Evaporation of alcohol droplets on surfaces in moist air.

Droplets of alcohol-based formulations are common in applications from sanitizing sprays to printing inks. However, our understanding of the drying dynamics of these droplets on surfaces and the influence of ambient humidity is still very limited. Here, we report the drying dynamics of picoliter droplets of isopropyl alcohol deposited on a surface under controlled humidity. Condensation of water vapor in the ambient environment onto alcohol droplets leads to unexpectedly complex drying behavior. As relative humidity (RH) increases, we observed a variety of phenomena including enhanced spreading, nonmonotonic changes in the drying time, the formation of pancake-like shapes that suppress the coffee-ring effect, and the formation of water-rich films around an alcohol-rich drop. We developed a lubrication model that accounts for the coupling between the flow field within the drop, the shape of the drop, and the vapor concentration field. The model reproduces many of the experimentally observed morphological and dynamic features, revealing the presence of unusually large spatial compositional gradients within the evaporating droplet and surface-tension-gradient-driven flows arising from water condensation/evaporation at the surface of the droplet. One unexpected feature from the simulation is that water can evaporate and condense concurrently in different parts of the drop, providing fundamental insights that simpler models based on average fluxes lack. We further observed rim instabilities at higher RH that are well-described by a model based on the Rayleigh-Plateau instability. Our findings have implications for the testing and use of alcohol-based disinfectant sprays and printing inks.

In Situ Self-Assembly of Nanoscale Particles into Macroscale Ordered Monolayers with Enhanced Memory Performance.

In situ fabrication of macroscale ordered monolayers of nanoparticles (NPs) on targeted substrates is highly desirable for precision electronic and optical devices, while it remains a great challenge. In this study, a solution is provided to address this challenge by developing a colloidal ink formulation and employing the direct-ink-writing (DIW) technique, where on-demand delivery of ink at a targeted location and directional evaporation with controllable rate are leveraged to precisely guide the deposition of polystyrene-grafted gold NPs (Au@PS NPs) into a macroscale monolayer with an ordered Au NP array embedded in a PS thin film. A 2D steady-state diffusion-controlled evaporation model, which explains the parameter dependence of the experimental results and gives semiquantitative agreement with the experimental evaporation kinetics is proposed. The ordered monolayer is used as both nanocrystal floating gates and the tunneling layer for nonvolatile memory devices. It shows significantly enhanced performance compared with a disordered NP film prepared by spin coating. This approach allows for fine control of NP self-assembly to print macroscaleordered monolayers directly onto substrates, which has great promise for application in broad fields, including microelectronic and photoelectronic devices, sensors, and functional coatings.

Counterions under a Surface-Adsorbed Cationic Surfactant Monolayer: Structure and Thermodynamics.

The surface adsorption of ionic surfactants is fundamental for many widespread phenomena in life sciences and for a wide range of technological applications. However, direct atomic-resolution structural experimental studies of noncrystalline surface-adsorbed films are scarce. Thus, even the most central physical aspects of these films, such as their charge density, remain uncertain. Consequently, theoretical models based on contradicting assumptions as for the surface films' ionization are widely used for the description and prediction of surface thermodynamics. We employ X-ray reflectivity to obtain the Ångström-scale surface-normal structure of surface-adsorbed films of the cationic surfactant cetyltrimethylammonium bromide (CTAB) in aqueous solutions at several different temperatures and concentrations. In conjunction with published neutron reflectivity data, we determine the surface-normal charge distribution due to the dissociated surfactants' headgroups. The distribution appears to be inconsistent with the Gouy-Chapman model yet consistent with a compact Stern layer model of condensed counterions. The experimental surfactant adsorption thermodynamics conforms well to classical, Langmuir and Kralchevsky, adsorption models. Furthermore, the Kralchevsky model correctly reproduces the observed condensation of counterions, allowing the values of the adsorption parameters to be resolved, based on the combination of the present data and the published surface tension measurements.

Comparative Study of Lipid- and Polymer-Supported Membranes Obtained by Vesicle Fusion.

