Three-Dimensional Deposits from Drying Particle-Laden Drops.

Formation of inhomogeneous (in the form of a "coffee ring") or homogeneous deposits accompanies the drying of a particle-laden drop. Invariably, this deposition occurs in a two-dimensional (2D) space (x, y plane) (and might have a finite thickness in z), where the evaporating drop is positioned. Here, we show an interesting extension of this problem: we demonstrate the occurrence of evaporation-mediated particle deposits that span three dimensions (x, y, and z). The extent of the span in this 3rd dimension (z) is comparable to the span in x and y and hence is much larger than the finite thickness (in z) of the 2D deposits. Particle-laden drops are introduced in an uncured and heavier (than the drop) polydimethysiloxane (PDMS) film, enabling the drop to come to the uncured PDMS surface and breach it and get partly exposed to the surrounding air enforcing the onset of evaporation. The subsequent curing of the drop-laden PDMS film ensures that the drop is occupying a three-dimensional (3D) cavity; as a consequence, the evaporation-driven flow field, depending on the particle sizes, leads to a deposition pattern that spans three dimensions. We consider particles of three different sizes: coffee particles (20-50 µm), silver nanoparticles (∼20 nm), and carbon nanotubes (CNTs) (1-2 µm). The coffee particles form a ring-like deposit in the x, y plane, while the much smaller silver nanoparticles (NPs) and CNTs form a 3D deposit that spans in x, y, and z directions. We anticipate that the present finding of the evaporation-triggered three-dimensional (3D) particle deposits will enable unprecedented self-assembly-driven fabrication of various materials, structures, and functional devices as well as patterning and coating in 3D spaces.

Applications, fluid mechanics, and colloidal science of carbon-nanotube-based 3D printable inks.

Additive manufacturing, also known as 3D printing (3DP), is a novel and developing technology, which has a wide range of industrial and scientific applications. This technology has continuously progressed over the past several decades, with improvement in productivity, resolution of the printed features, achievement of more and more complex shapes and topographies, scalability of the printed components and devices, and discovery of new printing materials with multi-functional capabilities. Among these newly developed printing materials, carbon-nanotubes (CNT) based inks, with their remarkable mechanical, electrical, and thermal properties, have emerged as an extremely attractive option. Various formulae of CNT-based ink have been developed, including CNT-nano-particle inks, CNT-polymer inks, and CNT-based non-nanocomposite inks (i.e., CNT ink that is not in a form where CNT particles are suspended in a polymer matrix). Various types of sensors as well as soft and smart electronic devices with a multitude of applications have been fabricated with CNT-based inks by employing different 3DP methods including syringe printing (SP), aerosol-jet printing (AJP), fused deposition modeling (FDM), and stereolithography (SLA). Despite such progress, there is inadequate literature on the various fluid mechanics and colloidal science aspects associated with the printability and property-tunability of nanoparticulate inks, specifically CNT-based inks. This review article, therefore, will focus on the formulation, dispersion, and the associated fluid mechanics and the colloidal science of 3D printable CNT-based inks. This article will first focus on the different examples where 3DP has been employed for printing CNT-based inks for a multitude of applications. Following that, we shall highlight the various key fluid mechanics and colloidal science issues that are central and vital to printing with such inks. Finally, the article will point out the open existing challenges and scope of future work on this topic.

Marangoni spreading and contracting three-component droplets on completely wetting surfaces.

SignificanceThe shape and dynamics of small sessile droplets are dictated by capillary forces. For liquid mixtures, evaporation adds spatio-temporal modulation to these forces that can either enhance or inhibit droplet spreading, depending on the direction of the resulting Marangoni flow. This work experimentally and numerically demonstrates the coexistence of two antagonistic Marangoni flows in a ternary mixture. Played against each other, they can choreograph a boomerang-like wetting motion: Droplets initially rapidly spread, then contract into a compact cap shape. While such a behavior has been impossible in wetting scenarios of simple liquids, it enables spread-retract-remove surface processing with the addition of a single small liquid volume, demonstrated here in a surface-cleaning experiment.

Thermal Marangoni Flow Impacts the Shape of Single Component Volatile Droplets on Thin, Completely Wetting Substrates.

