Effect Of Substrate Wettability Research Articles

Probing Into the Drying Pattern Dynamics of a Ferrofluid Droplet Under the Actuation of Magnetic Field

Evaporation of liquids, especially colloids, is a subject of research interest since decades. Evaporation patterns of colloidal particles can determine key factors about the liquid properties and the underlying physics. Particle deposition, self-assembly, and inkjet printing are various applications, where the control of droplet evaporation and its patterns can be useful. Here, kerosene-based ferrofluid droplets were allowed to evaporate in the presence of static magnetic field. Earlier, the evaporation pattern of the same fluid in ambient conditions was reported, where ring pattern with central accumulation was observed. This was attributed to thermal Marangoni flow. In this case, magnetic field helps in circumventing this pattern, and directional dependence on magnetic field in the final patterns was also observed. Magnetic field parallel to the substrate created chain structures in the direction of the field, whereas magnetic field perpendicular to the substrate resulted in completely uniform pattern. For higher volume fraction of the chosen samples, the influence of external magnetic field was seen earlier than the samples of lower volume fraction. Effect of substrate wettability was observed under similar conditions for glass substrate and Polydimethylsiloxane (PDMS) substrate. These experimental results are compared/supported by the available existing theoretical results/theories. The inferences obtained from this analysis are expected to be useful in the optimization of particle deposition of magnetic colloids, typically of demanding significance in fields, such as nanochromatography, lithography, surface engineering, self-assembly, inkjet printing, and so on.

Molecular Dynamics Study of Ionomer Aggregate Structure during Solvent Evaporation

Cathode catalyst layer (CL) is one of the principal components for polymer electrolyte fuel cells. CL is typically prepared by solution processing techniques where a catalyst ink consisting of Pt-C (platinum on a carbon powder support), ionomer, and solvents (water/alcohol mixtures) is exposed to evaporation. The structure and stability of ionomer aggregates in solutions are the key to the physical properties of ionomer thin films. Thus, understanding the ionomer aggregation behaviors in solutions is of great importance to gain a deep insight into fundamental morphological characteristics of ionomers. In our previous studies, we have observed ionomer formation into cylindrical bundle-like aggregates in solutions. The size of ionomer bundles decreases with increasing alcohol content, indicating that the ionomers tend to be more dispersed at higher alcohol content, which is in good agreement with the trend inferred from the scattering experiments. In this study, evolution of ionomer aggregate structure in water solutions have been investigated using solvent evaporation simulations based on the course-grained molecular dynamics method, as shown in FIgure 1. The balance between evaporation, sedimentation, and diffusion was controlled by the dimensionless Peclet number, providing a link between experiments and simulations. The effects of substrate wettability on the ionomer thin film structure will be discussed in the presentation. Figure 1

The effect of substrate wettability and modulus on gecko and gecko-inspired synthetic adhesion in variable temperature and humidity

Gecko adhesive performance increases as relative humidity increases. Two primary mechanisms can explain this result: capillary adhesion and increased contact area via material softening. Both hypotheses consider variable relative humidity, but neither fully explains the interactive effects of temperature and relative humidity on live gecko adhesion. In this study, we used live tokay geckos (Gekko gecko) and a gecko-inspired synthetic adhesive to investigate the roles of capillary adhesion and material softening on gecko adhesive performance. The results of our study suggest that both capillary adhesion and material softening contribute to overall gecko adhesion, but the relative contribution of each depends on the environmental context. Specifically, capillary adhesion dominates on hydrophilic substrates, and material softening dominates on hydrophobic substrates. At low temperature (12 °C), both capillary adhesion and material softening likely produce high adhesion across a range of relative humidity values. At high temperature (32 °C), material softening plays a dominant role in adhesive performance at an intermediate relative humidity (i.e., 70% RH).

Mineral Scaling in Microchips: Effect of Substrate Wettability on CaCO3 Precipitation

In the present work, the effect of wettability on CaCO3 precipitation was approached by monitoring crystal formation in microchips of different wettability degree. Solutions of calcium chloride and...

Effect of substrate wettability and flexibility on the initial stage of water vapor condensation.

Understanding the mechanisms of controlling vapor condensation on surfaces is of significant importance in many fields. Despite many efforts made in the investigation of vapor condensation, few studies concern the condensation on flexible substrates, especially in microscale. In this paper, the condensation of high temperature water vapor on substrate with various flexibilities and wettabilities is investigated using molecular dynamics simulation. The results indicate that when substrates with the same flexibility vary from hydrophobic to hydrophilic, the condensation rate increases and the condensation mode changes from no-condensation to dropwise condensation, incomplete filmwise condensation and filmwise condensation, and meanwhile, the heat exchange between the water vapor and the substrate becomes more efficient; when substrates with the same wettability vary from rigid to flexible, the nucleation density, the condensation rate and the heat exchange efficiency increase significantly. In particular, the condensation modes on rigid and flexible substrates with the same wettability are generally the same except for the substrates with εwater-Cu = 0.4 kcal mol-1; and the critical values of substrate spring constant for the condensation mode transition are about 80-100 kcal mol-1 Å-2 when εwater-Cu = 0.4 kcal mol-1. Therefore, changing the flexibility of the substrate is proposed as a new way to control the condensation mode at the initial stage of water vapor condensation to meet design requirements.

