Contact angle hysteresis of microbead suspensions

Numerical Study of the Coalescence and Mixing of Drops of Different Polymeric Materials

In this study, we employ direct numerical simulation (DNS) to investigate the solutal hydrodynamics dictating the three-dimensional coalescence of microscopic, identical-sized sessile drops of different but miscible shear-thinning polymeric liquids (namely, PVAc or polyvinyl acetate and PMMA or polymethylmethacrylate), with the drops being in partially wetted configuration. Despite the ubiquitousness of the interaction of different dissimilar droplets in a variety of engineering problems ranging from additive manufacturing to understanding the behavior of photonic crystals, coalescence of drops composed of different polymeric and non-Newtonian materials has not been significantly explored. Interaction of such dissimilar droplets often involves simultaneous drop spreading, coalescence, and mixing. The mixing dynamics of the dissimilar drops are governed by interphase diffusion, the residual kinetic energy of the drops stemming from the fact that coalescence starts before the spreading of the drops have been completed, and the solutal Marangoni convection. We provide the three-dimensional velocity fields and velocity vectors inside the completely miscible, dissimilar coalescing droplets. Our simulations explicate the relative influence of these different effects in determining the flow field at different locations and at different time instances and the consequent mixing behavior inside the interacting drops. We also show the non-monotonic (in terms of the direction of migration) propagation of the mixing front of the miscible coalescing drops over time. We also establish that the overall mixing (on either side of the mixing front) speeds up as the Marangoni effects dictate the mixing. We anticipate that our study will provide an important foundation for studying miscible multi-material liquid systems, which will be crucial for applications such as inkjet or aerosol jet printing, lab-on-a-chip, polymer processing, etc.

Features of Composite Layers Created Using an Aqueous Suspension of a Fluoropolymer

This paper presents a method for the formation of composite-polymer-containing coatings on MA8 Mg alloy by plasma electrolytic oxidation (PEO), followed by the deposition of a fluoropolymer from an aqueous suspension of superdispersed polytetrafluoroethylene. The Scanning Electron Microscope(SEM), Energy Dispersive Spectroscopy(EDS), and X-ray Diffraction(XRD) analyses established morphological features as well as elemental and phase composition of composite coatings. The fact that the pores are filled with a fluoropolymer has been experimentally confirmed. An assessment of the corrosion properties of formed composite coatings revealed a decrease in the corrosion current density by more than four orders of magnitude in comparison with the base PEO layer. The highest resistance to the damaging effects of a corrosive environment, according to the results of long-term exposure tests, was demonstrated by coatings after three treatments with polytetrafluoroethylene. The obtained polymer-containing coatings have antifriction properties, reducing the wear of the coatings by more than 27-fold in comparison with the base PEO layer. It was revealed that composite coatings have superhydrophobic properties: the value of the contact angle reaches 154°, and the hysteresis of the contact angle is less than 10°.

Colloidal Particles for Pickering Emulsion Stabilization Prepared via Antisolvent Precipitation of Lignin-Rich Cocoa Shell Extract

This study concerns the preparation and functionality testing of a new class of Pickering particles for food emulsion stabilization: colloidal lignin-rich particles (CLRPs) derived from ethanol-soluble extract of cocoa shell. A further goal was to achieve Pickering functionality without the need to add co-emulsifying surfactants during emulsion processing. Cocoa shell is a co-product of the food manufacturing industry. As such it is anticipated that the particles would be accepted as a natural food ingredient, provided no harmful solvents are used in any step of their processing. The cocoa shell particles were milled, dispersed in water and exposed to 250 °C for 1 h in a stainless-steel tubular reactor followed by ethanol extraction to obtain a lignin-rich extract (46% (w/w) lignin with the remainder predominantly lipids). CLRPs were then fabricated by the precipitation of ethanol-dissolved extract into water (antisolvent). By employing an agitated process and droplet dosing into a non-agitated process, four particle suspensions of a range of submicron diameters were obtained. All particle suspensions contained the same mass fraction of extract and were surface active, with surface tension decreasing with increasing particle size. The smallest particles were obtained when lipids were removed from the extract prior to particle processing. In contrast to the other four particle suspensions, this one failed to stabilize a 10% (w/w) sunflower oil-in-water emulsion. We hypothesize that the phospholipids indigenously present in these CLRP formulations are a critical component for Pickering functionality. It can be concluded that we have successfully introduced a new class of Pickering particles, fabricated from an industry co-product and anticipated to be food grade.

Microparticle Suspensions and Bacteria-Laden Droplets: Are They the Same in Terms of Wetting Signature?

