Solvation Forces Near Hydrophobic Surfaces: A Classical Density Functional Theory Study.

We use classical density functional theory (DFT) to model solvation interactions between hydrophobic surfaces, which we show to be characterized by depletion attraction at small surface to surface separations and a slowly decaying bipower law interaction at large separations. The solvation interaction originates from van der Waals (vdW) and Coulombic interactions between molecules in the polar solvent, e.g., water, and from the molecules thermal motion and finite volume. We investigate model hydrophobic surfaces represented by bubbles and nonpolar solids, e.g., aliphatic particles, and calculate in a DFT fashion the distribution of molecules in the interlaying solvent between two such surfaces and the hydrophobic excess force resulting from it. The interactions are largely attractive, which is well-known in measurement, albeit vdW attraction between molecules in solids and in the solvent may cause repulsion at certain interface to interface separations. We commence our analysis by suggesting an asymptotic analytical bipower law expression for the solvation interaction at large separations. Thereafter we present a full numerical solution, which is in good agreement with the analytical prediction and further explores the interaction at small surface to surface separations. Our theoretical results yield adhesion energies which agree with previous experiments.

Depletion-Induced Self-Assembly of Colloidal Particles on a Solid Substrate.

We investigate the depletion contributions to the self-assembly of microcolloids on solid substrates. The assembly is driven by the exclusion of nanoparticles and nonadsorbing polymers from the depletion zone between the microcolloids in the liquid and the underlying substrate. The model system consists of 1 µm polystyrene particles that we deposit on a flat glass slab in an electrolyte solution. Using polystyrene nanoparticles and poly(acrylic acid) polymers as depleting agents, we demonstrate in our experiments that nanoparticle concentrations of 0.5% (w/v) support well-ordered packing of microcolloids on glass, while the presence of polymers leads to irregular aggregate deposition structures. A mixture of nanoparticles and polymers enhances the formation of colloidal aggregate and particulate surface coverage compared to using the polymers alone as a depletion agent. Moreover, tuning the polymer ionization state from pH 4 to 9 modifies the polymer conformational state and radius of gyration, which in turn alters the microcolloid deposition from compact multilayers to flocculated structures. Our study provides entropic strategies for manipulating particulate assembly on substrates from dispersed to continuous coatings.

Particle Network Self-Assembly of Similar Size Sub-Micron Calcium Alginate and Polystyrene Particles Atop Glass.

Particle-mediated self-assembly, such as nanocomposites, microstructure formation in materials, and core-shell coating of biological particles, offers precise control over the properties of biological materials for applications in drug delivery, tissue engineering, and biosensing. The assembly of similar-sized calcium alginate (CAG) and polystyrene sub-micron particles is studied in an aqueous sodium nitrate solution as a model for particle-mediated self-assembly of biological and synthetic mixed particle species. The objective is to reinforce biological matrices by incorporating synthetic particles to form hybrid particulate networks with tailored properties. By varying the ionic strength of the suspension, the authors alter the energy barriers for particle attachment to each other and to a glass substrate that result from colloidal surface forces. The particles do not show monotonic adsorption trend to glass with ionic strength. Hence, apart from DLVO theory-van der Waals and electrostatic interactions-the authors further consider solvation and bridging interactions in the analysis of the particulate adsorption-coagulation system. CAG particles, which support lower energy barriers to attachment relative to their counterpart polystyrene particles, accumulate as dense aggregates on the glass substrate. Polystyrene particles adsorb simultaneously as detached particles. At high electrolyte concentrations, where electrostatic repulsion is largely screened, the mixture of particles covers most of the glass substrate; the CAG particles form a continuous network throughout the glass substrate with pockets of polystyrene particles. The particulate structure is correlated with the adjustable energy barriers for particle attachment in the suspension.

Acoustic resonance effects and cavitation in SAW aerosol generation.

The interaction of surface acoustic waves (SAWs) with liquids enables the production of aerosols with adjustable droplet sizes in the micrometer range expelled from a very compact source. Understanding the nonlinear acousto-hydrodynamics of SAWs with a regulated micro-scale liquid film is essential for acousto-microfluidics platforms, particularly aerosol generators. In this study, we demonstrate the presence of micro-cavitation in a MHz-frequency SAW aerosol generation platform, which is touted as a leap in aerosol technology with versatile application fields including biomolecule inhalation therapy, micro-chromatography and spectroscopy, olfactory displays, and material deposition. Using analysis methods with high temporal and spatial resolution, we demonstrate that SAWs stabilize spatially arranged liquid micro-domes atop the generator's surface. Our experiments show that these liquid domes become acoustic resonators with highly fluctuating pressure amplitudes that can even nucleate cavitation bubbles, as supported by analytical modeling. The observed fragmentation of liquid domes indicates the participation of three droplet generation mechanisms, including cavitation and capillary-wave instabilities. During aerosol generation, the cavitation bubbles contribute to the ejection of droplets from the liquid domes and also explain observed microstructural damage patterns on the chip surface eventually caused by cavitation-based erosion.

