the Action of Surface Tension and Electric Field: Linear and Nonlinear Theory and Experiment

The creeping-flow theory describing evolution and steady-state shape of two-dimensional ionic-conductor drops under the action of surface tension and the subcritical (in terms of the electric Bond number) electric field imposed in the substrate plane is developed. On the other hand, the experimental data are acquired for drops impacted or softly deposited on dielectric surfaces of different wettability and subjected to an in-plane subcritical electric field. Even though the experimental situation involves viscous friction of drops with the substrates and wettability-driven motion of the contact line, the comparison to the theory reveals that it can accurately describe the steady-state drop shape on a non-wettable substrate. In the latter case, the drop is sufficiently raised above the substrate, which diminishes the three-dimensional effects, making the two-dimensional description (lacking the no-slip condition at the substrate and wettability-driven motion of the contact line) relevant. Accordingly, it is demonstrated how the subcritical electric field deforms the initially circular drops until an elongated steady-state configuration is reached. In particular, the surface tension tends to round off the non-circular drops stretched by the electric Maxwell stresses imposed by the electrodes. A more pronounced substrate wettability leads to more elongated steady-state configurations observed experimentally than those predicted by the two-dimensional theory. The latter cases reveal significant three-dimensional effects in the electrically driven drop stretching. In the supercritical electric fields (corresponding to the supercritical electric Bond numbers), the electrical stretching of drops predicted by the present linearized two-dimensional theory results in splitting into two separate droplets. This scenario is corroborated by the predictions of the fully nonlinear results for similar electrically stretched bubbles in the creeping-flow regime available in the literature as well as by the present experimental results on a substrate with slip.

How to Achieve a Monostable Cassie State on a Micropillar-Arrayed Superhydrophobic Surface.

Superhydrophobic surfaces with a monostable Cassie state possess numerous interesting applications in many fields, such as microfluidics, oil-water separation, drag reduction, self-cleaning, heat dissipation, and so on. How to guarantee a monostable Cassie state of a superhydrophobic surface is still an interesting problem. In this paper, considering the influence of external interferences that may induce the possible wettability transition, the whole wetting process of a droplet on a trapezoidal micropillar-arrayed superhydrophobic surface is divided into six possible stages. Both the Gibbs-free energy in each stage and the energy barrier between adjacent stages are obtained and analyzed theoretically. It is interesting to find that the finally stable wettability of a trapezoidal micropillar-arrayed superhydrophobic surface significantly depends on the apparent contact angle of the lateral surface of a single micropillar, which can be divided into three regions from 0 to 180°, corresponding to the Wenzel state, metastable Cassie state, and monostable Cassie state. Furthermore, the size of each region is explicitly related to and can be well-tuned by the geometry of microstructures. Such a wettability classification is well verified by a number of existing experimental results and our numerical simulations. As a relatively general case, such a trapezoidal micropillar-arrayed superhydrophobic surface can also be conveniently degenerated to the triangular or rectangular micropillar-arrayed surface. All the results should be useful for the precise design of functional surfaces of different wettabilities.

Redox Switchable Polydopamine-Modified AFM-SECM Probes: A Probe for Electrochemical Force Spectroscopy.

Polydopamine (PDA) has high potential in biorelevant applications as a versatile thin film material, e.g., as adhesive coating for cell immobilization or for sensing applications due to the plethora of functional groups. In this study we present the modification of conductive colloidal atomic force-scanning electrochemical microscopy (AFM-SECM) probes with electrochemically deposited PDA resulting in functional probes for quantitative electrochemical adhesion studies. Surface functionality of PDA can be altered by oxidation or reduction of functional groups applying an appropriate potential to the PDA-modified AFM-SECM probe, thereby enabling adhesion measurements under potential control. This facilitates probing specific interactions of surface groups present in PDA with various surfaces of different wettabilities. The versatility of such switchable AFM-SECM probes is demonstrated for electrochemical force spectroscopic studies at model samples such as plasma-treated gold substrates, hydrophobic or hydrophilic self-assembled monolayers, and for adhesion measurements of bacteria in dependence of altered surface charges of the colloidal probe. The maximum obtained adhesion force of a positively polarized PDA-modified AFM-SECM probe was 6.2 ± 2.2 nN, and it was about 50% less (i.e., 2.6 ± 1.1 nN) for a negatively polarized probe at a hydrophilic OH-terminated gold surface. In situ control of the active surface groups enabled investigations on the influence of surface charges on adhesion. Furthermore, plateaus of constant force were observed, which are a characteristic of polymer structures. Finally, electrochemical force measurements with switchable probes were used for the first time during adhesion studies of bacterial cells (i.e., Pseudomonas fluorescens). Positively biased PDA-coated colloidal probes revealed adhesion forces of 6.0 ± 1.1 nN, whereas significantly reduced adhesion forces 1.1 ± 0.7 nN were observed for negatively biased PDA-modified colloidal probes.

