A key challenge encountered by printed electronics is that the conductivity of sintered metal nanoparticle (NP) traces is always several times smaller than the bulk metal conductivity. Identifying the relative roles of the voids and the residual polymers on NP surfaces in sintered NP traces, in determining such reduced conductivity, is essential. In this paper, we employ a combination of electron microscopy imaging and detailed simulations to quantify the relative roles of such voids and residual polymers in the conductivity of sintered traces of a commercial (Novacentrix) silver nanoparticle-based ink. High resolution transmission electron microscopy imaging revealed details of the morphology of the inks before and after being sintered at 150 °C. Prior to sintering, NPs were randomly close packed into aggregates with nanometer thick polymer layers in the interstices. The 2D porosity in the aggregates prior to sintering was near 20%. After heating at 150 °C, NPs sintered together into dense aggregates (nanoaggregates or NAgs) with sizes ranging from 100 to 500 nm and the 2D porosity decreased to near 10%. Within the NAgs, the NPs were mostly connected via sintered metal bridges, while the outer surfaces of the NAgs were coated with a nanometer thick layer of polymer. Motivated by these experimental results, we developed a computational model for calculating the effective conductivity of the ink deposit represented by a prototypical NAg consisting of NPs connected by metallic bonds and having a polymer layer on its outer surface placed in a surrounding medium. The calculations reveal that a NAg that is 35%-40% covered by a nanometer thick polymeric layer has a similar conductivity compared to prior experimental measurements. The findings also demonstrate that the conductivity is less influenced by the polymer layer thickness or the absolute value of the NAg dimensions. Most importantly, we are able to infer that the reduced value of the conductivity of the sintered traces is less dependent on the void fraction and is primarily attributed to the incomplete removal of the polymeric material even after sintering.

Nanoconfinement is known to drive phase separation (oftendenotedas microphase separation) of two highly miscible liquids by subjectingthe two liquids to disparate influences. Here, we propose a paradigmshift to this problem: we introduce the idea of "repeatability"in nanoconfinement-driven microphase separation. A drop consistingof two highly miscible liquids (A and B) is made to pass through ananochannel grafted with a collapsed layer of polymer that is philicto A but phobic to B. Subsequently, a significant number of moleculesof liquid A get imbibed into the polymer layer and the polymer layerpartially swells, while the molecules of liquid B mostly remain outof the polymeric layer and are carried away, emerging as a drop onthe other side of the polymer bilayer. This passage of drop (of liquidsA and B) is continued, and each time liquids A and B get separatedwith liquid A imbibing into the polymer layer and liquid B being carriedaway with the drop. This scenario, therefore, points to the repeatedoccurrence of the microphase separation of miscible binary liquidmixtures, enabling the processing of a much larger volume of liquid,given the fact that the presence of a grafted polymer layer continuesto provide a dynamically increasing space where liquid A can get localizedafter being separated from liquid B. We quantify such repeated microphaseseparation by noting the extent of separation (of liquid A) and extentof recovery (of liquid B) as functions of nanochannel height and numberof passes. Interestingly, we establish that this process also leadsto a distillation-like behavior (without any heat addition), wherethe concentration of liquid B (equivalent to the "less volatile"liquid in a standard distillation process) progressively increasesinside the drop after its passage through the nanochannel.