Despite surface energies dictating complete wetting, it has been classically observed that volatile alkanes do not spread completely on glass substrates, and faster evaporation rates lead to higher contact angles. Here we investigate how substrate thickness influences this behavior. For sufficiently thin substrates, we find alkanes evaporate slower and display higher apparent contact angles, at odds with the typical explanations involving just evaporation, capillarity, and viscous dissipation. We derive the droplet temperature distribution and use it as part of a criteria to show that thermal Marangoni contraction plays a significant role in establishing droplet shape on thin substrates.

Shape-driven arrest of coffee stain effect drives the fabrication of carbon-nanotube-graphene-oxide inks for printing embedded structures and temperature sensors.

Carbon nanotube (CNT) based binder-free, syringe-printable inks, with graphene oxide (GO) being used as the dispersant, have been designed and developed. We discovered that the printability of the ink is directly attributed to the uniform deposition of the GO-CNT agglomerates, as opposed to the 'coffee-staining' despite these aggregates being micron-sized. The ellipsoidal nature of the micron-scale GO-CNT agglomerates/particles enables these particles to severely perturb the air-water interface, triggering a large long-range capillary interaction that causes the uniform deposition by overcoming the "coffee-stain"-forming forces from the evaporation-mediated flows. We evaluated the properties of this ink and identified a temperature-dependent resistance with a negative temperature coefficient of resistance (TCR) α ranging from ∼-10-3 to -10-2/°C depending on ink compositions. Finally, the printing is conducted on flat and curved surfaces, for developing polymer-ink embedded structures that might serve as precursors to syringe-printable CNT-based nanocomposites, and for fabricating sensor-like patterns that for certain ink compositions demonstrate α∼-10-3/°C with a large averaged resistance drop (per unit temperature) of -3.5 Ω°C-1.

Interactions of gold and silica nanoparticles with plasma membranes get distinguished by the van der Waals forces: Implications for drug delivery, imaging, and theranostics.

Making a nanoparticle (NP) approach and interact with a plasma membrane (PM) through the receptor-ligand interaction is key for applications like targeted drug delivery, cellular imaging, and theranostics. In this paper, we show that the van der Waals (vdW) interactions dominate the electrostatics ensuring that a gold NP approached the PM more spontaneously as compared to a silica NP. The negative σ (charge density) of a PM induces a negative electrostatic potential at the surface of the approaching gold NP and the silica NP; however, there is very little difference between these induced values due to a small electric double layer at the physiological salt concentration (c∞). Hence there is very little difference in the electrostatic repulsion between the two cases, while the PM-NP vdW attraction is much more for the gold NP as a result of a larger Hamaker constant. Therefore, in comparison to the gold NP, the silica NP would (a) undergo a promotion of the specific adhesion and a prevention of the non-specific adhesion simultaneously for a larger σ - c∞ phase space including the physiological conditions, (b) necessitate a larger length of the ligands to trigger spontaneous receptor-ligand interactions, and (c) require a larger driving force for force-driven receptor-ligand interactions.

Ion at Air-Water Interface Enhances Capillary Wave Fluctuations: Energetics of Ion Adsorption.

Recent simulations provide the energetics of ion adsorption at the air-water (a/w) interface: The presence of the ion at the interface suppresses the fluctuations of the capillary waves (CWs) reducing the entropy and displaces the weakly interacting water molecules to the bulk causing a reduction in the enthalpy. Here, we provide atomistic simulation-based evidence that the inferences of the existing studies stem from considering a small simulation volume that pins the CWs. For an appropriate size of the simulation system, an ion at the a/w interface enhances the CW fluctuations. Furthermore, we discover that the characteristics of the waves governing these enhanced CW fluctuations ensure a significant decrease in the pressure-volume work causing the enthalpy decrease, while the same wave characteristics of the CWs become responsible for an entropy decrease. Overall, the paper revisits the free energy picture of this fundamental problem of ion adsorption at the a/w interface.

Polyelectrolyte brush bilayers in weak interpenetration regime: Scaling theory and molecular dynamics simulations.

We employ molecular dynamics (MD) simulations and develop scaling theories to quantify the equilibrium behavior of polyelectrolyte (PE) brush bilayers (BBLs) in the weakly interpenetrated regime, which is characterized by d_{0}

Effect of Plasma Membrane Semipermeability in Making the Membrane Electric Double Layer Capacitances Significant.

