Molecular Insights into the Effect of Crystal Planes on Droplet Wetting.

Dynamic wetting by liquids on solid surfaces depends on several aspects such as surface energy, roughness, and interfacial tension, among many others. Copper (Cu), gold (Au), aluminum (Al), and silicon (Si) are a few of the most important metals that are used extensively as substrates in various industrial and biomedical applications. Metals are etched frequently on different crystal planes for fabrication purposes. Etching exposes distinct crystal planes that may come in contact with the liquids when used for different applications. The interaction of the crystal planes with the liquid that comes in contact with the solid dictates the wetting behavior of the surface. This necessitates the importance of understanding how various crystal planes of the same metals behave under similar conditions. Herein, three specific crystal planes, namely, (1 0 0), (1 1 0), and (1 1 1), are investigated at a molecular scale for the above-mentioned metals. The dynamic contact angle and contact diameter trends revealed that the relatively hydrophobic surfaces (Cu, Si) tend to reach their equilibrium contact angle faster compared to the hydrophilic substrates (Al, Au). Molecular kinetic theory is used to estimate the three-phase contact line friction which is found to be higher for (1 1 1) planes. Further, a consistent potential energy distribution variation is observed for the crystal lattice of (1 0 0), (1 1 0), and (1 1 1). These findings can be used as a guideline to determine the factors needed to completely describe a dynamic wetting phenomenon of the droplet over the different crystal planes. The understanding will be of great use in deciding experimental strategies where fabricated different crystal planes would be required to have a liquid contact.

Early-Stage Liquid Infiltration in Nanoconfinements.

Liquid infiltration is one of the commonly adapted flow mechanisms in microscale/nanoscale heat-transfer applications. The theoretical modeling of dynamic infiltration profile in the microscale/nanoscale requires a deep study, because the acting forces are entirely different from those of a large-scale system. Herein, a model equation is developed from the fundamental force balance at the microscale/nanoscale level, to capture the dynamic infiltration flow profile. Molecular kinetic theory (MKT) is used to predict the dynamic contact angle. Molecular dynamics (MD) simulations are performed to study the capillary infiltration in two different geometries. The infiltration length is computed from the simulation results. The model is also evaluated over surfaces having different surface wettability. The generated model provides a better estimation of the infiltration length, compared to the well-established models. The developed model is expected to aid in the designing of microscale/nanoscale devices where liquid infiltration plays a key role.

Molecular Investigation of the Actuation of Electrowetted Nanodroplets.

It is well known that the wettability of a droplet on a solid substrate can be modified by the application of an electric field. The phenomenon of electrowetting along with the associated physics of droplet shape change and dynamics has traditionally been studied at the micro-scale leading to exciting applications. The present work is undertaken to explore the physics of electrowetting actuation of droplet movement at the molecular level. Molecular simulations are performed to obtain the dynamic spreading of the droplet under the action of a radially symmetric electric field on a silica substrate. The dynamic behavior of the contact diameter is found to be qualitatively similar to that observed at the laboratory scale. Further simulations of droplet actuation across an array of electrodes illustrated the dynamics of the center of mass, which is then used to estimate the contact line friction and compared with the predictions from a reduced-order model. A scaling analysis is used to probe the physics of the problem correlating the contact line friction coefficient and the droplet velocity after actuation. The results and understanding elicited from the fundamental approach have the potential to guide the development of quick and precise control of nano-sized droplets and may prove to be pivotal in the development of future nanofluidic systems, nanomanufacturing methodologies, and high-resolution optoelectronic devices.

A Review on Low-Dimensional Nanomaterials: Nanofabrication, Characterization and Applications.

The development of modern cutting-edge technology relies heavily on the huge success and advancement of nanotechnology, in which nanomaterials and nanostructures provide the indispensable material cornerstone. Owing to their nanoscale dimensions with possible quantum limit, nanomaterials and nanostructures possess a high surface-to-volume ratio, rich surface/interface effects, and distinct physical and chemical properties compared with their bulk counterparts, leading to the remarkably expanded horizons of their applications. Depending on their degree of spatial quantization, low-dimensional nanomaterials are generally categorized into nanoparticles (0D); nanorods, nanowires, and nanobelts (1D); and atomically thin layered materials (2D). This review article provides a comprehensive guide to low-dimensional nanomaterials and nanostructures. It begins with the classification of nanomaterials, followed by an inclusive account of nanofabrication and characterization. Both top-down and bottom-up fabrication approaches are discussed in detail. Next, various significant applications of low-dimensional nanomaterials are discussed, such as photonics, sensors, catalysis, energy storage, diverse coatings, and various bioapplications. This article would serve as a quick and facile guide for scientists and engineers working in the field of nanotechnology and nanomaterials.