We compare the fusion of giant lipid and block-copolymer vesicles on glass and poly(dimethylsiloxane) substrates. Both types of vesicles are similar in their ability to fuse to hydrophilic substrates and form patches with distinct heart or circular shapes. We use epifluorescence/confocal microscopy and atomic force microscopy on membrane patches to (i) characterize bilayer fluidity and patch-edge stability and (ii) follow the intermediate stages in the formation of continuous supported bilayers. Polymer membranes show much lower membrane fluidity and, unlike lipids, an inability of adjacent patches to fuse spontaneously into continuous membranes. We ascribe this effect to hydration repulsion forces acting between the patch edges, which can be diminished by increasing the sample temperature. We show that large areas of supported polymer membranes can be created by fusing giant vesicles on glass or poly(dimethylsiloxane) substrates and annealing their edges.

Evaporation of Binary-Mixture Liquid Droplets: The Formation of Picoliter Pancakelike Shapes.

Small multicomponent droplets are of increasing importance in a plethora of technological applications ranging from the fabrication of self-assembled hierarchical patterns to the design of autonomous fluidic systems. While often far away from equilibrium, involving complex and even chaotic flow fields, it is commonly assumed that in these systems with small drops surface tension keeps the shapes spherical. Here, studying picoliter volatile binary-mixture droplets of isopropanol and 2-butanol, we show that the dominance of surface tension forces at small scales can play a dual role: Minute variations in surface tension along the interface can create Marangoni flows that are strong enough to significantly deform the drop, forming micron-thick pancakelike shapes that are otherwise typical of large puddles. We identify the conditions under which these flattened shapes form and explain why, universally, they relax back to a spherical-cap shape toward the end of drop lifetime. We further show that the formation of pancakelike droplets suppresses the "coffee-ring" effect and leads to uniform deposition of suspended particles. The quantitative agreement between theory and experiment provides a predictive capability to modulate the shape of tiny droplets with implications in a range of technologies from fabrication of miniature optical lenses to coating, printing, and pattern deposition.

Wetting and Drying of Aqueous Droplets Containing Nonionic Surfactants CnEm.

This paper presents a systematic study of the wetting and drying of aqueous pico-liter droplets containing nonionic surfactants polyoxyethylene alkyl ethers (CnEm; n = 10, 12, 14, m = 6 or 8) in comparison with the anionic surfactant sodium dodecyl sulfate (SDS). The spreading and drying of droplets on hydrophilic substrates were studied by tracking the three-phase contact line (TCL) and by interferometry. CnEm droplets undergo phase separation during drying: a water-rich droplet retracts and leaves behind a thin film that is postulated to be a surfactant mesophase. This thin film either retracts or breaks up into small droplets on a longer time scale. The receding contact angle of the water-rich droplet on the thin film in the late stage of drying of CnEm droplets is independent of hydrophobicity of substrates, supporting the inference that a mesophase is present on the surface. Both CnEm and SDS solutions inhibit spreading on hydrophilic surfaces, which is attributed to Marangoni contraction as a result of a surface tension gradient across the gas-liquid interface. More pronounced suppression of spreading is observed in the case of CnEm solutions, possibly due to the phase transition of surfactant solution in the vicinity of the initial TCL leading to a viscous phase at the TCL that pins the droplet. Tracer particle measurements reveal that mild Marangoni flows exist for droplets with surfactant concentrations well above the critical micelle concentration (CMC). Origins of the surfactant gradients that result in Marangoni flows are discussed.

A general ink formulation of 2D crystals for wafer-scale inkjet printing.

Recent advances in inkjet printing of two-dimensional (2D) crystals show great promise for next-generation printed electronics development. Printing nonuniformity, however, results in poor reproducibility in device performance and remains a major impediment to their large-scale manufacturing. At the heart of this challenge lies the coffee-ring effect (CRE), ring-shaped nonuniform deposits formed during postdeposition drying. We present an experimental study of the drying mechanism of a binary solvent ink formulation. We show that Marangoni-enhanced spreading in this formulation inhibits contact line pinning and deforms the droplet shape to naturally suppress the capillary flows that give rise to the CRE. This general formulation supports uniform deposition of 2D crystals and their derivatives, enabling scalable and even wafer-scale device fabrication, moving them closer to industrial-level additive manufacturing.

