Frequency dependent deformation of liquid crystal droplets in an external electric field

Nematic drops suspended in the isotropic phase of the same substance were subjected to alternating electrical fields of varying frequency. The system was carefully kept in the isotropic-nematic coexistance region, which was broadened due to small amounts of non-mesogenic additives. Whereas the droplets remained spherical at low (order of 10 Hz) and high frequencies (in the kHz range), at intermediate frequencies, we observed a marked flattening of the droplet in the plane perpendicular to the applied field. The deformation of the liquid crystal (LC) droplets occurred both in substances with positive and negative dielectric anisotropy. The experimental data can be quantitatively modelled with a combination of the leaky dielectric model and screening of the applied electric field due to the finite conductivity.Comment: minor change

The Cassie-Wenzel transition of fluids on nanostructured substrates: Macroscopic force balance versus microscopic density-functional theory

Classical density functional theory is applied to investigate the validity of a phenomenological force-balance description of the stability of the Cassie state of liquids on substrates with nanoscale corrugation. A bulk free-energy functional of third order in local density is combined with a square-gradient term, describing the liquid-vapor interface. The bulk free energy is parameterized to reproduce the liquid density and the compressibility of water. The square-gradient term is adjusted to model the width of the water-vapor interface. The substrate is modeled by an external potential, based upon Lennard-Jones interactions. The three-dimensional calculation focuses on substrates patterned with nanostripes and square-shaped nanopillars. Using both the force-balance relation and density-functional theory, we locate the Cassie-to-Wenzel transition as a function of the corrugation parameters. We demonstrate that the force-balance relation gives a qualitatively reasonable description of the transition even on the nanoscale. The force balance utilizes an effective contact angle between the fluid and the vertical wall of the corrugation to parameterize the impalement pressure. This effective angle is found to have values smaller than the Young contact angle. This observation corresponds to an impalement pressure that is smaller than the value predicted by macroscopic theory. Therefore, this effective angle embodies effects specific to nanoscopically corrugated surfaces, including the finite range of the liquid-solid potential (which has both repulsive and attractive parts), line tension, and the finite interface thickness. Consistently with this picture, both patterns (stripes and pillars) yield the same effective contact angles for large periods of corrugation.Comment: 13 pages 9 figure

Printing surface charge as a new paradigm to program droplet transport

Directed, long-range and self-propelled transport of droplets on solid surfaces, especially on water repellent surfaces, is crucial for many applications from water harvesting to bio-analytical devices. One appealing strategy to achieve the preferential transport is to passively control the surface wetting gradients, topological or chemical, to break the asymmetric contact line and overcome the resistance force. Despite extensive progress, the directional droplet transport is limited to small transport velocity and short transport distance due to the fundamental trade-off: rapid transport of droplet demands a large wetting gradient, whereas long-range transport necessitates a relatively small wetting gradient. Here, we report a radically new strategy that resolves the bottleneck through the creation of an unexplored gradient in surface charge density (SCD). By leveraging on a facile droplet printing on superamphiphobic surfaces as well as the fundamental understanding of the mechanisms underpinning the creation of the preferential SCD, we demonstrate the self-propulsion of droplets with a record-high velocity over an ultra-long distance without the need for additional energy input. Such a Leidenfrost-like droplet transport, manifested at ambient condition, is also genetic, which can occur on a variety of substrates such as flexible and vertically placed surfaces. Moreover, distinct from conventional physical and chemical gradients, the new dimension of gradient in SCD can be programmed in a rewritable fashion. We envision that our work enriches and extends our capability in the manipulation of droplet transport and would find numerous potential applications otherwise impossible.Comment: 11 pages, 4 figure

Energieeffizienz in der Abluftreinigung (Schweinehaltung)

In der Studie wurden in der Intensivtierhaltung eingesetzte Abluftreinigungsanlagen einschließlich Lüftungssystem hinsichtlich ihres technischen Aufbaus, ihres Energieeinsatzes und ihrer Energieeffizienz untersucht. Auf dieser Basis wurden Vorschläge zur Steigerung der Energieeffizienz erarbeitet. Aus der Betrachtung von zwei sächsischen Schweinehaltungsbetrieben konnten Hinweise für die Anlagenbetreiber und für Neuanlagen abgeleitet werden

Tuning static drop friction

The friction force opposing the onset of motion of a drop on a solid surface is typically considered to be a material property for a fixed drop volume on a given surface. However, here we show that even for a fixed drop volume, the static friction force can be tuned by over 30% by preshaping the drop. The static friction usually exceeds the kinetic friction that the drop experiences when moving in a steady state. Both forces converge when the drop is prestretched in the direction of motion or when the drop shows low contact angle hysteresis. In contrast to static friction, kinetic friction is independent of preshaping the drop, that is, the drop history. Kinetic friction forces reflect the material properties

Contact-controlled amoeboid motility induces dynamic cell trapping in 3D-microstructured surfaces.

