Understanding the Formation of Anisometric Supraparticles: A Mechanistic Look Inside Droplets Drying on a Superhydrophobic Surface

Evaporating drops of nanoparticle suspensions on superhydrophobic surfaces can give anisotropic superaparticles. Previous studies implied the formation of a stiff shell that collapses, but the exact mechanism leading to anisotropy was unclear so far. Here we report on a new experiment using confocal laser scanning microscopy for a detailed characterization of particle formation from droplets of aqueous colloidal dispersions on superhydrophobic surfaces. In a customized setup, we investigated droplets of fumed silica suspensions using two different fluorescent dyes for independently marking silica and the water phase. Taking advantage of interfacial reflection, we locate the drop–air interface and extract normalized time-resolved intensity profiles for dyed silica throughout the drying process. Using comprehensive image analysis we observe and quantify shell-like interfacial particle accumulation arising from droplet evaporation. This leads to a buildup of a stiff fumed silica mantle of ∼20 μm thickness that causes deformation of the droplet throughout further shrinkage, consequently leading to the formation of solid anisometric fumed silica particles

Understanding the Formation of Anisometric Supraparticles: A Mechanistic Look Inside Droplets Drying on a Superhydrophobic Surface

Evaporating drops of nanoparticle suspensions on superhydrophobic surfaces can give anisotropic superaparticles. Previous studies implied the formation of a stiff shell that collapses, but the exact mechanism leading to anisotropy was unclear so far. Here we report on a new experiment using confocal laser scanning microscopy for a detailed characterization of particle formation from droplets of aqueous colloidal dispersions on superhydrophobic surfaces. In a customized setup, we investigated droplets of fumed silica suspensions using two different fluorescent dyes for independently marking silica and the water phase. Taking advantage of interfacial reflection, we locate the drop–air interface and extract normalized time-resolved intensity profiles for dyed silica throughout the drying process. Using comprehensive image analysis we observe and quantify shell-like interfacial particle accumulation arising from droplet evaporation. This leads to a buildup of a stiff fumed silica mantle of ∼20 μm thickness that causes deformation of the droplet throughout further shrinkage, consequently leading to the formation of solid anisometric fumed silica particles

Magnetically Actuated Micropatterns for Switchable Wettability

Arrays of actuated magnetic micropillars that can be tilted, twisted, and rotated in the presence of a magnetic field gradient were obtained. The type and extent of the movements are dependent on the distribution (isotropic, anisotropic) of the magnetizable particles inside the pillars and the strength and the direction of the magnetic field gradient. Independent motion of groups of pillars in the same or opposite directions or homogeneous motion of the whole pattern has been realized. Changing the pattern geometry causes changes in the roll-off angle (ROA) of water droplets on the surface. We show magnetically induced changes in the ROA and direction-dependent ROAs as a consequence of the anisotropy of tilted patterns. We also demonstrate transfer of microparticles between magnetically actuated neighboring pillars

Liquid Drops Impacting Superamphiphobic Coatings

The dynamics of liquid drops impacting superamphiphobic coatings is studied by high-speed video microscopy. Superamphiphobic coatings repel water and oils. The coating consists of a fractal-like hydrophobized silica network. Mixtures of ethanol–water and glycerin–water are chosen to investigate the influence of interfacial tension and viscosity on spreading and retraction dynamics. Drop spreading is dominated by inertia. At low impact velocity, the drops completely rebound. However, the contact time increases with impact velocity, whereas the restitution coefficient decreases. We suggest that the drop temporarily impales the superamphiphobic coating, although the drop completely rebounds. From an estimate of the pressure, it can be concluded that impalement is dominated by depinning rather than sagging. With increasing velocity, the drops partially pin, and an increasing amount of liquid remains on the coating. A time-resolved study of the retraction dynamics reveals two well-separated phases: a fast inertia-dominated phase followed by a slow decrease of the contact diameter of the drop. The crossover occurs when the diameter of the retracting drop matches the diameter of the drop before impact. We suggest that the depth of impalement increases with impact velocity, where impalement is confined to the initial impact zone of the drop. If the drop partially pins on the coating, the depth of impalement exceeds a depth, preventing the whole drop from being removed during the retraction phase

Liquid Drops Impacting Superamphiphobic Coatings

The dynamics of liquid drops impacting superamphiphobic coatings is studied by high-speed video microscopy. Superamphiphobic coatings repel water and oils. The coating consists of a fractal-like hydrophobized silica network. Mixtures of ethanol–water and glycerin–water are chosen to investigate the influence of interfacial tension and viscosity on spreading and retraction dynamics. Drop spreading is dominated by inertia. At low impact velocity, the drops completely rebound. However, the contact time increases with impact velocity, whereas the restitution coefficient decreases. We suggest that the drop temporarily impales the superamphiphobic coating, although the drop completely rebounds. From an estimate of the pressure, it can be concluded that impalement is dominated by depinning rather than sagging. With increasing velocity, the drops partially pin, and an increasing amount of liquid remains on the coating. A time-resolved study of the retraction dynamics reveals two well-separated phases: a fast inertia-dominated phase followed by a slow decrease of the contact diameter of the drop. The crossover occurs when the diameter of the retracting drop matches the diameter of the drop before impact. We suggest that the depth of impalement increases with impact velocity, where impalement is confined to the initial impact zone of the drop. If the drop partially pins on the coating, the depth of impalement exceeds a depth, preventing the whole drop from being removed during the retraction phase