In Situ Fabrication of Polymeric Microcapsules by Ink-Jet Printing of Emulsions.

Phase separation driven by solvent evaporation of emulsions can be used to create polymeric microcapsules. The combination of emulsion solvent evaporation with ink-jet printing allows the rapid fabrication of polymeric microcapsules at a target location on a surface. The ink is an oil-in-water emulsion containing in the dispersed phase a shell-forming polymer, a core-forming fluid that is a poor solvent for the polymer, and a low-boiling good solvent. After the emulsion is printed onto the substrate, the good solvent evaporates by diffusion through the aqueous phase, and the polymer and the poor solvent phase separate to form microcapsules. The continuous aqueous phase contains polyvinyl alcohol that serves as an emulsifier and a binder of the capsules to the substrate. This method is demonstrated for microcapsules with various shell-forming polymers (polystyrene, poly(methylmethacrylate) and poly(l-lactide)) and core-forming poor solvents (hexadecane and a 4-heptanone/sunflower oil mixture). Cargoes such as fluorescent dyes (Nile Red and tetracyanoquinodimethane) or active ingredients (e.g., the fungicide tebuconazole) can be encapsulated. Uniform microcapsules are obtained by printing emulsions containing monodisperse oil droplets produced in a microfluidic device. We discuss the physical parameters that need to be controlled for the successful fabrication of microcapsules in inkjet printing. The method for rapid, in situ encapsulation could be useful for controlled-release applications such as in agrochemical sprays, fragrances, functional coatings, and topical medicines.

Fabrication of monolayers of uniform polymeric particles by inkjet printing of monodisperse emulsions produced by microfluidics.

Emulsion solvent evaporation is a well-established method for generating microparticles from solutions of polymers in volatile organic solvents dispersed in an aqueous medium. Previous work has shown that this approach can also be used to deposit particles by inkjet printing where the particles are formed during the drying of a liquid ink on a substrate. The particle size distribution, however, was very broad. Here we demonstrate that inkjet printing of oil-in-water emulsions produced by microfluidics can generate micron-sized particles with a narrow size distribution (coefficient of variation <6%) and that these particles can self-assemble into ordered arrays with hexagonal packing. The conditions under which drops can be printed with a minimum of break up and coalescence of the oil droplets in the emulsion are explored. Factors affecting the size of the particles and the morphology of the deposit are described. This study uses polystyrene in dichloromethane as a model system, but the approach can be generalized to the production of structured and functional particles.

Drying of Ethanol/Water Droplets Containing Silica Nanoparticles.

The evaporation of colloidal drop on a substrate with a pinned contact line usually results in a ring stain (the so-called coffee-ring effect). In this paper, we present an investigation of the evaporation of sessile picoliter droplets of binary solvent mixtures containing fumed silica nanoparticles (NPs). The internal flows in ethanol/water droplets are suppressed, and a uniform deposit morphology is achieved with a low loading (0.2-0.5 vol %) of hydrophobic fumed silica NPs. The effective control of the particle deposit morphology is based on a rapid sol-gel transition assisted by preferential evaporation of ethanol. For droplets of dilute suspensions, the fumed silica NPs tend to agglomerate and form an elastic network quickly, starting from the region close to the three-phase contact line and below the gas-liquid interface and growing toward the interior of the droplet as the solvents evaporate and the surface descends. Higher silica particle concentrations, lower ethanol concentrations, and weaker Marangoni flows all contribute to the sol-gel transition and hence to the suppression of the coffee-ring effect.

Effect of Surface Freezing on Stability of Oil-in-Water Emulsions.

Penetration of alkane molecules into the adsorbed film of a cationic surfactant gives rise to a surface freezing transition at the alkane-water interface upon cooling. In this paper, we show that surface freezing of hexadecyltrimethylammonium chloride (CTAC) at the tetradecane-water interface stabilizes oil-in-water (OW) emulsions. For concentrations of CTAC near the critical micelle concentration, an OW emulsion coalesced readily above the surface freezing transition whereas the OW emulsion was stable in the surface frozen state. There was a discontinuous change in the stability of the OW emulsion at a temperature very close to the surface phase transition temperature as determined by interfacial tensiometry and ellipsometry on a planar oil-water interface. The mechanical elasticity of the surface frozen layer opposes film drainage and density fluctuations that could lead to rupture and is the most likely cause of the enhanced emulsion stability.