We develop a combined numerical (using direct numericalsimulation)and scaling model to study the dynamics of two identical photocurable,shear-thinning polymeric drops undergoing spreading and coalescence(on a substrate) in the presence of in situ curing.We only consider the shear-thinning nature of the drops but do notaccount for any changes in the drop elasticity during the in situ photocuring process. Two separate cases are studied:(1) & tau;(s) MUCH LESS-THAN & tau;(p) (case 1) and(2) & tau;(s) & SIM; & tau;(p) (case 2) (& tau;(s) and & tau;(p) represent the spreading and photocuringtimescales, respectively). Photopolymerization-driven increase inthe drop viscosity significantly delays the spreading and the coalescenceevents for case 2. Furthermore, for case 2, at any given time duringthe coalescence process, both the height and the width of the "bridge"that forms due to coalescence are always smaller than those for case1. Our scaling analysis establishes three distinct regimes characterizingthe bridge growth for both cases 1 and 2: the initial regime (viscousand capillary forces balance each other, and the bridge growth rateis weak), the intermediate regime (inertia and capillary forces balanceeach other, and the bridge growth rate is enhanced), and the lateregime (viscous and capillary forces balance each other, and the bridgegrowth rate is weak). Also, in regimes where viscous forces play arole (initial and late regimes), the bridge growth is weaker for case2, while in the intermediate regime, the rate of bridge growth issimilar between cases 1 and 2. We also provide the temperature variationand the evolution of the curing front inside the coalescing dropsfor cases 1 and 2: the significantly rapid rate of polymerizationfor case 2 manifests in a noticeable temperature drop, fast propagationof the curing front, and the fluid flow (associated with coalescence)affecting the migration of the curing front inside the coalescingdrops. We anticipate that our study will be crucial in designing polymericdroplet-based additive manufacturing systems and understanding thebehavior of polymeric blends and polymeric emulsions.

Interaction between nanoparticles (NPs) and a layer of grafted and solvated polymer molecules has been widely explored for a variety of applications ranging from fabrication of nanocomposites and sensors to developing nanocoating for virus deactivation. In all of these applications, the solvated polymer molecules are necessarily philic to the NPs, and consequently, driven by the favorable NP-polymer interactions, there is the formation of numerous stable direct (i.e., without any intervening solvent molecule) NP-monomer (monomer of the polymer) contact pairs. In this paper, we propose a paradigm shift in this problem: we employ molecular dynamics (MD) simulations and establish that under appropriate conditions, it is possible to develop numerous stable direct contacts between a NP and a solvated polymer layer even when the polymer molecules are extremely phobic to the NP. Here, by "stable" contacts, we refer to the NP-Polymer contacts that remain intact for a finite duration of time; of course, such contacts, after being intact for a finite time duration, might get broken and reformed. In terms of the mechanism of the process, the NP is driven inside a grafted layer of collapsed (in the absence of solvent) and phobic (to the NP) polymer molecules by a liquid drop (polymer is philic to the liquid). Subsequently, the liquid molecules imbibe and diffuse inside the polymer layer, but the NPs, due to the large steric effect imposed by the polymer molecules, remain localized within the polymer layer. This ensures the establishment of several stable direct contacts between the NP and the highly phobic polymer molecules. We quantify these contacts by their numbers, stability, and frequency of occurrences as well as their dependences on the NP-polymer interaction energies and NP sizes. We also quantify the associated NP dynamics inside the polymeric layer. Finally, we argue that our finding will open up avenues for leveraging NP-polymer interactions for a myriad of applications even for cases where the polymer molecules are phobic to the NPs.

In this paper, we develop a theory to establish that the thermo-osmotic (TOS) effects, induced by the application of an axial temperature gradient, lead to a massive enhancement in liquid transport in nanochannels grafted with charged polyelectrolyte (PE) brushes. We quantify the TOS transport by quantifying the induced electric field and the induced TOS flow field. The different components of the electric field, namely the ionic component, the thermal component and the osmotic component, as well as the contributions of different ions to these components, are quantified. Furthermore, we express the TOS velocity as a combination of chemiosmotic (COS), thermal and electro-osmotic (EOS) components. The COS and the thermal components augment each other and the overall strength and direction of the TOS flow are dictated by the direction and the relative strength of the EOS component. Most importantly, we compare the cases of brush-grafted nanochannels with those of the brush-free nanochannels of identical surface charge densities: the TOS transport is massively augmented in the brush-grafted nanochannels attributed to the combination of the localization of the electric double layer (EDL) (and hence any body force that depends on the EDL charge density) away from the nanochannel wall (i.e. the location of the maximum drag force) and the presence of a possible molecular slip (experienced by the liquid) along the brush surface.