Molecular Investigation of Contact Line Movement in Electrowetted Nanodroplets.

Molecular dynamics (MD) simulation of an electrowetted nanodroplet is performed to understand the fundamental origin of the involved parameters resulted from the molecular movement in the vicinity of the three-phase contact line (TPCL). During the spreading of the droplet, contact line friction (CLF) force is found to be the controlling one among all other resistive forces. Being molecular in nature, MD study is required to unveil the CLF, which is manifested by the TPCL friction coefficient ζ. The combined effect of temperature, electric field, and surface wettability, manifested by the solid-liquid Lennard-Jones interaction parameter, is studied to explore the droplet spreading. The entire droplet wetting dynamics is divided into two different regimes, namely, spreading regime and equilibrium regime. The molecular frequency during the TPCL movement in the equilibrium regime is affected by the presence of any external perturbation and results in an alteration of ζ. The predetermined knowledge of the alteration of CLF due to the coupling effect of electric field and temperature will have a potential application towards designing electric field-inspired droplet movement devices.

Evaporation-Driven Micromixing in Sessile Droplets for Miniaturized Absorbance-Based Colorimetry.

We demonstrate the use of an evaporating, sessile droplet on a nonwetting substrate as a miniature micromixing device to conduct sample-dye reactions for absorbance-based colorimetry. The nonwetting substrate supports buoyancy-induced mixing inside the droplet for rapid completion of the measurement. The Bradford assay is used as a proof of concept, where a protein-containing sample is reacted with a reagent dye to measure the protein concentration. Viability of absorbance measurement through the droplet is first established using droplets in which the reactants are mixed prior to their deposition onto the substrate. In a second set of experiments involving in situ mixing, the reagent is directly added to a sessile droplet of the protein-containing sample, allowing the reactants to mix while the absorbance is being measured. Interplay between buoyancy-induced mixing, protein-reagent reaction, and protein adsorption onto the substrate leads to a complex temporal absorbance measurement signal. Videos corresponding to the signal data show that each of these mechanisms dominates during different phases of droplet evolution, causing a signal pattern containing peaks and valleys having a strong monotonic trend with the protein concentration. Overall, the second absorbance peak at which the reaction nears completion is the most sensitive to sample concentration. Heating of the substrate is demonstrated to dramatically speed up the mixing process. These protein concentration measurements, obtained with a simpler system and low reactant volumes, demonstrate that this droplet micromixing concept is a viable alternative to microtiter plates for colorimetric applications.

Electrodewetting and Wetting of an Extended Meniscus.

Here, we report the intriguing movements of an extended liquid meniscus on a silicon substrate under the influence of sinusoidal alternating current (AC) voltages at different operating frequencies. As opposed to droplet electrowetting, wherein the droplet spreads and experiences oscillations at the free surface, the application of AC voltage to a thin liquid film results in distinct and uniform dewetting, in conjunction with augmented wetting. Image analyzing interferometry is used for the precise measurement of the film thickness profile and other associated parameters. We postulate that the classic Young-Lippmann equation fails to explain the dynamics of an extended meniscus and evince that the dynamics of film displacement could be successfully explained by considering the product of the applied electric field and its gradient, as opposed to the existing consideration of a square dependence on the applied voltage. The physics of the hitherto unreported phenomena is elucidated by developing a mathematical model, taking into consideration all of the germane forces governing the dynamics of the thin liquid film. We affirm that the present study would serve as a fundamental background for a fascinating mode of liquid actuation, with inherent application potential in several existing and novel microfluidic systems.

Multiscale Modeling of the Three-Dimensional Meniscus Shape of a Wetting Liquid Film on Micro-/Nanostructured Surfaces.