On flat substrates, several cell types exhibit amoeboid migration, which is characterized by restless stochastic successions of pseudopod protrusions. The orientation and frequency of new membrane protrusions characterize efficient search modes, which can respond to external chemical stimuli as observed during chemotaxis in amoebae. To quantify the influence of mechanical stimuli induced by surface topography on the migration modes of the amoeboid model organism Dictyostelium discoideum, we apply high resolution motion analysis in microfabricated pillar arrays of defined density and geometry. Cell motion is analyzed by a two-state motility-model, distinguishing directed cellular runs from phases of isotropic migration that are characterized by randomly oriented cellular protrusions. Cells lacking myosin II or cells deprived of microtubules show significantly different behavior concerning migration velocities and migrational angle distribution, without pronounced attraction to pillars. We conclude that microtubules enhance cellular ability to react with external 3D structures. Our experiments on wild-type cells show that the switching from randomly formed pseudopods to a stabilized leading pseudopod is triggered by contact with surface structures. These alternating processes guide cells according to the available surface in their 3D environment, which we observed dynamically and in steady-state situations. As a consequence, cells perform "home-runs" in low-density pillar arrays, crawling from pillar to pillar, with a characteristic dwell time of 75 s. At the boundary between a flat surface and a 3D structured substrate, cells preferentially localize in contact with micropillars, due to the additionally available surface in the microstructured arrays. Such responses of cell motility to microstructures might open new possibilities for cell sorting in surface structured arrays

Rheological properties of viscoelastic drops on superamphiphobic substrates

The rheological properties of microliter sized drops of polymer solutions were investigated using measurements of their mechanical vibrational response. Drops were suspended on superamphiphobic substrates and vibrated by the application of a short mechanical impulse. Surface vibrations were monitored by refracting laser light through the drops and focusing the refracted light onto the surface of a photodiode. Time dependent variations in the photodiode output were Fourier transformed to obtain the frequency and spectral width of the mechanical resonances of the drops. These quantities were related to the frequency dependent shear storage and loss moduli (G′ and G″, respectively) using a simple theoretical model. The resulting rheological properties were found to be in agreement with microrheology measurements of the same solutions. Drop vibration therefore provides a fast and accurate method of quantifying the rheological properties of single drops

Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions

Fabrics treated to repel water, superhydrophobic, and water and oil, superamphiphobic, have numerous industrial and consumer-level benefits. However, the liquid repellency decreases in the course of time. This is largely due to chemical or physical changes of the coating due to prolonged exposure to relatively harsh environments. To develop more durable fabric treatments for specific applications, it is necessary to measure the extent to which the treated fabrics retain their low-wettability after being subjected to controlled aggressive environmental conditions. In this study, plain weave fabrics made from polyester filaments and coated with silicone nanofilaments in-solution were exposed to aerodynamic icing conditions. The coated fabrics showed superhydrophobic behavior, or superamphiphobic for those that were fluorinated. The wettability of the fabrics was progressively evaluated by contact angle and roll-off-angle measurements. The coated fabrics were able to maintain their low-wettability characteristics after exposure to water droplet clouds at airspeeds up to 120 m/s, despite damage to the silicone nanofilaments, visible through scanning electron microscopy

Structure of the stationary phase survival protein YuiC from B.subtilis

- Background: Stationary phase survival proteins (Sps) were found in Firmicutes as having analogous domain compositions, and in some cases genome context, as the resuscitation promoting factors of Actinobacteria, but with a different putative peptidoglycan cleaving domain. - Results: The first structure of a Firmicute Sps protein YuiC from B. subtilis, is found to be a stripped down version of the cell-wall peptidoglycan hydrolase MltA. The YuiC structures are of a domain swapped dimer, although some monomer is also found in solution. The protein crystallised in the presence of pentasaccharide shows a 1,6-anhydrodisaccharide sugar product, indicating that YuiC cleaves the sugar backbone to form an anhydro product at least on lengthy incubation during crystallisation. - Conclusions: The structural simplification of MltA in Sps proteins is analogous to that of the resuscitation promoting factor domains of Actinobacteria, which are stripped down versions of lysozyme and soluble lytic transglycosylase proteins