Geometry Dependence of the Receding Angle of a Droplet on a Solid Cylindrical Surface.

Droplets that partially wet solid surfaces exhibit hysteresis in their contact angle. The values of the minimum (receding) and maximum (advancing) angles, θ and θ, are empirically well-defined and thought to be unique for a given set of materials. We measured the contact angles of water droplets hanging from hydrophobic, PDMS-functionalized glass and found that the value of θ varies with the curvature of the glass.

Triggered interactions between nanoparticles and lipid membranes: design principles for gel formation or disruption-and-release.

Lipid bilayer vesicles offer exciting possibilities for stimulated response, taking advantage of the membrane's flexibility and impermeability. We show how synergistic interactions between vesicles and polymer-based nanoparticles can be triggered at the nanoscale using UV light. This interaction leads either to adhesion and a membrane-based gel, or to nanoscale wrapping of the particles by the membrane and then vesicle destruction.

Membrane Remodeling by DNA Origami Nanorods: Experiments Exploring the Parameter Space for Vesicle Remodeling.

Inspired by the ability of cell membranes to alter their shape in response to bound particles, we report an experimental study of long, slender nanorods binding to lipid bilayer vesicles and altering the membrane shape. Our work illuminates the role of particle concentration, adhesion strength, and membrane tension in determining the membrane morphology. We combined giant unilamellar vesicles with oppositely charged nanorods, carefully tuning the adhesion strength, membrane tension, and particle concentration.

Vesicle-Based Gel via Polyelectrolyte-Induced Adhesion: Structure, Rheology, and Response.

We describe an experimental study of soft solids composed of micron-scale lipid bilayer vesicles that adhere to one another through electrostatic attraction to an oppositely charged polymer (PDADMAC). As the polymer concentration was increased, we found a fluid phase, a solid gel phase, and a gel composed of internally reorganized vesicles. Optical microscopy images showed a nearly close-packed structure of adhered vesicles that retained their closed-cell morphology.

Response of a raft of particles to a local indentation.

Interfaces that are coated with a layer of adsorbed particles (particle "rafts") are common in natural and industrial settings. Particle-coated interfaces may be useful in part because the particulate structure can endow the fluid interface with physical properties distinct from molecular surfactants. We study the mechanics of particulate assemblies by measuring the raft's response to indentation in the vertical direction by a flat, circular disc.

Nanoparticles binding to lipid membranes: from vesicle-based gels to vesicle tubulation and destruction.

While cells offer numerous inspiring examples in which membrane morphology and function are controlled by interactions with viruses or proteins, we still lack design principles for controlling membrane morphology in synthetic systems. With experiments and simulations, we show that spherical nanoparticles binding to lipid-bilayer membrane vesicles results in a remarkably rich set of collective morphologies that are controllable via the particle binding energy. We separately study cationic and anionic particles, where the adhesion is tuned by addition of oppositely charged lipids to the vesicles.

Wetting, meniscus structure, and capillary interactions of microspheres bound to a cylindrical liquid interface.

Wetting, meniscus structure, and capillary interactions for polystyrene microspheres deposited on constant curvature cylindrical liquid interfaces, constructed from nonvolatile ionic or oligomeric liquids, were studied by optical interferometry and optical microscopy. The liquid interface curvature resulted from the preferential wetting of finite width lines patterned onto planar silicon substrates. Key variables included sphere diameter, nominal (or average) contact angle, and deviatoric interfacial curvature.

Wax Spreading in Paper under Controlled Pressure and Temperature.

This work describes a novel rapid method to fabricate high-resolution paper-based microfluidic devices using wax-ink-based printing. This study demonstrates that both temperature and pressure are important knobs in controlling the device resolution. High-resolution lines and patterns were obtained by heating the paper asymmetrically from one side up to 110 °C while applying pressure up to 49 kPa.

Deformation of the contact line around spherical particles bound at anisotropic fluid interfaces.

When a particle adsorbs at a liquid interface, the 3-phase contact line geometry depends on the shape of the particle and of the liquid interface. The shape of the contact line is the key to controlling capillary forces among particles, and is therefore a useful means to direct assembly of interfacial particles. We measured the shape of the contact line around millimeter-sized PDMS-coated glass spheres at water/air interfaces with anisotropic shapes.

