111 research outputs found
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
Wetting on silicone surfaces
Silicone is frequently used as a model system to investigate and tune wetting on soft materials. Silicone is biocompatible and shows excellent thermal, chemical, and UV stability. Moreover, the mechanical properties of the surface can be easily varied by several orders of magnitude in a controlled manner. Polydimethylsiloxane (PDMS) is a popular choice for coating applications such as lubrication, self-cleaning, and drag reduction, facilitated by low surface energy. Aiming to understand the underlying interactions and forces, motivated numerous and detailed investigations of the static and dynamic wetting behavior of drops on PDMS-based surfaces. Here, we recognize the three most prevalent PDMS surface variants, namely liquid-infused (SLIPS/LIS), elastomeric, and liquid-like (SOCAL) surfaces. To understand, optimize, and tune the wetting properties of these PDMS surfaces, we review and compare their similarities and differences by discussing (i) the chemical and molecular structure, and (ii) the static and dynamic wetting behavior. We also provide (iii) an overview of methods and techniques to characterize PDMS-based surfaces and their wetting behavior. The static and dynamic wetting ridge is given particular attention, as it dominates energy dissipation, adhesion, and friction of sliding drops and influences the durability of the surfaces. We also discuss special features such as cloaking and wetting-induced phase separation. Key challenges and opportunities of these three surface variants are outlined
Energieeffizienz in der Abluftreinigung (Schweinehaltung)
In der Studie wurden in der Intensivtierhaltung eingesetzte Abluftreinigungsanlagen einschlieĂlich LĂŒftungssystem hinsichtlich ihres technischen Aufbaus, ihres Energieeinsatzes und ihrer Energieeffizienz untersucht. Auf dieser Basis wurden VorschlĂ€ge zur Steigerung der Energieeffizienz erarbeitet. Aus der Betrachtung von zwei sĂ€chsischen Schweinehaltungsbetrieben konnten Hinweise fĂŒr die Anlagenbetreiber und fĂŒr 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 (GâČ and Gâł, 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
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