Electric double layers (or EDLs) formed at the membrane-electrolyte interface (MEI) and membrane-cytosol interface (MCI) of a charged lipid bilayer plasma membrane develop finitely large capacitances. However, these EDL capacitances are often much larger than the intrinsic capacitance of the membrane, and all of these capacitances are in series. Consequently, the effect of these EDL capacitances in dictating the overall membrane-EDL effective capacitance Ceff becomes negligible. In this paper, we challenge this conventional notion pertaining to the membrane-EDL capacitances. We demonstrate that, on the basis of the system parameters, the EDL capacitance for both the permeable and semipermeable membranes can be small enough to influence Ceff. For the semipermeable membranes, however, this lowering of the EDL capacitance can be much larger, ensuring a reduction of Ceff by more than 20-25%. Furthermore, for the semipermeable membranes, the reduction in Ceff is witnessed over a much larger range of system parameters. We attribute such an occurrence to the highly nonintuitive electrostatic potential distribution associated with the recently discovered phenomena of charge-inversion-like electrostatics and the attainment of a positive zeta potential at the MCI for charged semipermeable membranes. We anticipate that our findings will impact the quantification and the identification of a large number of biophysical phenomena that are probed by measuring the plasma membrane capacitance.

Interaction between a water drop and holey graphene: retarded imbibition and generation of novel water-graphene wetting states.

Transport of water through a holey or nanoporous graphene structure in presence of an external force (e.g., a pressure gradient) has paved the way for a myriad of applications ranging from water desalination to water-alcohol separation. In this study, on the other hand, we employ molecular dynamics (MD) simulations to probe no-external-force imbibition/permeation of a water nanodrop through a multilayer holey graphene structure. We carry out MD simulations in a two-dimensional set up; consequently, the holes in the graphene sheets appear as lateral separations between finite-length graphene layers, while the inter-layer distances appear as vertical separations. Interplay of lateral and vertical separations triggers combined spreading-imbibition dynamics, enforcing highly non-trivial water-graphene wetting states such as wetting-induced encapsulation of a complete graphene stack by the water drop, simultaneous wetting of multiple surfaces of multiple graphene stacks, and wetting with pinned and unpinned contact lines on vertically separated graphene layers. Finally, under certain combinations of lateral and vertical ISSs, there can be wetting of a much larger surface area of graphene, which is useful in applications (e.g., enhanced energy storage or heat transfer) favored by enhanced graphene-water contact area. Therefore, the present study has a twofold objective: first, it unravels, for the first time, water-holey graphene imbibition dynamics, and second, it identifies imbibition-triggered novel water-graphene wetting states for improving realistic applications involving graphene and water.

Compression of polymer brushes in the weak interpenetration regime: scaling theory and molecular dynamics simulations.

We employ scaling theory and Molecular Dynamics (MD) simulations to probe the compression of the semi-dilute polymer brush bilayers (BBLs) in the weak interpenetration (IP) regime. Such a regime is characterized by two layers of interacting polymer brushes grafted on opposing planar surfaces having a separation dg, such that d0 < dg < 2d0, with d0 being the unperturbed brush height. Currently, scaling theories are known for polymer BBLs with a much larger degree of IP (i.e., dg < d0) - in such regimes, the brush height can be quantified by the corresponding IP length δ. On the other hand, we show that in the weak IP regime, the brush height is not solely dictated by δ. We develop new scaling theories to show that δ in this weak IP regime is different from that in the strong IP regime. Secondly, we establish that the compressed brush height in this weakly IP regime can be described as d ∼ Nχ with χ < 1 and varying monotonically with dg/d0. MD simulations are carried out to quantify δ and χ and the results match excellently with our new scaling theory predictions. Finally, we establish that our scaling theory can reasonably predict the experimentally witnessed variation of the interaction energy dictating the compressive force between the interpenetrating brushes in this weakly IP regime.

Elasto-electro-capillarity: drop equilibrium on a charged, elastic solid.

We present a study here on elasto-electro-capillarity - for the first time, the matter of drop equilibrium on a soft (elastic and incompressible) and charged solid has been studied. Charges on the elastic solid induce an electric double layer or EDL at the solid-drop interface. Our analysis accounts for the electrostatic wetting contribution of the EDL in the overall energy balance. Our results reveal that (a) with an increase in "softness", the equilibrium solid-liquid contact angles show transition from the EDL-modified Young's law (rigid limit) to the EDL-modified Neumann's law (soft limit); (b) the EDL effects invariably enhance solid deformation and lower the apparent contact angle made by the drop with the undeformed solid; (c) the solid contact angles increase and the cusp made by the deformed solid undergoes enhanced rotation due to the EDL effects; and (d) the EDL effects are more prominent for the case where the solid-vapor surface energy exceeds the solid-liquid surface energy. The fact that the EDL effects invariably decrease the overall wetting energy of the system, thereby supporting a larger increase in the elastic energy associated with a larger solid deformation, explains all these findings and establishes that surface charges enhance the "softness" of a soft surface in the context of elastocapillarity.