Confined Assemblies of Colloidal Particles with Soft Repulsive Interactions.

We investigate the microconfinement of charged silica nanoparticles dispersed in refractive index matching monomers in poly(dimethylsiloxane) (PDMS) porous membrane. Here, the silica colloidal particles interact with each other and the pore wall via electrostatic double layer forces. Different from the hard sphere systems where the assembled morphologies are prescribed by the diameter ratio between the cylindrical confinement and the nanoparticles, here we observe a much richer variety of assemblies that are highly sensitive to both bulk and local nanoparticle concentration with fixed particle size and channel size.

Osmotically-induced tension and the binding of N-BAR protein to lipid vesicles.

The binding affinity of a curvature-sensing protein domain (N-BAR) is measured as a function of applied osmotic stress while the membrane curvature is nearly constant. Varying the osmotic stress allows us to control membrane tension, which provides a probe of the mechanism of binding. We study the N-BAR domain of the Drosophila amphiphysin and monitor its binding on 50 nm-radius vesicles composed of 90 mol% DOPC and 10 mol% PIP.

Effect of interface shape on advancing and receding fluid-contact angles around spherical particles.

The angle of contact between a solid surface and a fluid interface plays a key role in wetting and is therefore a focus in studies of a wide range of natural phenomena and fluidic technologies. The contact angle ranges between two values, a maximum (advancing) angle and a minimum (receding) angle. These limiting angles are thought to be properties of the fluids and of the chemistry or topography of the solid.

Solid particles adsorbed on capillary-bridge-shaped fluid polystyrene surfaces.

Particles adsorbed on microscopic polystyrene (PS) capillary bridge surfaces were observed to investigate their motion under capillary forces arising from a nonuniform shape. Capillary bridges were created by placing thin PS films, heated above the glass transition temperature (Tg), between two electrodes with an air gap between the surface of the PS and the upper electrode. Silica particles, 100 nm in diameter, were placed on the surface of the PS capillary bridges, and the sample was heated above the Tg of PS to enable particle motion.

Morphology-dependent electronic properties in cross-linked (P3HT-b-P3MT) block copolymer nanostructures.

Combined Kelvin probe force microscopy and wavelength-resolved photoluminescence measurements on individual pre- and post-cross-linked poly(3-hexylthiophene)-b-poly(3-methyl alcohol thiophene) (P3HT-b-P3MT) nanofibers have revealed striking differences in their optical and electronic properties driven by structural perturbation of the crystalline aggregate nanofiber structures after cross-linking. Chemical cross-linking from diblock copolymer P3HT-b-P3MT using a hexamethylene diisocyanate cross-linker produces a variety of morphologies including very small nanowires, nanofiber bundles, nanoribbons, and sheets, whose relative abundance can be controlled by reaction time and cross-linker concentration. While the different cross-linked morphologies have almost identical photophysical characteristics, KPFM measurements show that the surface potential contrast, related to the work function of the sample, depends sensitively on nanostructure morphology related to chain-packing disorder.

Change of line tension in phase-separated vesicles upon protein binding.

We measured the effect of a model membrane-binding protein on line tension and morphology of phase-separated lipid-bilayer vesicles. We studied giant unilamellar vesicles composed of a cholesterol/dioleoylphosphatidylcholine/palmitoylsphingomyelin mixture and a controlled mole fraction of a Ni-chelating lipid. These vesicles exhibited two coexisting fluid-phase domains at room temperature.

Adsorption energy of nano- and microparticles at liquid-liquid interfaces.

We study experimentally the energy of adsorption, DeltaE, of nanoparticles and microparticles at the oil-water interface by monitoring the decrease of interfacial tension as the particles bind. For citrate-stabilized gold nanoparticles assembling on a droplet of octafluoropentyl acrylate, we find DeltaE = -5.1 k(B)T for particle radius R = 2.

Self-assembled shells composed of colloidal particles: fabrication and characterization.

We construct shells with tunable morphology and mechanical response with colloidal particles that self-assemble at the interface of emulsion droplets. Particles self-assemble to minimize the total interfacial energy, spontaneously forming a particle layer that encapsulates the droplets. We stabilize these layers to form solid shells at the droplet interface by aggregating the particles, connecting the particles with adsorbed polymer, or fusing the particles.