Numerical Simulations of Polymer Solution Droplet Impact on Surfaces of Different Wettabilities

This paper presents a physically based numerical model to simulate droplet impact, spreading, and eventually rebound of a viscoelastic droplet. The simulations were based on the volume of fluid (VOF) method in conjunction with a dynamic contact model accounting for the hysteresis between droplet and substrate. The non-Newtonian nature of the fluid was handled using FENE-CR constitutive equations which model a polymeric fluid based on its rheological properties. A comparative simulation was carried out between a Newtonian solvent and a viscoelastic dilute polymer solution droplet. Droplet impact analysis was performed on hydrophilic and superhydrophobic substrates, both exhibiting contact angle hysteresis. The effect of substrates’ wettability on droplet impact dynamics was determined the evolution of the spreading diameter. While the kinematic phase of droplet spreading seemed to be independent of both the substrate and fluid rheology, the recoiling phase seemed highly influenced by those operating parameters. Furthermore, our results implied a critical polymer concentration in solution, between 0.25 and 2.5% of polystyrene (PS), above which droplet rebound from a superhydrophobic substrate could be curbed. The present model could be of particular interest for optimized 2D/3D printing of complex fluids.

Janus Soft Actuators with On–Off Switchable Behaviors for Controllable Manipulation Driven by Oil

Soft actuators have tremendous applications in diverse fields. Facile preparation, rapid actuation, and versatile actions are always pursued when developing new types of soft actuators. In this paper, we present a facile method integrating laser etching and mechanical cutting to prepare Janus actuators driven by oil. A Janus film with superhydrophobic and hydrophobic sides was fabricated successfully. By cutting the functional layer at the desired positions, a number of quintessential oil-driven soft devices were demonstrated. Furthermore, Janus actuators with surfaces of different wettabilities exhibited different swelling behaviors, and different media manifested different surface extensions; thus, these actuators are promising candidates for soft actuators and also realized on-off switchability between an oil/water mixture and ethanol. This study offers novel insight into the design of soft actuators, and this insight may be helpful for developing an oil-driven soft actuator that can be operated like a human finger to manipulate any object and extending stimuli-responsive applications for soft robotics.

Division of Ferrofluid Drops Induced by a Magnetic Field.

We report a comprehensive study of the division of ferrofluid drops caused by their interaction with a permanent magnet. As the magnet gradually approaches the sessile drop, the drop deforms into a spiked cone and then divides into two daughter droplets. This process is the result of a complex interplay between the polarizing effect caused by the magnetic field and the magnetic attraction due to the field gradient. As a first attempt to describe it, during each scan we identify two characteristic distances between the magnet and the drop: zmax, corresponding to the drop reaching its maximum height, and zsaddle, corresponding to the formation of a saddle point on the drop peak identifying the beginning of the drop breakup. We have investigated the location of these two points using sessile drops of ferrofluid water solutions at various concentrations and volumes, deposited on four surfaces of different wettability. An empirical scaling law based on dimensionless variables is found to accurately describe these experimental observations. We have also measured the maximum diameter of the drops right before the division and found that it is very close to a critical size, which depends on the magnetic attraction.

Erratum: “Tribological Properties of Textured Cemented Carbide Surfaces of Different Wettability Produced by Pulse Laser” [ASME J. Micro Nano-Manuf., 2018, 6(2), p. 021001; DOI: 10.1115/1.4038629

Erratum: “Tribological Properties of Textured Cemented Carbide Surfaces of Different Wettability Produced by Pulse Laser” [ASME J. Micro Nano-Manuf., 2018, 6(2), p. 021001; DOI: 10.1115/1.4038629

Tribological Properties of Textured Cemented Carbide Surfaces of Different Wettability Produced by Pulse Laser