Wetting dynamics of a water nanodrop on graphene.

Water-graphene wetting interactions are central to several applications such as desalination, water filtration, electricity generation, biochemical sensing, fabrication of fuel cells, and many more. While substantial attention has been devoted to probe the wetting statics of a water drop on graphene, unraveling the possible wetting translucency nature of graphene, very little research has been done on the dynamics of wetting of water drops on graphene-coated solids or free-standing graphene layers. In this paper, we employ molecular dynamics (MD) simulations to study the contact and the spreading of a water nanodrop, quantifying its wetting dynamics, on supported and free-standing graphene. We demonstrate that nanoscale water drops establish contact with graphene by forming patches on graphene, and this patch formation is hastened for graphene layer(s) supported on hydrophilic solids. More importantly, our results demonstrate that the nanodrop spreading dynamics, regardless of the number of graphene layers or the nature of the underlying solid, obey the half-power law, i.e., r∼t(1/2) (where r is the wetting contact radius and t is the spreading time) for the entire timespan of spreading except towards the very end of the spreading lifetime when the spreading stops. Such a spreading behavior is exactly analogous to the spreading dynamics of nanodroplets for standard solids - this is in sharp contrast to the wetting statics of graphene where the wetting translucency effect makes graphene different from other standard solids.

Polyelectrolyte brushes: theory, modelling, synthesis and applications.

Polyelectrolyte (PE) brushes are a special class of polymer brushes (PBs) containing charges. Polymer chains attain "brush"-like configuration when they are grafted or get localized at an interface (solid-fluid or liquid-fluid) with sufficiently close proximity between two-adjacent grafted polymer chains - such a proximity triggers a particular nature of interaction between the adjacent polymer molecules forcing them to stretch orthogonally to the grafting interface, instead of random-coil arrangement. In this review, we discuss the theory, synthesis, and applications of PE brushes. The theoretical discussion starts with the standard scaling concepts for polymer and PE brushes; following that, we shed light on the state of the art in continuum modelling approaches for polymer and PE brushes directed towards analysis beyond the scaling calculations. A special emphasis is laid in pinpointing the cases for which the PE electrostatic effects can be de-coupled from the PE entropic and excluded volume effects; such de-coupling is necessary to appropriately probe the complicated electrostatic effects arising from pH-dependent charging of the PE brushes and the use of these effects for driving liquid and ion transport at the interfaces covered with PE brushes. We also discuss the atomistic simulation approaches for polymer and PE brushes. Next we provide a detailed review of the existing approaches for the synthesis of polymer and PE brushes on interfaces, nanoparticles, and nanochannels, including mixed brushes and patterned brushes. Finally, we discuss some of the possible applications and future developments of polymer and PE brushes grafted on a variety of interfaces.

Streaming potential and electroviscous effects in soft nanochannels: towards designing more efficient nanofluidic electrochemomechanical energy converters.

In this paper we provide analytical solutions for the streaming potential and electroviscous effects in soft nanochannels. The analysis is based on the solution of the linearized Poisson-Boltzmann equation, valid for small electrostatic potentials. We identify the important dimensionless parameters that dictate these two effects. Results are provided for a large range of electric double layer (EDL) thickness values, spanning from the case of very thin to very large overlapped EDL thicknesses. We compare the results with those of a rigid nanochannel, having zeta potential equal to the electrostatic potential at the solid-polyelectrolyte interface of the soft nanochannels. For the soft nanochannel, the streaming potential varies very weakly with the EDL thickness and can be substantially larger than that corresponding to the rigid nanochannel. The electroviscous effects for the soft nanochannel, unlike the rigid nanochannel, virtually always exhibit a monotonic decrease with the EDL thickness, and for certain parameter ranges can be several times larger than that for a rigid nanochannel. Most importantly, for the soft nanochannels the electrochemomechanical energy conversion, associated with the generation of streaming potential, is found to be highly efficient, with the efficiency being several times higher than that of a rigid nanochannel.