Connecting Colloidal Forces to the Equilibrium Thickness of Particulate Deposits on a Substrate in Contact with a Suspension Using Classical Density Functional Theory.

We study contributions of colloidal forces, i.e., hydrophobic, van der Waals, and electrical double layer interactions, to the thickness of a colloidal deposit in equilibrium with an aqueous suspension by using classical density functional theory, which we expand to obtain a Ginzburg-Landau type energy functional. We regard colloidal particles as clusters of molecular segments-a reminiscent of polymer statistical physics and of the classic Hamaker treatment of van der Waals interactions between particles. This approach appropriately accounts for the integral interaction energy between colloidal particles, which may take magnitudes of many times the characteristic molecular thermal energy kBT (Boltzmann constant times temperature). The analysis highlights the well-known insight that entropy is mostly governed by the solvent molecules and gives physical values to the statistical coefficients in a Ginzburg-Landau type energy functional.

First-Principle Colloidal Gate for Controlling Liquid and Molecule Flow Using 2D Claylike Nanoparticles.

Herein, we exploit the natural tendency of two-dimensional (2D) clay nanoparticles to self-assemble and restrict water permeability in soils to fabricate a first of its kind synthetic, pH-activated, reversible, and tunable colloidal flow gate. To realize this, we studied the effect of the pH level of a suspension of claylike layered double hydroxide (LDH) nanoparticles on the LDH coagulation process. We then packed the LDH into a fixed-bed column and examined the effect of pH on mass transport through the column. We found that the 2D platelike LDH particles coagulate in an edge-to-edge configuration, which renders highly nonisotropic aggregates, pivotal for obstructing the transport of liquid and molecules therein. We showed that the coagulation and flow through the column may be regulated by imposing various pH levels as an external stimulus to affect LDH zeta potential. Hence, this work shows that the flow through a column comprising a 2D particle bed can be regulated in a reversible manner by simply alternating the pH of the wash solution, equilibration time, or gate dimensions. Furthermore, we show that, subject to pH treatment, we may open and close the colloidal gate for the transport of large molecules and provide selective transport thereof.

Crystallinity Dependence of PLLA Hydrophilic Modification during Alkali Hydrolysis.

Poly(L-lactic acid) (PLLA) has been extensively used in tissue engineering, in which its surface hydrophilicity plays an important role. In this work, an efficient and green strategy has been developed to tailor surface hydrophilicity via alkali hydrolysis. On one hand, the ester bond in PLLA has been cleaved and generates carboxyl and hydroxyl groups, both of which are beneficial to the improvement of hydrophilicity. On the other hand, the degradation of PLLA increases the roughness on the film surface. The resultant surface wettability of PLLA exhibits crucial dependence on its crystallinity. In the specimen with high crystallinity, the local enrichment of terminal carboxyl and hydroxyl groups in amorphous regions accelerates the degradation of ester group, producing more hydrophilic groups and slit valleys on film surface. The enhanced contact between PLLA and water in aqueous solution (i.e., the Wenzel state) contributes to the synergistic effect between generated hydrophilic groups and surface roughness, facilitating further degradation. Consequently, the hydrophilicity has been improved significantly in the high crystalline case. On the contrary, the competition effect between them leads to the failure of this strategy in the case of low crystallinity.

Surface Acoustic Wave Mitigation of Precipitate Deposition on a Solid Surface─An Active Self-Cleaning Strategy.

We demonstrate the application of a 20 MHz frequency surface acoustic wave (SAW) in a solid substrate to render its surface "self-cleaning", redirecting the deposition of precipitating mass onto a nearby inert substrate. In our experiment, we confine a solution of poly(methyl methacrylate) polymer and a volatile toluene solvent between two substrates, lithium niobate and glass, at close proximity. We render the glass surface low energy by employing hydrophobic coating. In the absence of SAW excitation, we observe that the evaporation of the solvent yields polymer coating on the higher energy lithium niobate surface, while the glass surface is mostly devoid of polymer deposits. The application of a propagating SAW in the lithium niobate substrate mitigates the deposition of the polymer on its surface. As a response, we observe an increase in the deposition of the polymer precipitates on glass. Above a SAW power threshold, the polymer appears to deposit solely on glass, leaving the surface of the lithium niobate substrate devoid of polymer mass.