The design of structured surfaces for increasing the heat flux dissipated during boiling and evaporation processes via enhanced liquid rewetting requires prediction of the liquid meniscus shape on these surfaces. In this study, a general continuum model is developed to predict the three-dimensional meniscus shape of liquid films on micro/nanostructured surfaces based on a minimization of the system free energy that includes solid-liquid van der Waals interaction energy, surface energy, and gravitational potential. The continuum model is validated at the nanoscale against molecular dynamics simulations of water films on gold surfaces with pyramidal indentations, and against experimental measurements of water films on silicon V-groove channels at the microscale. The validated model is used to investigate the effect of film thickness and surface structure depth on the meniscus shape. The meniscus is shown to become more conformal with the surface structure as the film thickness decreases and the structure depth increases. Assuming small interface slope and small variation in film thickness, the continuum model can be linearized to obtain an explicit expression for the meniscus shape. The error of this linearized model is quantitatively assessed and shown to increase with increasing structure depth and decreasing structure pitch. The model developed can be used for accurate prediction of three-dimensional meniscus shape on structured surfaces with micro/nano-scale features, which is necessary for determining the liquid delivery rate and heat flux dissipated during thin-film evaporation. The linearized model is useful for rapid prediction of meniscus shape when the structure depth is smaller than or comparable to the liquid film thickness.

Fibrillar disruption by AC electric field induced oscillation: A case study with human serum albumin.

The effect of oscillation induced by a frequency-dependent alternating current (AC) electric field to dissociate preformed amyloid fibrils has been investigated. An electrowetting-on-dielectric type setup has been used to apply the AC field of varying frequencies on preformed fibrils of human serum albumin (HSA). The disintegration potency has been monitored by a combination of spectroscopic and microscopic techniques. The experimental results suggest that the frequency of the applied AC field plays a crucial role in the disruption of preformed HSA fibrils. The extent of stress generated inside the droplet due to the application of the AC field at different frequencies has been monitored as a function of the input frequency of the applied AC voltage. This has been accomplished by assessing the morphology deformation of the oscillating HSA fibril droplets. The shape deformation of the oscillating droplets is characterized using image analysis by measuring the dynamic changes in the shape dependent parameters such as contact angle and droplet footprint radius and the amplitude. It is suggested that the cumulative effects of the stress generated inside the HSA fibril droplets due to the shape deformation induced hydrodynamic flows and the torque induced by the intrinsic electric dipoles of protein due to their continuous periodic realignment in presence of the AC electric field results in the destruction of the fibrillar species.

Magnetowetting of Ferrofluidic Thin Liquid Films.

An extended meniscus of a ferrofluid solution on a silicon surface is subjected to axisymmetric, non-uniform magnetic field resulting in significant forward movement of the thin liquid film. Image analyzing interferometry is used for accurate measurement of the film thickness profile, which in turn, is used to determine the instantaneous slope and the curvature of the moving film. The recorded video, depicting the motion of the film in the Lagrangian frame of reference, is analyzed frame by frame, eliciting accurate information about the velocity and acceleration of the film at any instant of time. The application of the magnetic field has resulted in unique changes of the film profile in terms of significant non-uniform increase in the local film curvature. This was further analyzed by developing a model, taking into account the effect of changes in the magnetic and shape-dependent interfacial force fields.

Contact Line Dynamics during the Evaporation of Extended Colloidal Thin Films: Influence of Liquid Polarity and Particle Size.

Exercising control over the evaporation of colloidal suspensions is pivotal to modulate the coating characteristics for specific uses, wherein the interactions among the liquid, the particles, and the substrate control the process. In the present study, the contact line dynamics of a receding colloidal liquid film consisting of particles of distinctly different sizes (nominal diameters 0.055 and 1 µm and surface unmodified) during evaporation is analyzed. The role of the liquid polarity is also investigated by replacing the polar liquid (water) with a relatively nonpolar liquid (isopropyl alcohol) in the colloidal suspension. The characteristics of the evaporating receding meniscus, namely, the film thickness and the curvature are experimentally evaluated using an image-analyzing interferometry technique. The experimental results are assessed in conjunction with the augmented Young-Laplace equation, highlighting the roles of the relevant components of the disjoining pressure and the polarity of the liquid involved in the colloidal suspension.

Evaluating the morphology of erythrocyte population: An approach based on atomic force microscopy and flow cytometry.