Slippery Surface Based on Photoelectric Responsive Nanoporous Composites with Optimal Wettability Region for Droplets' Multifunctional Manipulation.

The development of responsive slippery surfaces is important because of the high demand for such materials in the fields of liquid manipulation on biochips, microfluidics, microreactions, and liquid‐harvesting devices. Although great progress has been achieved, the effect of substrate wettability on slippery surfaces stability is overlooked by scientists. In addition, current responsive slippery surfaces generally function utilizing single external stimuli just for imprecisely controlling liquid motion, while advanced intelligences are always expected to be integrated into one smart interface material for widespread multifunctional applications. Therefore, designing slippery surfaces that collaboratively respond to complex external stimuli and possess sophisticated composite function for expanding applications from controlling droplets motion to patterned writing is urgently needed but remains a challenge. Here, a photoelectric cooperative‐responsive slippery surface based on ZnO nanoporous composites is demonstrated. First, the effect of composite surface wettability on slippery surface stability is systematically researched and the optimum wettability region for fabricating stable slippery surfaces is determined. Furthermore, controllable droplet motion and patterned writing are realized on the same slippery surfaces under photoelectric cooperative stimuli, and the related response mechanism is also deeply studied. This kind of material has potential applications in biochips, microfluidics, in situ patterning, and water‐harvesting systems.

Experimental study and mechanism analysis on the effect of substrate wettability on graphene sheets distribution morphology within uniform printing droplets

Uniform graphene films and micro-patterns are the cornerstones of graphene-based printed electronics. However, disk-like reduced graphene oxide (RGO) sheets trend to concentrate at the edge of the drop because of the famous coffee-ring effect, resulting in non-uniform patterns. To understand the physics of coffee-ring formation for RGO droplets on hydrophilic substrates, we propose a mechanical model to elucidate the influence and its mechanism of substrate wettability on the solute migration behavior and solute distribution morphology of RGO droplets. Stronger coffee-ring morphology and a slower velocity transition on the PMMA can be observed as compared to that on the glass slides. An explanation based on the mechanical model is provided as the large contact angle on the PMMA leads to a small hindrance force and finally results in more significant coffee-ring morphology. Remarkably, we have revealed one underlying mechanism by which the hydrophilic substrate with better wettability will form more uniform patterns. This study can provide fundamental insights into the relationship between substrate wettability and RGO sheets distribution morphology. It might contribute to morphology regulation of RGO droplets in the DOD printing of graphene films and micro-patterns.

Thermal singularity and contact line motion in pool boiling: Effects of substrate wettability.

The dynamic van der Waals theory [Phys. Rev. E 75, 036304 (2007)] is employed to model the growth of a single vapor bubble in a superheated liquid on a flat homogeneous substrate. The bubble spreading dynamics in the pool boiling regime has been numerically investigated for one-component van der Waals fluids close to the critical point, with a focus on the effect of the substrate wettability on bubble growth and contact line motion. The substrate wettability is found to control the apparent contact angle and the rate of bubble growth (the rate of total evaporation), through which the contact line speed is determined. An approximate expression is derived for the contact line speed, showing good agreement with the simulation results. This demonstrates that the contact line speed is primarily governed by (1) the circular shape of interface (for slow bubble growth), (2) the constant apparent contact angle, and (3) the constant bubble growth rate. It follows that the contact line speed has a sensitive dependence on the substrate wettability via the apparent contact angle which also determines the bubble growth rate. Compared to hydrophilic surfaces, hydrophobic surfaces give rise to a thinner shape of bubble and a higher rate of total evaporation, which combine to result in a much faster contact line speed. This can be linked to the earlier formation of a vapor film and hence the onset of boiling crisis.

Multiscale patterning of nanocomposite polyelectrolyte/nanoparticle films using inkjet printing and AFM scratching

The fabrication of structured polymer/nanoparticle composite films through a combination of additive, subtractive and self-assembly methodologies is investigated. Consumer grade inkjet printing hardware is employed to deposit cationic polyelectrolytes on (i) hydrophilic and (ii) hydrophobised glass substrates. The hydrophobisation process controls the spreading of the droplets and hence the lateral size of printed features. The printed cationic polyelectrolyte regions are used as a template to direct the self-assembly of negatively charged gold nanoparticles onto the surface. Micro-scale features are created in the polyelectrolyte/nanoparticle films using AFM scratching to selectively displace material. The effect of substrate wettability on film morphology is discussed.

The effect of substrate wettability on the breakdown of a locally heated fluid film

The effect of the equilibrium contact angle of wetting on the dynamics of the dry patch propagation and on the critical heat flux upon the breakdown of a water film that is heated locally from the substrate side is studied experimentally. The equilibrium contact angle is varied from 27° ± 6° to 74° ± 9° (with no changes in the thermophysical properties of the system) through the use of different types of surface grinding. The studies are performed for three flow modes: (a) a fluid film that freely flows down along a substrate with an inclination of 5° to the horizon, (b) a film that moves along a horizontal substrate under the influence of hydrostatic pressure, and (c) a static film on a horizontal substrate. It is found that the substrate wettability has a significant effect on the dry patch propagation rate and its final size in all these cases, but has almost no effect on the threshold heat flux at which the breakdown of a film occurs.