The micro/nanotextured cemented carbide surface of different wettability was produced by laser scanning and fluorinated treatment. The tribological properties of the untextured, oleophobic and oleophilic micro/nanotextured surface were investigated experimentally including the effects of crank speed and contact pressure by a reciprocating friction and a wear tester. For all tested surfaces, the friction coefficient of the surface decreased as both the increasing crank speed and contact pressure increased. Compared to the untextured surface, the friction coefficient of the micro/nanotextured surface was significantly decreased, being sensitive to the wettability of the surface. Besides, the tribological properties of the oleophobic micro/nanotextured surface were superior to the oleophilic micro/nanotextured surface under the same experimental conditions. The improvement in tribological properties of the oleophobic micro/nanotextured surface could be attributed to the low wettability, which was beneficial to rapid accumulation of the lubricating oil on the surface.

Densification of frost on hydrophilic and hydrophobic substrates – Examining the effect of surface wettability

The properties of a growing frost layer were analyzed and compared for surfaces of different wettability to determine the effect that the surface energy has on the frost mass, thickness, and density. Three surfaces were tested – an uncoated, untreated aluminum plate (Surface 1), an identical plate coated with a hydrophobic coating (Surface 2), and a plate containing a hydrophilic coating (Surface 3). For these experiments, the frost layer was grown for a three-hour period inside a Plexiglas environmental test chamber where the relative humidity was held constant (i.e. 60%, 80%) using an ultrasonic humidifier. The surface temperature of the plate was fixed using a thermoelectric cooler (TEC) and monitored by four thermocouples affixed to the surface and stage. Frost thickness was determined from images of the frost layer taken using a CCD camera mounted directly overhead. A reduction in frost density of 37–41% was observed on the hydrophobic surface (Surface 2), whereas an increase of 20–26% was consistently observed on the hydrophilic surface (Surface 3) as compared to the baseline surface. Frost layer property data were also compared against models found in the literature. Reasonably good agreement was observed when comparing against data from the baseline surface; however, the agreement was not generally as good when compared against the hydrophilic and hydrophobic surfaces suggesting the need for surface wettability to be included as a parameter in future frost densification models.

Plantigrade settlement of the musselMytilus coruscusin response to natural biofilms on different surfaces

Surface properties affect the attachment of micro- and macroscopic marine organisms. The current study examined the settlement response of the musselMytilus coruscusplantigrades to natural biofilms formed on surfaces of different wettability. The percentages of plantigrade settlement were not influenced by the biofilms formed on variously wettable surfaces in the short term, but after 10 days, the plantigrade settlement rates decreased on biofilms formed on lower wettability surfaces. In general, lower wettability of the surfaces resulted in the decrease of the dry weight, bacterial and diatom density and the thickness of natural biofilms when compared to high wettability surfaces. In contrast, chlorophyll-aconcentration in biofilms was independent of the initial wettability of the surfaces. Comparative cluster analysis of bacterial denaturing gradient gel electrophoresis patterns revealed that high variability existed between the bacterial community on high wettability surfaces and that on low wettability surfaces. Thus, surface wettability affects the formation of natural biofilms, and this variation in biofilms developed on different wettability surfaces may explain the discrepancy in their corresponding inducing activities onM. coruscusplantigrade settlement. This finding provides new insight into interactions between mussel settlement, biofilm characteristics and surface properties.

125I-Radiolabeling, Surface Plasmon Resonance, and Quartz Crystal Microbalance with Dissipation: Three Tools to Compare Protein Adsorption on Surfaces of Different Wettability

The extent of protein adsorption is an important consideration in the biocompatibility of biomaterials. Various experimental methods can be used to determine the quantity of protein adsorbed, but the results usually differ. In the present work, self-assembled monolayers (SAMs) were used to prepare a series of model gold surfaces varying systematically in water wettability, from hydrophilic to hydrophobic. Three commonly used methods, namely, surface plasmon resonance (SPR), quartz crystal microbalance with dissipation (QCM-D), and (125)I-radiolabeling, were employed to quantify fibrinogen (Fg) adsorption on these surfaces. This approach allows a direct comparison of the mass of Fg adsorbed using these three techniques. The results from all three methods showed that protein adsorption increases with increasing surface hydrophobicity. The increase in the mass of Fg adsorbed with increasing surface hydrophobicity in the SPR data was parallel to that from (125)I-radiolabeling, but the absolute values were different and there does not seem to be a "universally congruent" relationship between the two methods for surfaces with varying wettability. For QCM-D, the variation in protein adsorption with wettability was different from that for SPR and radiolabeling. On the more hydrophobic surfaces, QCM-D gave an adsorbed mass much higher than from the two other methods, possibly because QCM-D measures both the adsorbed Fg and its associated water. However, on the more hydrophilic surfaces, the adsorbed mass from QCM-D was slightly greater than that from SPR, and both were smaller than from (125)I-radiolabeling; this was true no matter whether the Sauerbrey equation or the Voigt model was used to convert QCM-D data to adsorbed mass.