Combining Inkjet Printing with Emulsion Solvent Evaporation to Pattern Polymeric Particles.

We demonstrate a concept to produce deposits of polymer in the form of particles by inkjet printing an emulsion in which the discrete phase evaporates preferentially. An oil/water emulsion with polymer contained inside the oil phase is used as ink for printing. Circular deposits of spherical polymer particles with uniform thickness are obtained. The effects of the hydrophobicity of substrates and the physical properties of the oil on the morphology of the deposits are explored. The deposit of aggregated polymeric particles can be transformed into a uniform film by annealing if required. This strategy for the patterning of polymer materials in the form of either particles or a film works for mixtures of polymers and functional cargoes.

Ink-Jet Printing of High-Molecular-Weight Polymers in Oil-in-Water Emulsions.

Ink-jet printing of high-molecular-weight polymers is inhibited by the extensional elasticity of the polymer chains. This article describes how emulsions can be used to print high-molecular-weight polymers at much higher concentrations than is possible in a binary solution. The polymers are dissolved in the dispersed phase of an oil-in-water emulsion. The surface tension of the oil-water interface opposes deformation of the oil droplets during ejection from the nozzle and leads to the extensional strain occurring in the polymer-free continuous phase. We show that a solution of polystyrene (PS) (Mn = 419 kDa, polydispersity index = 1.21) can be printed at an overall concentration of 3.8 wt % in a sodium dodecyl sulfate-stabilized emulsion of methyl benzoate in water, an order of magnitude higher in concentration than previously reported for PS dissolved in binary solutions [ Hoath , S. D. , Harlen , O. G. , and Hutchings , I. M. J. Rheol. 2012 , 56 ( 5 ), 1109 - 1127 ]. Factors influencing the formulation of emulsions for ink-jet printing of polymers are discussed.

Mechanical Characterization of Ultralow Interfacial Tension Oil-in-Water Droplets by Thermal Capillary Wave Analysis in a Microfluidic Device.

Measurements of the ultralow interfacial tension and surfactant film bending rigidity for micron-sized heptane droplets in bis(2-ethylhexyl) sodium sulfosuccinate-NaCl aqueous solutions were performed in a microfluidic device through the analysis of thermally driven droplet interface fluctuations. The Fourier spectrum of the stochastic droplet interface displacement was measured through bright-field video microscopy and a contour analysis technique. The droplet interfacial tension, together with the surfactant film bending rigidity, was obtained by fitting the experimental results to the prediction of a capillary wave model. Compared to existing methods for ultralow interfacial tension measurements, this contactless, nondestructive, all-optical approach has several advantages, such as fast measurement, easy implementation, cost-effectiveness, reduced amount of liquids, and integration into lab-on-a-chip devices.

Time-resolved phase-sensitive second harmonic generation spectroscopy.

A methodology based on time-resolved, phase-sensitive second harmonic generation (SHG) for probing the excited state dynamics of species at interfaces is presented. It is based on an interference measurement between the SHG from the sample and a local oscillator generated at a reference together with a lock-in measurement to remove the large constant offset from the interference. The technique is characterized by measuring the phase and excited state dynamics of the dye malachite green at the water/air interface. The key attributes of the technique are that the observed signal is directly proportional to sample concentration, in contrast to the quadratic dependence from non-phase sensitive SHG, and that the real and imaginary parts of the 2nd order non-linear susceptibility can be determined independently. We show that the method is highly sensitive and can provide high quality excited state dynamics in short data acquisition times.

Printing small dots from large drops.

Printing of droplets of pure solvents containing suspended solids typically leads to a ring stain due to convective transport of the particles toward the contact line during evaporation of the solvent. In mixtures of volatile solvents, recirculating cells driven by surface tension gradients are established that lead to migration of colloidal particles toward the center of the droplet. In favorable cases, a dense disk of particles forms with a diameter much smaller than that of the droplet. In the latter stages of drying, convective transport of the particles radially toward the contact line still occurs. Two strategies are described to fix the distribution of particles in a compact disk much smaller than the initial diameter of the drying droplet. First, a nanoparticulate clay is added to induce an evaporation-driven sol-gel transition that inhibits convective flow during the latter stages of drying. Second, a nonadsorbing polymer is added to induce depletion flocculation that restricts particle motion after the particles have been concentrated near the center of the droplet. The area of the resulting deposit can be as little as 10% of the footprint of the printed droplet.