We employ molecular dynamics (MD) simulations to study the spreading and imbibition of a liquid drop on a porous, soft, solvophilic, and responsive surface represented by a layer of polymer molecules grafted on a solvophilic solid. These polymer molecules are in a crumpled and collapsed globule-like state before the interaction with the drop but transition to a "brush"-like state as they get wetted by the liquid drop. We hypothesize that for a wide range of densities of polymer grafting (sigma(g)), the drop spreading is dictated by the balance of the driving inertial pressure and balancing viscoelastic dissipation (associated with the spreading of the liquid drop on the polymer layer that undergoes globule-to-brush transition and serves as the viscoelastic solid). Using the well-known idea that the viscoelastic resisting force exerted by the viscoelastic solid on a spreading drop scales as u(n) (where n is the index of the power-law-like rheology of the polymer layer serving as the viscoelastic solid and u is the spreading velocity of the drop on this viscoelastic solid) and considering n = 2/3, we show that the scaling calculation recovers the MD simulation prediction of r similar to t(1/4) and req similar to sigma(-1/3)(g) (where r and r(eq) are the instantaneous and equilibrium spreading radii, respectively). We further describe the wicking behavior of the drop through the polymer layer by appropriately accounting for the manner in which the progressive time-dependent swelling of the grafted polymer molecules provides larger space for the wicking. Third, we quantify, possibly for the first time, the temporal dynamics of the "brush"-forming process (i.e., capture the dynamics of wetting-mediated globule-to-brush transition). We show that the dynamics of the polymer chain swelling depends on sigma(g) and is faster for sparser grafting. Most importantly, we confirm that the height of the relaxed polymer chains approximately scales as sigma(1/3)(g), confirming the attainment of brush-like configuration by the polymer molecules as they are wetted by the liquid drop. Finally, we argue that our simulations raise the possibility of designing soft, "responsive", and widely deployable liquid-infused surfaces where the polymer grafted solid, with the polymer undergoing a globule-to-brush transition, serves as the responsive "surface".

In this paper, we employ the direct numerical simulation (DNS) method for probing three-dimensional, axisymmetric coalescence of microscale, power-law-obeying, and shear-thinning polymeric liquid drops of identical sizes impacting a solid, solvophilic substrate with a finite velocity. Unlike the cases of drop coalescence of Newtonian liquid drops, coalescence of non-Newtonian polymeric drops has received very little attention. Our study bridges this gap by providing (1) the time-dependent, three-dimensional (3D) velocity field and 3D velocity vectors inside two coalescing polymeric drops in the presence of a solid substrate and (2) the effect of the drop impact velocity (on the solid substrate), quantified by the Weber number (We), on the coalescence dynamics. Our simulations reveal that the drop coalescence is qualitatively similar for different We values, although the velocity magnitudes involved, the time required to attain different stages of coalescence, and the time needed to attain equilibrium vary drastically for finitely large We values. Finally, we provide detailed simulation-based, as well as physics-based, scaling laws describing the growth of the height and the width of the bridge (formed due to coalescence) dictating the 3D coalescence event. Our analyses reveal distinct scaling laws for the growth of bridge height and width for early and late stages of coalescence as a function of We. We also provide simulation-based coalescence results for the case of two unequal sized drops impacting on a substrate (nonaxisymmetric coalescence) as well as results for axisymmetric coalescence for drops of different rheology. We anticipate that our findings will be critical in better understanding events such as inkjet or aerosol jet polymer printing, dynamics of polymer blends, and many more.

This paper presents an all-atom molecular dynamics (MD) simulation of highly charged and densely grafted polyelectrolyte (PE) brushes on a flat surface. Simulation findings for the variation of brush height with polymer size and grafting density are explained by the scaling laws for brushes in the non-linear osmotic regime. More importantly, this study establishes the triggering of an ultraconfinement effect by the densely grafted brushes. This effect leads to significant changes in the distribution, structure, and properties as well as a massive mobility reduction of both the counterions and water. Furthermore, the interplay of the ultraconfinement effect and the large counterion-PE electrostatic interactions trigger a “water-in-salt”-like scenario (witnessed in highly concentrated aqueous electrolyte solutions), which is characterized by the counterion-PE-functional group serving as the “salt” with the “salt” overwhelming the water in both mass and volume and affecting the solvation structure of the counterions by replacing the solvation water with the PE-functional group.