Adhesion behavior of microbial pathogens on commonly encountered surfaces is one of the most pertinent questions now. We present the characterization of bacteria-laden droplets and quantify the adhesion forces on highly repellent surfaces with the help of a simple experimental setup. Comparing the force signature measured directly using an in-house capillary deflection-based droplet force apparatus, we report an anomalous adhesion behavior of live bacteria (E. coli)-laden droplets on repellent surfaces, which stands in stark contrast to the observed adhesion signature when the doping agent is changed to inert microparticles or the same bacteria in an incapacitated state. We showed that the regular contact angle measurements using optical goniometry is unable to differentiate between the live bacteria and the dead ones (including microparticles) and thus delineate its limitations and the complementary nature of the adhesion measurements in understanding the fundamental interfacial interaction of living organisms on solid surfaces.

Physicochemical parameters that underlie inkjet printing for medical applications

One of the most common types of 3D printing technologies is inkjet printing due to its numerous advantages, including low cost, programmability, high resolution, throughput, and speed. Inkjet printers are also capable of fabricating artificial tissues with physiological characteristics similar to those of living tissues. These artificial tissues are used for disease modeling, drug discovery, drug screening, and replacements for diseased or damaged tissues. This paper reviews recent advancements in one of the most common 3D printing technologies, inkjet dispensing. We briefly consider common printing techniques, including fused deposition modeling (FDM), stereolithography (STL), and inkjet printing. We briefly discuss various steps in inkjet printing, including droplet generation, droplet ejection, interaction of droplets on substrates, drying, and solidification. We also discuss various parameters that affect the printing process, including ink properties (e.g., viscosity and surface tension), physical parameters (e.g., internal diameter of printheads), and actuation mechanisms (e.g., piezoelectric actuation and thermal actuation). Through better understanding of common 3D printing technologies and the parameters that influence the printing processes, new types of artificial tissues, disease models, and structures for drug discovery and drug screening may be prepared. This review considers future directions in inkjet printing research that are focused on enhancing the resolution, printability, and uniformity of printed structures.

Friction and Adhesion of Microparticle Suspensions on Repellent Surfaces

With the recent advancements in the development and application of repellent surfaces, both in air and under liquid medium, accurate characterization of repellence behavior is critical in understanding the mechanism behind many observed phenomena and to exploit them for novel applications. Conventionally, the repellence behavior of a surface is characterized by the optical measurement of the dynamic contact angle of the target (to be repelled) liquid on the test surface. However, as already established in the literature, optical measurements are prone to appreciable error, especially for repellent surfaces with high contact angles. Here, we present an alternative, more accurate force-based characterization method of both friction and adhesion forces of microparticle-laden aqueous droplets over various repellent surfaces, where the force signature is captured by probing the surface with a droplet of the test liquid mounted at the tip of a flexible cantilever and then tracking the deflection of the tip of the cantilever as the probe droplet interacts with the surface. A systematic investigation of the response of repellent surfaces toward droplets with different microparticle concentrations reveals the dependency and sensitivity of measured adhesion and friction signature toward particle concentration. A comparison with the theoretical estimate from optical goniometry highlights the deviation of the theoretical data from experimentally measured values and further substantiates the need for such a force-based approach for accurate characterization of repellence behavior.

Robust Contact Angle Determination for Needle-in-Drop Type Measurements

One of the main approaches for contact angle determination using sessile drops with a missing apex (e.g., because of the presence of the needle tip) is the polynomial drop-profile fitting method. The major disadvantage of this fitting procedure is that the derived contact angle is highly sensitive to the polynomial order and the number of pixels involved in the actual fit. In the present work, an easily implementable method is introduced to effectively tackle these drawbacks. Instead of fitting the drop-profile itself, the polynomial fitting is applied to the difference between the drop profile and the circumcircle, independently for both sides of the drop. The derivative value of this difference at the contact point is used to correct the slope obtained analytically from the circumcircle. It is shown that this approach allows the robust determination of the contact angle with high (≤0.6°) accuracy in a straightforward manner, and the results are not affected by the actual contact angle, drop volume, or the resolution of the captured image. Validation of this new approach is also given in the contact angle range of 20°-150° by comparing the results to the values calculated by the Young-Laplace fit.