Revisiting the Electroacoustic Phenomenon in the Presence of Surface Acoustic Waves.

Recently, one has been observing abundant studies on the application of surface acoustic waves (SAWs) in solid substrates for manipulating liquids and particulates in micron-to-nanometer thick films and channels and in porous media. At these length scales, contributions of SAWs to the electrical double layer (EDL) of ions and of the latter to particulates and flow may become appreciable. However, the nature of the interplay between SAWs and EDLs is unknown. We demonstrate the contribution of a SAW to the near-equilibrium electrical and ion-concentration fields in an EDL near inert and piezoelectric substrates. In particular, we concentrate on the leakage of transient and steady components of electrical potential through the excited EDL. Far from the solid, the leakage may be interpreted by different models of the EDL to give information about the EDL characteristic relaxation time, ζ-potential, and the Stern layer therein. In addition, the analysis given here may explain observed SAW-induced electrochemical effects on piezoelectric substrates.

Powerful Acoustogeometric Streaming from Dynamic Geometric Nonlinearity.

Past forms of acoustic streaming, named after their progenitors Eckart (1948), Schlichting (1932), and Rayleigh (1884), serve to describe fluid and particle transport phenomena from the macro to micro-scale. Governed by the fluid viscosity, traditional acoustic streaming arises from second-order nonlinear coupling between the fluid's density and particle velocity, with the first-order acoustic wave time averaging to zero. We describe a form of acoustogeometric streaming that has a nonzero first-order contribution. Experimentally discovered in nanochannels of a height commensurate with the viscous penetration depth of the fluid in the channel, it arises from nonlinear interactions between the surrounding channel deformation and the leading order acoustic pressure field, generating flow pressures three orders of magnitude greater than any known acoustically mediated mechanism. It enables the propulsion of fluids against significant Laplace pressure, sufficient to produce 6 mm/s flow in a 130-150 nm tall nanoslit. We find quantitative agreement between theory and experiment across a variety of fluids and conditions, and identify the maximum flow rate with a channel height 1.59 times the viscous penetration depth.

Polymer dewetting in solvent-non-solvent environment- new insights on dynamics and lithography-free patterning.

HYPOTHESIS: We show that one may employ polymer dewetting in solvent-non-solvent environment to obtain lithography-free fabrication of well-defined nano- to micro- scale polymer droplets arrays from pre-patterned polymer films. The polymer droplet pattern may be converted to a series of hybrid organic-inorganic and inorganic well-defined nano-patterns by using sequential infiltration synthesis (SIS). In particular, we scrutinize the physical parameters which govern the dewetting of flat and striped polymer thin films, which is the key to obtaining our objective of lithography-free ordered nano-patterns. EXPERIMENTS: We immerse polystyrene (PS) and polymethyl methacrylate (PMMA) thin films in water in the presence of chloroform vapors. We study the ensuing polymer dewetting dynamics and the pattern formation of nanospheres by employing in-situ light microscopy and scanning electron microscopy. We then investigate pattern formation by dewetting of polymer stripes, fabricated by directed solvent evaporation, and SIS of AlOx from vapor phase precursors, trimethyl aluminum (TMA) and H2O, within the nanosphere patterns. FINDINGS: We find that solvent- non-solvent environments render film dewetting rates, which are an order of magnitude faster than solvent vapor dewetting, and supports the formation of small solid polymer droplets, down to sub-100 nm droplet size, of large contact angles with the solid substrate. Pre-patterned polymer film stripes support the formation of highly ordered structures of polymer droplets, which are easily transformed to hybrid polymer-AlOx nanosphere patterns and templated AlOx nanosphere via SIS.

Coupling between wetting dynamics, Marangoni vortices, and localized hot cells in drops of volatile binary solutions.

HYPOTHESIS: A sessile drop comprising a mixture of volatile solvents supports spatial variations in interfacial energy, which gives rise to solutal Marangoni flow, alongside evaporative loss of drop mass. Both the Marangoni flow and evaporation bring about a dance of concurrent and inter-connected phenomena: internal Marangoni vortices, localized hot cells, and complex wetting dynamics. EXPERIMENT: We employ Particle Image Velocimetry and Infra-Red Microscopy to visualize Marangoni vortices, temperature variations, and the wetting dynamics of drops of toluene and ethanol mixtures. FINDINGS: The intensity of the measured phenomena vary concurrently in time and in like manner according with the initial composition of drops. In particular, we observe maximum intensity levels when the initial toluene proportion in the drops is 60%, and none of these phenomena in the case of pure toluene. Moreover, the drops initially expand on the solid in response to Marangoni flow, then contract due to evaporation; between these dynamic wetting regimes, we further observe a regime of one or periodic wetting/de-wetting cycles at low toluene concentrations. Our findings indicate that both the solutal Marangoni flow and evaporation drive the different phenomena we observe and confirm the connection between Marangoni vortices and the formation of localized hot cells.