Novel whole cell adhesion assays of three isolates of the fouling diatom Amphora coffeaeformis reveal diverse responses to surfaces of different wettability

Whole cell, strength of adhesion assays of three different isolates of the fouling diatom Amphora coffeaeformis were compared using a hydrophilic surface viz. acid washed glass (AWG), and a hydrophobic surface viz. a self assembled monolayer (SAM) of undecanethiol (UDT). Assays were performed using a newly designed turbulent flow channel that permits direct observation and recording of cell populations on a test surface. Exposure to continuous shear stress over 3 h revealed that the more motile isolate, WIL2, adhered much more strongly to both test surfaces compared to the other two strains. When the response of the isolates to shear stress after 3 h was compared, there was no significant difference in the percentage of cells removed, irrespective of surface wettability. Cells of the three isolates of A. coffeaeformis varied significantly in their response to different surfaces during initial adhesion, indicating the presence of a wide range of ‘physiological races’ within this species.

Interaction Between Triblock Copolymer Surfactant and Low- κ Dielectric Surfaces Relevant to CMP

We examine the effects of the nonionic triblock copolymer surfactant Pluronic P103 on three surfaces of different wettability relevant to chemical mechanical planarization (CMP). Two of the surfaces are low-k organosilicate glass (OSG) films, Coral and Black Diamond; the third is a silica surface. Atomic force microscopy (AFM) force curves were used to probe the forces over each surface in solutions of P103. Each surface was also examined in potassium sulfate solutions to investigate the effect of ionic strengths. The AFM force curves show that both P103 and potassium sulfate eliminate adhesive forces at sufficiently high concentrations. DLVO theory was used to fit the AFM approach curves in order to calculate estimated surface potentials. Interestingly, the force curves suggest that molecular orientation of the P103 is different on surfaces of different wettability. The P103 was found to adopt a flat conformation on the hydrophilic silica surface while more extended structures formed on the more hydrophobic Coral and Black Diamond surfaces. These results provide a molecular-level understanding to aid the development of CMP formulations that will provide greater control on dielectric removal rate and reduce the overall non-uniformity in film thickness across the wafer.

Influence of wettability and surface charge on the interaction between an aqueous electrolyte solution and a solid surface

The hydrodynamic drainage force of a Newtonian aqueous electrolyte solution squeezed between two surfaces of different wettability was measured using the AFM colloidal probe technique. The surface hydrophobicity, roughness, polarity and approach velocity, and thus the shearing rate of the liquid, were controlled. A direct relationship between the mobility of the aqueous electrolyte solution close to the surfaces and the hydrophobicity of the surfaces was not established. We predict that the mobility of the liquid depends in a more complex fashion on the polarity and charge of the surfaces and on the properties of the electrolyte.

Computer simulation of polypeptide adsorption on model biomaterials

When biomaterials are inserted in a biological environment, for instance in a body implant, proteins do quickly adsorb on the exposed surface. Such process is of fundamental importance, since it directs the subsequent cell adhesion. Here we review recent advances in this field obtained with molecular simulations. While coarse-grained models can provide important general results, as it has long been recognized in polymer science, the hierarchical structure of a very complex copolymer such as a protein, together with the nature of the biomaterial surface suggest that atomistic models are better suited to investigate these phenomena. Thus, after briefly mentioning some common features of coarse-grained and atomistic force fields, we first discuss early theoretical and coarse-grained simulation results about protein adsorption, and then we highlight the main results recently obtained by us with atomistic models. In particular, we discuss some conformational and energetic aspects of the adsorption of protein fragments with different secondary structure on surfaces of different wettability, including hydrophobic graphite and hydrophilic poly(vinylalcohol). We also consider other features, such as the simulation of the materials wettability, the hydration of the adsorbed fragments, their kinetics of spreading, and the sequential adsorption of two protein fragments on top of each other, highlighting the results of general interest.