Liquid Drops Impacting Superamphiphobic Coatings

The dynamics of liquid drops impacting superamphiphobic coatings is studied by high-speed video microscopy. Superamphiphobic coatings repel water and oils. The coating consists of a fractal-like hydrophobized silica network. Mixtures of ethanol–water and glycerin–water are chosen to investigate the influence of interfacial tension and viscosity on spreading and retraction dynamics. Drop spreading is dominated by inertia. At low impact velocity, the drops completely rebound. However, the contact time increases with impact velocity, whereas the restitution coefficient decreases. We suggest that the drop temporarily impales the superamphiphobic coating, although the drop completely rebounds. From an estimate of the pressure, it can be concluded that impalement is dominated by depinning rather than sagging. With increasing velocity, the drops partially pin, and an increasing amount of liquid remains on the coating. A time-resolved study of the retraction dynamics reveals two well-separated phases: a fast inertia-dominated phase followed by a slow decrease of the contact diameter of the drop. The crossover occurs when the diameter of the retracting drop matches the diameter of the drop before impact. We suggest that the depth of impalement increases with impact velocity, where impalement is confined to the initial impact zone of the drop. If the drop partially pins on the coating, the depth of impalement exceeds a depth, preventing the whole drop from being removed during the retraction phase

Interfacial Energy and Glass Temperature of Polymers Confined to Nanoporous Alumina

We report on the effect of interfacial energy on the glass temperature, <i>T</i>_g, of several amorphous polymers with various glass temperatures and polymer/substrate interactions confined within self-ordered nanoporous alumina (AAO). The polymers studied include poly­(phenyl­methyl­siloxane) (PMPS), poly­(vinyl acetate) (PVAc), 1,4-polybutadiene (PB), oligostyrene (PS), and poly­(dimethyl­siloxane) (PDMS). The segmental dynamics and associated <i>T</i>_g’s are studied by means of dielectric spectroscopy. The interfacial energy for the polymer/substrate interface, γ_SL, is calculated with Young’s equation whereas the AAO membrane surface energy is obtained by measuring contact angles for several reference liquids. We find that interfacial energy plays a significant role in the segmental dynamics of polymers under confinement within AAO. There is a trend for a <i>decreasing</i> glass temperature relative to the bulk with <i>increasing</i> interfacial energy. PDMS exhibits the highest interfacial energy and the highest reduction in glass temperature within AAO. Other effects that may also contribute to changes in <i>T</i>_g are discussed

Picosatellites for maritime security applications – the Lambdasat Case

This study explores the potential deployment of small satellites for maritime interdiction and security applications by investigating the available solutions and formulating a generic proposal to optimize the use of those short-lived space assets in support of these operations. The operational background is analyzed with respect to the potential use of these systems by field officers. An analysis for operational micro and picosatellite characteristics is executed, and a brief outlook on the vulnerabilities of those for Low Earth Orbits is given. Moreover, a real scenario has been implemented, and the obtained computational results provide useful insight into how these space systems can be used for maritime security operations. Particular reference is given to the state-of-the-art status of propulsion systems capable of enhancing the lifetime of the satellites. Similarly, a literature survey has been conducted collecting all available picosatellites in orbit today that deal with maritime security applications. The study also explores the application of Lambdasat picosatellite (currently in orbit) to demonstrate the capability of exchanging alert messages between ground stations in Greece and in the US and vessels in the middle of the ocean. With these experiments we will demonstrate the ability magnitude of a picosatellite to support maritime operations

Wetting on the Microscale: Shape of a Liquid Drop on a Microstructured Surface at Different Length Scales

Describing wetting of a liquid on a rough or structured surface is a challenge because of the wide range of involved length scales. Nano- and micrometer-sized textures cause pinning of the contact line, reflected in a hysteresis of the contact angle. To investigate contact angles at different length scales, we imaged water drops on arrays of 5 μm high poly­(dimethylsiloxane) micropillars. The drops were imaged by laser scanning confocal microscopy (LSCM), which allowed us to quantitatively analyze the local and large-scale drop profile simultaneously. Deviations of the shape of drops from a sphere decay at two different length scales. Close to the pillars, the amplitude of deviations decays exponentially within 1–2 μm. The drop profile approached a sphere at a length scale 1 order of magnitude larger than the pillars’ height. The height and position dependence of the contact angles can be understood from the interplay of pinning of the contact line, the principal curvatures set by the topography of the substrate, and the minimization of the air–water interfaces