The Electrical Double Layer Force between Spherical Particles Which Are Partially Submerged in Water.

We study the EDL force between two colloidal particles that are adsorbed to the surface of an electrolyte solution. The attachment of colloidal particles to a free surface of an electrolyte solution, which may interface with another liquid or vapor phase, is a well-known phenomenon that is employed in many scientific and industrial applications, the most well-known of which is the Pickering emulsion. In addition to capillary stresses, the particles will experience an electrical double layer (EDL) force when they are close to each other. The force originates from the overlap of the diffusive layers of ions that appear in the electrolyte solution next to the charged surfaces of the particles and the charged surface of the electrolyte solution, which is free of particles. Here, we elucidate the contribution of the free surface of the electrolyte solution to the EDL force between two spherical particles, which are half-submerged in the electrolyte solution. We solve the linearized Poisson-Boltzmann equation for the excess electrical potential near the particles and integrate over the resulting excess Maxwell and osmotic stresses on the particles. We further give corresponding Páde approximations, thus enabling the use of simple formulas for the EDL force between interacting particles in cases similar to the ones in this study without the need to repeat the mathematical procedure employed here.

Enabling Rapid Charging Lithium Metal Batteries via Surface Acoustic Wave-Driven Electrolyte Flow.

Both powerful and unstable, practical lithium metal batteries have remained a difficult challenge for over 50 years. With severe ion depletion gradients in the electrolyte during charging, they rapidly develop porosity, dendrites, and dead Li that cause poor performance and, all too often, spectacular failure. Remarkably, incorporating a small, 100 MHz surface acoustic wave device (SAW) solves this problem. Providing acoustic streaming electrolyte flow during charging, the device enables dense Li plating and avoids porosity and dendrites. SAW-integrated Li cells can operate up to 6 mA cm-2 in a commercial carbonate-based electrolyte; omitting the SAW leads to short circuiting at 2 mA cm-2 . The Li deposition is morphologically dendrite-free and close to theoretical density when cycling with the SAW. With a 245 µm thick Li anode in a full Li||LFP (LiFePO4 ) cell, introducing the SAW increases the uncycled Li from 145 to 225 µm, decreasing Li consumption from 41% to only 8%. A closed-form model is provided to explain the phenomena and serve as a design tool for integrating this chemistry-agnostic approach into batteries whatever the chemistry within.

Deposition of nanoparticles from a volatile carrier liquid.

HYPOTHESIS: Traversing length scales in a volatile suspension alters the various contributions to particle deposition from conjoining and disjoining surface forces and from convective and liquid evaporative effects, which is apparent in the deposit morphology. EXPERIMENT: We investigate the particulate structures to result from the self-assembly of nanoparticles following the evaporation of a volatile carrier liquid from the level of the single particle and up to a level which is apparent to the naked eye, while quantifying the contributions of the main mechanisms that are involved in the deposition process. FINDINGS: We show that from the level of the nanoparticles in our experiment and up to a length scale of approximately 10 µm, the morphology of the deposit is particularly sensitive to particle adhesion to the substrate and to liquid evaporation. At greater length scales, the morphology of the deposit is well correlated with the finite volume of particles and with particle convection effects. The particulate structures are in the form of detached particles and particle islands, stripes, and continuous coating, which may vary at different length scales of the same deposit.

Analysis of the oscillatory wetting-dewetting motion of a volatile drop during the deposition of polymer on a solid substrate.

A recent experimental work revealed an oscillatory wetting-dewetting motion of the three phase contact line during the deposition of polymer from a volatile solution. Here we employ a theoretical model to explain the wetting-dewetting motion of the contact line by incorporating opposing evaporation and Marangoni induced flows in the deposition process. We take into account the contribution of polymer concentration to the surface tension of the volatile drop and show that by changing the different parameters of the system we are able to traverse the dynamics of the three phase contact line from a simple dewetting regime to the wetting-dewetting regime, observed in experiment. We further show that deposition patterns, which were previously attributed to stick and stick-slip modes of the contact line motion may be generated by the wetting-dewetting mode. We summarize our theoretical findings in phase diagrams, which show the expected regimes of contact line motion and the resulting types of patterned deposits which are to be obtained under different physical conditions.