Erythrocyte morphology is gaining importance as a powerful pathological index in identifying the severity of any blood related disease. However, the existing technique of quantitative microscopy is highly time consuming and prone to personalized bias. On the other hand, relatively unexplored, complementary technique based on flow cytometry has not been standardized till date, particularly due to the lack of a proper morphological scoring scale. In this article, we have presented a new approach to formulate a non-empirical scoring scale based on membrane roughness (R(rms)) data obtained from atomic force microscopy. Subsequently, the respective morphological quantifier of the whole erythrocyte population, commonly known as morphological index, was expressed as a function of highest correlated statistical parameters of scattered signal profiles generated by flow cytometry. Feed forward artificial neural network model with multilayer perceptron architecture was used to develop the intended functional form. High correlation coefficient (R(2) = 0.95), even for model-formulation exclusive samples, clearly indicates the universal validity of the proposed model. Moreover, a direct pathological application of the proposed model has been illustrated in relation to patients, diagnosed to be suffering from a wide variety of cancer.

Molecular Dynamics Study of Thermally Augmented Nanodroplet Motion on Chemical Energy Induced Wettability Gradient Surfaces.

Droplet motion on a surface with chemical energy induced wettability gradient has been simulated using molecular dynamics (MD) simulation to highlight the underlying physics of molecular movement near the solid-liquid interface including the contact line friction. The simulations mimic experiments in a comprehensive manner wherein microsized droplets are propelled by the surface wettability gradient against forces opposed to motion. The liquid-wall Lennard-Jones interaction parameter and the substrate temperature are varied to explore their effects on the three-phase contact line friction coefficient. The contact line friction is observed to be a strong function of temperature at atomistic scales, confirming their experimentally observed inverse functionality. Additionally, the MD simulation results are successfully compared with those from an analytical model for self-propelled droplet motion on gradient surfaces.

Effect of Surface Wettability on Crack Dynamics and Morphology of Colloidal Films.

The effect of surface wettability on the dynamics of crack formation and their characteristics are examined during the drying of aqueous colloidal droplets (1 µL volume) containing nanoparticles (53 nm mean particle diameter, 1 w/w %). Thin colloidal films, formed during drying, rupture as a result of the evaporation-induced capillary pressure and exhibit microscopic cracks. The crack initiation and propagation velocity as well as the number of cracks are experimentally evaluated for substrates of varying wettability and correlated to their wetting nature. Atomic force and scanning electron microscopy are used to examine the region in the proximity of the crack including the particle arrangements near the fracture zone. The altered substrate-particle Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions, as a consequence of the changed wettability, are theoretically evaluated and found to be consistent with the experimental observations. The resistance of the film to cracking is found to depend significantly on the substrate surface energy and quantified by the critical stress intensity factor, evaluated by analyzing images obtained from confocal microscopy. The results indicate the possibility of controlling crack dynamics and morphology by tuning the substrate wettability.

Electrowetting of partially wetting thin nanofluid films.

It is observed that the presence of negatively charged, suspended nanoparticles significantly changes the electric-field-induced spreading and contact line dynamics of partially wetting liquid films. Image-analyzing interferometry is used to accurately measure the meniscus profile, including the spatial change in the meniscus curvature. The nanoparticle-containing meniscus exhibits enhanced spreading with an increase in the particle size and weight fraction. The instantaneous contact line velocities are measured using video microscopy and a frame-by-frame analysis of the extracted images. The effects of electric field polarity reversal on the flow toward the contact line are explored as well. The movement of the meniscus is analyzed taking into account the capillary forces and Maxwell-stress-induced flows. An analytical model based on the Young-Laplace equation is used to analyze the electric-field-induced contact line motion, and the model-predicted velocities are compared to the experiments.

Electric field enhanced spreading of partially wetting thin liquid films.

Equilibrium and dynamic electrowetting behavior of ultrathin liquid films of surfactant (SDS) laden water over silicon substrate (with native oxide) is investigated. A nonobtrusive optical method, namely, image analyzing interferometry, is used to measure the meniscus profile, adsorbed film thickness, and the curvature of the capillary meniscus. Significant advancement of the contact line of the liquid meniscus, as a result of the application of electric field, is observed even at relatively lower values of applied voltages. The results clearly demonstrate the balance of intermolecular and surface forces with additional contribution from Maxwell stress at the interline. The singular nature of Maxwell stress is exploited in this analysis to model the equilibrium meniscus profile using the augmented Young-Laplace equation, leading to the in situ evaluation of the dispersion constant. The electrowetting dynamics has been explored by measuring the velocity of the advancing interline. The interplay of different forces at the interface is modeled using a control volume approach, leading to an expression for the interline velocity. The model-predicted interline velocities are successfully compared with the experimentally measured velocities. Beyond a critical voltage, contact line instability resulting in emission of droplets from the curved meniscus has been observed.