Effects of living cells on the bioink printability during laser printing

Laser-induced forward transfer has been a promising orifice-free bioprinting technique for the direct writing of three-dimensional cellular constructs from cell-laden bioinks. In order to optimize the printing performance, the effects of living cells on the bioink printability must be carefully investigated in terms of the ability to generate well-defined jets during the jet/droplet formation process as well as well-defined printed droplets on a receiving substrate during the jet/droplet deposition process. In this study, a time-resolved imaging approach has been implemented to study the jet/droplet formation and deposition processes when printing cell-free and cell-laden bioinks under different laser fluences. It is found that the jetting behavior changes from no material transferring to well-defined jetting with or without an initial bulgy shape to jetting with a bulgy shape/pluming/splashing as the laser fluence increases. Under desirable well-defined jetting, two impingement-based deposition and printing types are identified: droplet-impingement printing and jet-impingement printing with multiple breakups. Compared with cell-free bioink printing, the transfer threshold of the cell-laden bioink is higher while the jet velocity, jet breakup length, and printed droplet size are lower, shorter, and smaller, respectively. The addition of living cells transforms the printing type from jet-impingement printing with multiple breakups to droplet-impingement printing. During the printing of cell-laden bioinks, two non-ideal jetting behaviors, a non-straight jet with a non-straight trajectory and a straight jet with a non-straight trajectory, are identified mainly due to the local nonuniformity and nonhomogeneity of cell-laden bioinks.

Effects of interplay of nanoparticles, surfactants and base fluid on the surface tension of nanocolloids

A systematically designed study has been conducted to understand and demarcate the degree of contribution by the constituting elements to the surface tension of nanocolloids. The effects of elements such as surfactants, particles and the combined effects of these on the surface tension of these complex fluids are studied employing the pendant drop shape analysis method by fitting the Young-Laplace equation. Only the particle has shown an increase in the surface tension with particle concentration in a polar medium like DI water, whereas only a marginal effect of particles on surface tension in weakly polar mediums like glycerol and ethylene glycol has been demonstrated. Such behaviour has been attributed to the enhanced desorption of particles to the interface and a theory has been presented to quantify this. The combined particle and surfactant effect on the surface tension of a complex nanofluid system showed a decreasing behaviour with respect to the particle and surfactant concentration with a considerably feeble effect of particle concentration. This combined colloidal system recorded a surface tension value below the surface tension of an aqueous surfactant system at the same concentration, which is a counterintuitive observation as only the particle results in an increase in the surface tension and only the surfactant results in a decrease in the surface tension. The possible physical mechanism behind such an anomaly happening at the complex fluid air interface has been explained. Detailed analyses based on thermodynamic, mechanical and chemical equilibrium of the constituents and their adsorption-desorption characteristics as extracted from the Gibbs adsorption analysis have been provided. The present paper conclusively explains several physical phenomena observed, yet hitherto unexplained, in the case of the surface tension of such complex fluids by segregating the individual contributions of each component of the colloidal system.

Wetting characteristics of underwater micro-patterned surfaces

Wetting phenomena of underwater systems do not always follow the classical Wenzel and Cassie–Baxter configurations.

Dynamics of Electrically Modulated Colloidal Droplet Transport

Electrically actuated transport dynamics of colloidal droplets, on a hydrophobic dielectric film covering an array of electrodes, is studied here. Specifically, the effects of the size and electrical properties (zeta-potential) of the colloidal particles on such transport characteristics are investigated. For the colloidal droplets, the application of an electrical voltage leads to additional attenuation of the local dielectric-droplet interfacial tension. This is due to the electrically triggered enhanced colloidal particle adsorption at the dielectric-droplet interface, in the immediate vicinity of the droplet three-phase contact line (TPCL). The extent of such interfacial particle adsorption, and hence, the extent of the consequential reduction in the interfacial tension, is dictated by the combined effects of the three-phase contact line spreading, particle size, the interfacial electrostatic interaction between the colloidal particles (if charged) and the charged dielectric surface above the activated electrode, and the interparticle electrostatic repulsion. The electrical driving force of varying magnitude, stemming from this altered solid-liquid interfacial tension gradient in the presence of the colloidal particles, culminates in different droplet transport velocity and droplet transfer frequency for different colloidal droplets. We substantiate the inferences from our experimental results by a quasi-steady state force balance model for colloidal droplet transport. We believe that the present work will provide an accurate framework for determining the optimal design and operational parameters for digital microfluidic chips handling colloidal droplets, as encountered in a plethora of applications.

Plasma treatment induced wetting transitions on biological tissue (pigeon feathers)

We report first the wetting transition from superhydrophobicity to superhydrophilicity observed on nitrogen and air plasma irradiated biological tissue (pigeon feathers). Non-irradiated feathers demonstrate pronounced Cassie-Baxter ("fakir") wetting characterized by high apparent contact angles and low sliding angles. Plasma-irradiated feathers are superhydrophilic. Plasma treatment does not affect the barbs/barbules keratin-built network constituting pigeon pennae, but it changes the Young, advancing and receding angles of the tissue. The mechanism of wetting transition is discussed.