In this paper, we develop a theory for describing the thermodynamics, configuration, and electrostatics of strongly-stretched, pH-responsive polyelectrolyte (PE) brushes in the presence of large salt concentrations. The aim of the paper, therefore, is to study the properties of a PE brush in a salt concentration regime (namely, large concentrations of several molars) that has been hitherto unexplored theoretically in the context of PE brushes but can be routinely encountered in molecular scale simulations of the problem. The brushes are modelled using our recently developed augmented Strong Stretching Theory (SST), while the effect of the presence of the large salt concentration is accounted for by including the contributions of three different types of non-Poisson-Boltzmann (non-PB) effects to the free energy description of the PE brush induced electric double layer (EDL). These non-PB effects are ionic non-mean-field ion-ion correlations, solvent polarization, and the finite size effect of the ions and water dipoles. We study the individual influences of these different effects and show that the ion-ion correlations and solvent polarization effect reduce the brush height which consequently enhances the monomer density and leads to an electrostatic potential distribution of the brush induced EDL that has a larger magnitude at near-wall locations and becomes zero at shorter distances from the wall. The finite size effect, on the other hand, increases the brush height and therefore, weakens the monomer density and leads to a smaller near-wall magnitude of the EDL potential that becomes zero at larger distances from the wall. Eventually, we consider the impact of all the three non-PB effects simultaneously and show that the ion-ion correlations and solvent polarization effect dominate the size effects and dictate the overall brush configuration and the EDL electrostatics. We also point out that the influence of all the three non-PB effects becomes the largest for a larger salt concentration and a smaller bulk pH. Finally, we compare our theoretical predictions with those obtained from our recently developed all-atom MD simulation model and obtain an excellent match.

We demonstrate that functionalizing nanoslits with pH-responsive polyelectrolyte brushes can lead to extremely fast electro-osmotic (EOS) water transport, where the maximum centreline velocity and the volume flow rate can be an order of magnitude larger than these quantities in identically charged brush-free nanochannels for a wide range of system parameters. Such an enhancement is most remarkable given that the brushes have been known to retard the transport by imparting additional drag on the fluid flow. We argue that this enhancement stems from the localization of the charge density of the brush-induced electric double layer (and, hence, the EOS body force) away from the nanochannel wall (or the location of the wall-induced drag force). This ensures a much larger impact of the EOS body force triggering such fast water transport. Finally, the calculated flux values for the present brush-grafted nanochannels are found to be significantly larger than those for a wide range of nanofluidic membranes and channels, suggesting that the brush functionalization can be considered as a mechanism for enabling such superfast nanofluidic transport.

Development of anti-biofouling coating has attracted immense attention for reducing the massively detrimental effects of biofouling in systems ranging from ship hulls and surgical instruments to catheters, implants, and stents. In this paper, we propose a model to quantify the role of electrostatic and van der Waals (vdW) forces in dictating the efficacy of dielectric coating for preventing the nonspecific adhesion mediated biofouling in salty systems. The model considers a generic charged lipid-bilayer encapsulated vesicle-like structure representing the bio-organism. Also, we consider the fouling caused by the nonspecific adhesion of the bio-organism on the substrate, without accounting for the explicit structures (e.g., pili, appendages) or conditions (e.g., surface adhesins secreted by the organisms) involved in the adhesion of specific microorganism. The model is tested by considering the properties of actual coating materials and biofouling causing microorganisms (bacteria, fungi, algae). Results show that while the electrostatic-vdW effect can be significant in anti-biofouling action for cases where the salt concentration is relatively low (e.g., saline solution for surgical instruments), it might not be effective for marine environment where the salt concentration is much higher. The findings, therefore, point to a hitherto unexplored driving mechanism of anti-biofouling action of the coating. Such an identification will also enable the appropriate choices of the coating materials (e.g., possible dielectric material with volume charge) and other system parameters (e.g., salinity of the solution for storing the surgical instruments) that will significantly improve the efficiency of the coatings in preventing the nonspecific adhesion mediated biofouling.