Control of the particle distribution in inkjet printing through an evaporation-driven sol-gel transition.

A ring stain is often an undesirable consequence of droplet drying. Particles inside evaporating droplets with a pinned contact line are transported toward the periphery by radial flow. In this paper, we demonstrate how suspensions of laponite can be used to control the radial flow inside picoliter droplets and produce uniform deposits. The improvement in homogeneity arises from a sol-gel transition during evaporation. Droplets gel from the contact line inward, reducing the radial motion of particles and thus inhibiting the formation of a ring stain. The internal flows and propagation of the gelling front were followed by high-speed imaging of tracer particles during evaporation of the picoliter droplets of water. In the inkjet nozzle, the laponite network is broken down under high shear. Recovery of the low shear viscosity of laponite suspensions was shown to be fast with respect to the lifetime of the droplet, which was instrumental in controlling the deposit morphology. The radial and vertical particle distributions within dried deposits were measured for water droplets loaded with 1 and 5 wt % polystyrene spheres and various concentrations of laponite. Aggregation of the polystyrene spheres was suppressed by the addition of colloidal silica. The formulation can be tuned to vary the deposit profile from a ring to a pancake or a dome.

Theoretical analysis for the optical deformation of emulsion droplets.

We propose a theoretical framework to predict the three-dimensional shapes of optically deformed micron-sized emulsion droplets with ultra-low interfacial tension. The resulting shape and size of the droplet arises out of a balance between the interfacial tension and optical forces. Using an approximation of the laser field as a Gaussian beam, working within the Rayleigh-Gans regime and assuming isotropic surface energy at the oil-water interface, we numerically solve the resulting shape equations to elucidate the three-dimensional droplet geometry. We obtain a plethora of shapes as a function of the number of optical tweezers, their laser powers and positions, surface tension, initial droplet size and geometry. Experimentally, two-dimensional droplet silhouettes have been imaged from above, but their full side-on view has not been observed and reported for current optical configurations. This experimental limitation points to ambiguity in differentiating between droplets having the same two-dimensional projection but with disparate three-dimensional shapes. Our model elucidates and quantifies this difference for the first time. We also provide a dimensionless number that indicates the shape transformation (ellipsoidal to dumbbell) at a value ≈ 1.0, obtained by balancing interfacial tension and laser forces, substantiated using a data collapse.

Total internal reflection spectroscopy for studying soft matter.

Total internal reflection (TIR) spectroscopy is a widely used technique to study soft matter at interfaces. This tutorial review aims to provide researchers with an overview of the principles, experimental design and applications of TIR spectroscopy to enable them to understand how this class of techniques might be used in their research. It also highlights limitations and pitfalls of TIR techniques, which will assist readers in critically analysing the literature. Techniques covered include attenuated total reflection infrared spectroscopy (ATR-IR), TIR fluorescence, TIR Raman scattering and cavity-enhanced techniques. Other related techniques are briefly described.

Adsorption kinetics of non-ionic surfactants in micellar solutions: effects of added charge.

The kinetics of adsorption of micellar solutions of non-ionic surfactants have been studied in an overflowing cylinder. The addition of small amounts (< 10% of the total surfactant concentration) of ionic surfactants, CTAB and STS, to solutions of C16E8 causes a dramatic reduction in the rate of adsorption of the nonionic surfactant. The results are rationalised by a combination of monomer and micelle adsorption to the air-water interface. In the presence of trace ionic surfactants, only uncharged micelles adsorb. The adsorption kinetics are independent of the sign of the charge on the micelles, only on its magnitude. The influence of ionic surfactants on the adsorption rate is reversed by addition of millimolar concentrations of salt. Electrolyte screens the repulsions between micelles and the adsorbed monolayer and allows charged micelles to adsorb without first breaking down into monomers.