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

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.

Droplet duos on water display pairing, autonomous motion, and periodic eruption

Under non-equilibrium conditions, liquid droplets dynamically couple with their milieu through the continuous flux of matter and energy, forming active systems capable of self-organizing functions reminiscent of those of living organisms. Among the various dynamic behaviors demonstrated by cells, the pairing of heterogeneous cell units is necessary to enable collective activity and cell fusion (to reprogram somatic cells). Furthermore, the cyclic occurrence of eruptive events such as necroptosis or explosive cell lysis is necessary to maintain cell functions. However, unlike the self-propulsion behavior of cells, cyclic cellular behavior involving pairing and eruption has not been successfully modeled using artificial systems. Here, we show that a simple droplet system based on quasi-immiscible hydrophobic oils (perfluorodecalin and decane) deposited on water, mimics such complex cellular dynamics. Perfluorodecalin and decane droplet duos form autonomously moving Janus or coaxial structures, depending on their volumes. Notably, the system with a coaxial structure demonstrates cyclic behavior, alternating between autonomous motion and eruption. Despite their complexity, the dynamic behaviors of the system are consistently explained in terms of the spreading properties of perfluorodecalin/decane duplex interfacial films.

Diphenylalanine Deposition by Dip-Coating from Acidic Solutions: Fibers and Closed Homogeneous Films

A simple but precisely controllable strategy by molecular assembly that enables the construction of biomaterials is always in the development. Dip-coating deposition of diphenylalanine (FF) onto planar solid substrates from aqueous acidic (acetic, propanoic, formic, and HCl) solutions is studied as a function of the process control parameters (deposition speed, initial concentration of FF and acids, and external gas flow). The results are studied by optical microscopy, AFM, and ellipsometry. For low acidity and low FF concentrations, FF forms microfibers, nanofibers, or stripes of fiber aggregates. For higher acidity and FF concentrations, closed films of FF of remarkably smooth surfaces are found. The thickness of these films can be well-controlled by the FF concentration and the deposition speed and explained by the evaporation regime. These unusual results provide new possibilities to fabricate more abundant structures by a simple strategy and develop a candidate for biological membrane areas.

Precipitation dominated thin films of acetaminophen fabricated by meniscus guided coating

Crystallization above the solvent boiling point facilitates the identification of a new precipitation dominant morphology during meniscus guided coating.

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.

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.

Ring patterns generated by an expanding colloidal meniscus

The drop-and-dry is a common technique allowing for creation of periodic nanoparticle (NP) structures for sensing, photonics, catalysis, etc. However, the reproducibility and scalability of this approach for fabrication of NP-based structures faces serious challenges due to the complexity of the simple, at first glance, evaporation process. In this work we study the effect of the spatial confinement on the NP self-assembly under slow solvent evaporation, when the air-liquid-substrate contact line (CL) expands from the center towards the walls of a cylindrical cell, forming a toroid. Using in situ video monitoring of the stick-slip CL motion, we find regular hydrodynamic perturbations in the meniscus, and reveal fine details of the formation of quasiperiodic rings of close packed NP layers. We report that drying of the toroidal NP droplet has a number of important differences from drying of the classical hemispherical colloidal drops. In toroidal drops we observe linear-in-time average meniscus motion, in contrast to the hemispherical drops where the meniscus moves as a square root of time. While both droplet geometries produce NP ring patterns, the ring width for the toroidal drop decreases with increasing ring radius, while it decreases with decreasing the radius of the hemispherical drop. We suggest that free ligands are the main cause of the Marangoni instabilities driving the periodic vorticity in the meniscus. In addition, we show that the usually ignored contact line tension may yield a considerable contribution to the CL pinning causing the CL slip-stick motion and the ring formation.

Applying droplets and films in evaporative lithography

This review covers experimental results of evaporative lithography and analyzes existing mathematical models of this method. Evaporating droplets and films are used in different fields, such as cooling of heated surfaces of electronic devices, diagnostics in health care, creation of transparent conductive coatings on flexible substrates, and surface patterning. A method called evaporative lithography emerged after the connection between the coffee ring effect taking place in drying colloidal droplets and naturally occurring inhomogeneous vapor flux densities from liquid-vapor interfaces was established. Essential control of the colloidal particle deposit patterns is achieved in this method by producing ambient conditions that induce a nonuniform evaporation profile from the colloidal liquid surface. Evaporative lithography is part of a wider field known as "evaporative-induced self-assembly" (EISA). EISA involves methods based on contact line processes, methods employing particle interaction effects, and evaporative lithography. As a rule, evaporative lithography is a flexible and single-stage process with such advantages as simplicity, low price, and the possibility of application to almost any substrate without pretreatment. Since there is no mechanical impact on the template in evaporative lithography, the template integrity is preserved in the process. The method is also useful for creating materials with localized functions, such as slipperiness and self-healing. For these reasons, evaporative lithography attracts increasing attention and has a number of noticeable achievements at present. We also analyze limitations of the approach and ways of its further development.

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.

Drying Droplets with Soluble Surfactants

We propose a theory for the drying of liquid droplets of surfactant solutions. We show that the added surfactant hinders droplet receding and facilitates droplet spreading, causing a complex behavior of the contact line of an evaporating droplet: the contact line first recedes, then advances, and finally recedes again. We also show that the surfactant can change the deposition pattern from mountain-like to volcano-like and then to coffee-ring-like. Specially, when the contact line motion undergoes a clear receding-advancing transition, a two-ring pattern is formed. The mechanism of the two-ring formation is different from the stick-slip mechanism proposed previously and may be tested experimentally.

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.