Transitions between different motion regimes of the three-phase contact line during the pattern deposition of polymer from a volatile solution.

HYPOTHESIS: The interplay between different transport mechanisms during polymer deposition from a volatile solution determines the motion regime of the three-phase contact line, e.g. monotonous slip, stick-slip, and oscillatory wetting-dewetting regimes of motion, which define the morphology of the deposit. EXPERIMENT: To investigate the transitions between the motion regimes of the contact line, we evaporate solutions of Poly-methyl-methacrylate (PMMA) and Poly-dimethyl-siloxane (PDMS) in toluene. The solutions are confined in a well-defined (micro-chamber) geometry, where we adjust the system temperature, initial polymer concentration and molecular mass, and precisely determine the rate of evaporation. FINDINGS: We show that transitions between particular motion regimes of the contact line are connected to two types of competition between physical mechanisms. A transport competition between polymer diffusion and convection determines the distribution of polymer in the volatile meniscus and hence of spatial variations in the surface energy of the solution. A competition between evaporative and surface energy stresses in the liquid meniscus determines the motion of the contact line. We report the temporal variations of the contact line position during each motion regime and give a phase diagram to quantify the physical parameters that are responsible to transitions between the different regimes.

Connecting Monotonic and Oscillatory Motions of the Meniscus of a Volatile Polymer Solution to the Transport of Polymer Coils and Deposit Morphology.

We study the deposition mechanisms of polymer from a confined  meniscus of volatile liquid. In particular, we investigate the physical processes that are responsible for qualitative changes in the pattern deposition of polymer and the underlying interplay of the state of pattern deposition, motion of the meniscus, and the transport of polymer within the meniscus. As a model system we evaporate a solution of poly(methyl methacrylate) (PMMA) in toluene. Different deposition patterns are observed when varying the molecular mass,  the initial concentration of the solute, and temperature; these  are systematically presented in the form of morphological phase diagrams. The modi of deposition and meniscus motion are correlated. They vary with the ratio between the evaporation-driven convective flux and the diffusive flux of the polymer coils in the solution. In the case of a diffusion-dominated solute transport, the solution monotonically dewets the solid substrate by evaporation, supporting continuous contact line motion and continuous polymer deposition. However, a convection-dominated transport results in an oscillatory ratcheting dewetting-wetting motion of the contact line with more pronounced dewetting phases. The deposition process is then periodic and produces a stripe pattern. The oscillatory motion of the meniscus differs from the well documented stick-slip motion of the meniscus, observed as well, and is attributed to the opposing influences of evaporation and Marangoni stresses, which alternately dominate the deposition process.

Signatures of van der Waals and Electrostatic Forces in the Deposition of Nanoparticle Assemblies.

We evaporate aqueous suspensions in a microchamber to explore the connection between the morphology of the nanoparticle deposits at nanometer resolutions and at micrometer and hundreds of micrometers resolutions. Repulsive or weakly attractive electrical double-layer and van der Waals surface forces render the deposition of detached particles and small aggregates at nanometer resolutions. However, strongly attractive surface forces render the dense deposition of large aggregates. At greater length resolutions, the deposit morphology is further governed by evaporation-mediated transport of particles in the volatile suspension. We use experiment and theory to show that the contributions of the different mechanisms to the deposit morphology are mediated by particle coagulation and by particle adsorption to the substrate. The nanometer deposit morphology and particle transport render the morphology of the deposits at greater length resolutions, where it may take the shape of crude or smooth particulate micropatterns or continuous particulate coating layers.

Simulations of the dynamic deposition of colloidal particles from a volatile sessile drop.

HYPOTHESIS: The deposition of particles from a volatile liquid drop atop a substrate is primarily governed by the advection and diffusion of the particles in the liquid. Colloidal particles may further coagulate and adsorb to the substrate during the deposition process. The external geometry and the internal composition of the particulate deposit are then determined by an interplay between these four mechanisms. SIMULATION: We simulate the process of deposition by solving the governing transport equations. We explore the interplay between the different mechanisms mentioned above. In particular, we study the contribution of the diffusion of colloidal particles and aggregates to the morphology of the deposit, which was neglected in a previous study. FINDINGS: The rates of diffusion and coagulation of each specific aggregate are dependent on its size. Hence, the transport equation uniquely correlates to each population of aggregates. The overall transport problem, alongside the rates of particle and aggregate adsorption and liquid evaporation, determines the geometry of the deposit. Moreover, the local rate of particle coagulation determines the internal composition of the different aggregate populations in the deposit. Our results appear to be in qualitative agreement with previous experimental findings.