The Effect of Substrate Wettability on Hydrazine-Processed CZTS Thin Films

ABSTRACTWe report a study on the wetting and spreading of hydrazine-CZTS solution on a series of solid surfaces. The work of adhesion between a hydrazine solution and soda-lime glass, Si, graphite, ITO, SnO2, ZnO, CdS, In2S3, Cu, Au, Ag, Al, Ni, Mo, and carbon single-walled nanotubes was calculated using observed contact angles and the areas of the interface. The surface roughness of drop-casted CZTS precursor films was lower on surfaces with better hydrazine wettability. This suggests that the surface roughness of solution-processed films can be controlled by altering the wetting behavior of the solution on the substrate.

Substrate wettability induced alterations in convective heat transfer characteristics in microchannel flows: An order parameter approach

In this paper, we analyze the consequences of substrate wettability on the convective heat transfer characteristics in microchannel flows. However, in sharp contrast to other reported studies, we do not presume a slip-length a priori, but take an explicit accounting of the substrate wettability through an order parameter consideration. The order parameter formalism also allows us to consider co-existences of bulk and interfacial phases and takes the consequent thermo-physical property variations aptly into account. Our results reveal explicit dependence of the heat transfer characteristics, such as the thermal entrance length and the Nusselt number, on the equilibrium contact angle. Further, we present an analysis that theoretically justifies the role of wettability by decomposing the thermophysical properties and flow variables into a base part and a wettability-dependant part, with rigorous justification from a perturbation-based analysis of the order parameter evolution equations. These considerations, along with our simulation predictions, reveal that the effect of substrate wettability on bulk thermal characteristics of microchannel transport is primarily advection-dominated, although a combined advection–diffusion mechanism controls the corresponding interfacial transport in the near-wall regime. Our results bring out an explicit dependence of the heat transfer characteristics on the substrate wettability, and in particular, reveal a trend of decreasing Nusselt number with increasing value of the contact angle.

Effect of Substrate Wettability and Surface Structure on Nucleation of Crystal

The heterogeneous nucleation behaviors of NH4Cl crystal on rough aluminum substrate surface immerged in NH4Cl-H2O solution were experimentally analyzed, and the influence mechanism of the micro/nano-scale surface structures on heterogeneous nucleation was investigated. It has been shown that wettability and nucleation are affected by substrate surface condition. The intrinsic wetting properties between nucleus and substrate surface, and the surface structure of certain geometrical scales, both impose effects on the heterogeneous nucleation properties. For a nucleus-wetting substrate surface, heterogeneous nucleation is promoted by a higher complexity of the surface morphology; but for a nucleus-nonwetting substrate surface, heterogeneous nucleation is inhibited by a higher complexity of the surface morphology.

Consistent description of electrohydrodynamics in narrow fluidic confinements in the presence of hydrophobic interactions

Electrohydrodynamics in the presence of hydrophobic interactions in narrow confinements is traditionally represented from a continuum viewpoint by a Navier slip-based conceptual paradigm, in which the slip length carries the sole burden of incorporating the effects of substrate wettability on interfacial electromechanics, precluding any explicit dependence of the interfacial potential distribution on the substrate wettability. Here we show that this traditional way of treating electrokinetics-wettability coupling may lead to serious discrepancies in predicting the resultant transport characteristics as manifested through an effective zeta potential. We suggest that an alternative consistent description of the underlying physics through a free-energy-based formalism, in conjunction with considerations of hydrodynamic and electrical property variations consistent with the pertinent phase-field description, may represent the underlying consequences in a more rational manner, as compared to the traditional slip-based model coupled with a two-layer description. Our studies further reveal that the above discrepancies may not occur solely due to the slip-based route of representing the interfacial wettability, but may be additionally attributed to the act of "discretizing" the interfacial phase fraction distribution through an artificial two-layer route.

Effect of substrate wettability in liquid dielectrophoresis (LDEP) based droplet generation: Theoretical analysis and experimental confirmation

The generation of droplets for biological reactions at the microscale can be achieved by many techniques, among which the so-called liquid dielectrophoresis technique (LDEP). This is not a new process, but the parameters influencing actuation voltage still need further insight: size and geometry (electrodes width and gap, dielectric thickness), materials (dielectric constant), liquids (surface tension, dielectric constant, conductivity), working conditions (voltage, frequency) and substrate wettability (contact angle). This large experimental space is firstly reduced using non dimensional numbers and then studied in a systematic way thanks to the design of experiments. The contact angle influence is explained thanks to a new analytical model. To summarize, this paper recalls analytical models used to predict the voltage threshold required to develop a liquid rivulet from a mother drop, taking the contact angle into account and providing a large set of experimental results.