In this paper, we study the diffusioosmotic (DOS) transport in a nanochannel grafted with pH-responsive polyelectrolyte (PE) brushes and establish brush-functionalization-driven enhancement in induced nanofluidic electric field and electrokinetic transport. The PE brushes are modeled using our recently developed augmented strong stretching theory. We consider the generation of the DOS transport due to the imposition of a salt concentration gradient along the length of the nanochannel. The presence of the salt concentration gradient induces an electric field that has an osmotic (associated with the flow-driven migration of the ions in the induced electric double layer) and an ionic (associated with the conduction current) component. These two components evolve in a manner such that the electric field in the brush-grafted nanochannel is larger (smaller) in magnitude than that in the brush-less nanochannels for the case where the electric field is positive (negative). Furthermore, we quantify the DOS flow velocity and establish that for most of the parameter choices, the DOS velocity, which is a combination of the induced pressure-gradient-driven chemiosmotic component and the induced electric field driven electroosmotic transport, is significantly larger for the nanochannels grafted with backbone-charged PE brushes (i.e., brushes where the charge is distributed along the entire length of the brushes) as compared to brush-free nanochannels or nanochannels grafted with PE brushes containing charges on their non-grafted ends.

Continuous release of gas bubbles in large numbers from a localized source in a liquid column, popularly known as "bubble plumes", is very relevant in nature and industries. The bubble plumes morphologically consist of a long continuous stem supporting a dispersed head. Through our direct numerical simulations using two-way coupled Euler-Lagrangian framework, we show that a bubble plume rising in a quiescent liquid column develops clusterlike instabilities for the Grashof numbers, Gr>145. For levels Gr<100, the stem is continuous with a small plume head, whereas at high buoyancy (Gr>350), the plume stem shows intermittently passing puffing instabilities in the form of bubble clusters. The clusters are a group of bubbles localized in space with high concentration that travel upward with speed C_{ph}=0.45U_{C} and are separated by a distance of at least 5L_{0}, where U_{C} is the characteristic velocity and L_{0} is the characteristic length based on the injection conditions. The bubble rise Reynolds numbers in the steady state for both the plume head and the stem shows Re∝Gr^{0.45±0.03}, and the proportionality constant is ten times higher in the plume stem than in the plume head. In the plume core, the spatial acceleration due to the bubble motion generates the turbulent production, whereas, at the plume edge, the small-scale fluctuations generate the mean vorticity. At high Gr, the clusters evolve due to the lift forces acting on the bubbles as a result of increase in the mean vorticity. While rising, bubbles entrain the liquid from the surroundings, and we found that the entrainment rate is not as strong as compared to the classical thermal plumes.

In this paper, we provide a theory to quantify the ionic current ( iion ) in nanochannels grafted with pH-responsive polyelectrolyte (PE) brushes. We consider the PE brushes to be modeled by our recently proposed augmented strong stretching theory (SST) model that improves the existing SST models by incorporating the effects of excluded volume interactions and an extended mass action law. Use of such augmented SST for this problem implies that this is the first study on computing iion in PE brush-grafted nanochannels accounting for the appropriate coupled configuration-electrostatic description of the PE brushes. iion is obtained as functions of PE brush grafting density, medium pH and salt concentration ( c∞ ), and the density of polyelectrolyte chargeable sites (PECS). For large c∞ , iion increases linearly with c∞ (as for such c∞ , iion becomes independent of the PE charge and is dominated by the bulk mobility and number density of the electrolyte ions), whereas iion is independent of c∞ at small c∞ (where the electric double layer electrostatics and the total number of ions in the system is dominated by the hydrogen ions). We further witness an enhancement of iion for smaller pH and larger grafting density at low and moderate c∞ , while there is little to no effect of the PECS density on the ionic current except for weakly grafted brushes at low c∞ . We anticipate that this study will serve as a theoretical foundation for a large number of applications that are based on the brush-induced modification of the ionic current in a nanochannel.