Analysis of the equilibrium droplet shape based on an ellipsoidal droplet model

The extent of a droplet's spreading over a flat, smooth solid substrate and its equilibrium height in the presence of gravity are determined approximately, without a numerical solution of the governing nonlinear differential equation, by assuming that the droplet takes on the shape of an oblate spheroidal cap and by minimizing the corresponding free energy. The comparison with the full numerical evaluations confirms that the introduced approximation and the obtained results are accurate for contact angles below about 120° and for droplet sizes on the order of the capillary length of the liquid. The flattening effect of gravity is to increase the contact radius and decrease the height of the droplet, with these being more pronounced for higher values of the Bond number.

Effect of submicron particles on electrowetting on dielectrics (EWOD) of sessile droplets

The present study elucidates the effects of included submicron-sized particles on the wetting behavior of sessile droplets under the influence of applied electric field in an electro-wetting-on-dielectric (EWOD) configuration. A thermodynamic description using an energy minimization approach is used to analyze the experimental results related to the effects of the included particles on the EWOD phenomenon, considering the effects of line tension as well. The effects of particle size and concentration on interfacial areas are included in the model to analyze the wetting characteristics. Experiments are also conducted with submicron-sizes latex beads, in an effort to elucidate the related phenomena. It is further postulated that these beads act as suspended dielectrics in the droplet, thereby mimicking a system of two capacitors in series. An effective electrical permittivity of the composite medium is used to study the experimental results related to contact angle changes at different concentrations and diameters of submicron particles in the droplet.

Anomalous contact angle hysteresis of a captive bubble: advancing contact line pinning

Contact angle hysteresis of a sessile drop on a substrate consists of continuous invasion of liquid phase with the advancing angle (θ(a)) and contact line pinning of liquid phase retreat until the receding angle (θ(r)) is reached. Receding pinning is generally attributed to localized defects that are more wettable than the rest of the surface. However, the defect model cannot explain advancing pinning of liquid phase invasion driven by a deflating bubble and continuous retreat of liquid phase driven by the inflating bubble. A simple thermodynamic model based on adhesion hysteresis is proposed to explain anomalous contact angle hysteresis of a captive bubble quantitatively. The adhesion model involves two solid–liquid interfacial tensions (γ(sl) > γ(sl)′). Young’s equation with γ(sl) gives the advancing angle θ(a) while that with γ(sl)′ due to surface rearrangement yields the receding angle θ(r). Our analytical analysis indicates that contact line pinning represents frustration in surface free energy, and the equilibrium shape corresponds to a nondifferential minimum instead of a local minimum. On the basis of our thermodynamic model, Surface Evolver simulations are performed to reproduce both advancing and receding behavior associated with a captive bubble on the acrylic glass.

Fluorescence and morphology modulation in a photochromic diarylethene self-assembly system

Switchable supramolecular self-assemblies on the basis of interaction between melamine group containing photochromic diarylethene unit (DTE) and naphthalimide derivate (1) were designed and fabricated. 1 can gelate several aprotic solvents with different morphologies. The gel turned into partial gel in ethyl acetate with the addition of DTE as a guest molecule. Both the absorption and fluorescence spectra of the assembly can be reversibly switched by alternating UV/visible light irradiation. Meanwhile, the morphology of the coassembly of 1(2)·DTE changed to film from original pieces of gel 1 in ethyl acetate. When 1(2)·DTE was irradiated by UV light, the film morphology was converted into aggregated flakes. Moreover, the surface wettability of the complex can also be switched by light irradiation. The photochromic diarylethene unit is able to modulate the fluorescence and morphology of the assembled system only by virtue of light irradiation. Therefore, these results provide further insights into fluorescence and morphology controlling, especially application in upscale smart responsive materials.

Anodic growth of large-diameter multipodal TiO2 nanotubes

We report on the formation of a new class of nanostructures, namely, multipodal hollow titania nanotubes possessing two or more legs, achieved during the electrochemical anodization of titanium in diethylene glycol (DEG)-based electrolytes. The unique multipodal porous structure is expected to extend and enhance the applications of TiO(2) nanotube arrays. Multipodal nanotubes form by a process we term "nanotube combination", which only occurs in viscous electrolytes at high anodization potentials in the presence of a low concentration of fluoride-bearing species. The mechanism of formation of multipodal nanotubes is considered, and the tube length at which nanotube combination occurs is predicted theoretically using a simplified analytical model. The results suggest that capillary forces strong enough to bend the TiO(2) nanotubes by tens of degrees are generated during the imbibition of electrolyte into and out of the intertubular spaces between adjacent tapered nanotubes.