67 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
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Mesoporous Coatings with Simultaneous LightâTriggered Transition of Water Imbibition and Droplet Coalescence
A systematic study of gating water infiltration and condensation into ceramic nanopores by carefully adjusting the wetting properties of mesoporous films using photoactive spiropyran is presented. Contact angle measurements from the side reveal significant changes in wettability after irradiation due to spiropyran/merocyanine-isomerization, which induce a wetting transition from dry to wet pores. The change in wettability allows the control of water imbibition in the nanopores and is reflected by the formation of an imbibition ring around a droplet. Furthermore, the photoresponsive wettability is able to overcome pinning effects and cause a movement of a droplet contact line, facilitating droplet coalescence, as recorded by high-speed imaging. The absorbed light not only effectuates droplet merging but also causes flows inside the drop due to heat absorption by the spiropyran, which results in gradients in the surface tension. IR imaging and particle tracking is used to investigate the heat absorption and temperature-induced flows, respectively. These flows can be used to manipulate, for example, molecular movement inside water and deposition inside solid mesoporous materials and are therefore of great importance for nanofluidic devices as well as for future water management concepts, which include filtering by imbibition and collection by droplet coalescence. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb
Forced dynamic dewetting of structured surfaces: Influence of surfactants
We analyse the dewetting of printing plates for gravure printing with
well-defined gravure cells. The printing plates were mounted on a rotating
horizontal cylinder that is half immersed in an aqueous solution of the anionic
surfactant sodium 1-decanesulfonate. The gravure plates and the presence of
surfactants serve as one example of a real-world dewetting situation. When
rotating the cylinder, a liquid meniscus was partially drawn out of the liquid
forming a dynamic contact angle at the contact line. The dynamic contact angle
is decreased on a structured surface as compared to a smooth one. This is due
to contact line pinning at the borders of the gravure cells. Additionally,
surfactants tend to decrease the dynamic receding contact angle. We consider
the interplay between these two effects. We compare the height differences of
the meniscus on the structured and unstructured area as function of dewetting
speeds. The height difference increases with increasing dewetting speed. With
increasing size of the gravure cells this height difference and the induced
changes in the dynamic contact angle increased. By adding surfactant, the
height difference and the changes in the contact angle for the same surface
decreased. We further note that although the liquid dewets the printing plates
some liquid is always left in the gravure cell. At high enough surfactant
concentrations or high enough dewetting speed, the dynamic contact angles in
the structured surface approach those in flat surfaces. We conclude that
surfactant reduces the influence of surface structure on dynamic dewetting
Reversible magnetomechanical collapse: virtual touching and detachment of rigid inclusions in a soft elastic matrix
Soft elastic composite materials containing particulate rigid inclusions in a
soft elastic matrix are candidates for developing soft actuators or tunable
damping devices. The possibility to reversibly drive the rigid inclusions
within such a composite together to a close-to-touching state by an external
stimulus would offer important benefits. Then, a significant tuning of the
mechanical properties could be achieved due to the resulting mechanical
hardening. For a long time, it has been argued whether a virtual touching of
the embedded magnetic particles with subsequent detachment can actually be
observed in real materials, and if so, whether the process is reversible. Here,
we present experimental results that demonstrate this phenomenon in reality.
Our system consists of two paramagnetic nickel particles embedded at finite
initial distance in a soft elastic polymeric gel matrix. Magnetization in an
external magnetic field tunes the magnetic attraction between the particles and
drives the process. We quantify the scenario by different theoretical tools,
i.e., explicit analytical calculations in the framework of linear elasticity
theory, a projection onto simplified dipole-spring models, as well as detailed
finite-element simulations. From these different approaches, we conclude that
in our case the cycle of virtual touching and detachment shows hysteretic
behavior due to the mutual magnetization between the paramagnetic particles.
Our results are important for the design and construction of reversibly tunable
mechanical damping devices. Moreover, our projection on dipole-spring models
allows the formal connection of our description to various related systems,
e.g., magnetosome filaments in magnetotactic bacteria.Comment: 14 pages, 7 figure
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Sperm-Driven Micromotors Moving in Oviduct Fluid and Viscoelastic Media
Biohybrid micromotors propelled by motile cells are fascinating entities for autonomous biomedical operations on the microscale. Their operation under physiological conditions, including highly viscous environments, is an essential prerequisite to be translated to in vivo settings. In this work, a sperm-driven microswimmer, referred to as a spermbot, is demonstrated to operate in oviduct fluid in vitro. The viscoelastic properties of bovine oviduct fluid (BOF), one of the fluids that sperm cells encounter on their way to the oocyte, are first characterized using passive microrheology. This allows to design an artificial oviduct fluid to match the rheological properties of oviduct fluid for further experiments. Sperm motion is analyzed and it is confirmed that kinetic parameters match in real and artificial oviduct fluids, respectively. It is demonstrated that sperm cells can efficiently couple to magnetic microtubes and propel them forward in media of different viscosities and in BOF. The flagellar beat pattern of coupled as well as of free sperm cells is investigated, revealing an alteration on the regular flagellar beat, presenting an onâoff behavior caused by the additional load of the microtube. Finally, a new microcap design is proposed to improve the overall performance of the spermbot in complex biofluids. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
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Transparent model concrete with tunable rheology for investigating flow and particle-migration during transport in pipes
The article describes the adaption and properties of a model concrete for detailed flow studies. To adapt the yield stress and plastic viscosity of the model concrete to the corresponding rheological properties of real concrete, the model concrete is made of a mixture of glass beads and a non-Newtonian fluid. The refractive index of the non-Newtonian fluid is adjusted to the refractive index of the glass beads by the addition of a further constituent. The rheological properties of the model concrete are characterised by measurements in concrete rheometers. Finally, the first exemplary results from experiments with the model concrete are presented, which give incipient impressions of the complex internal dynamics in flowing concrete
Mechanofluorescent Polymer Brush Surfaces that Spatially Resolve Surface Solvation
Polymer brushes, consisting of densely end-tethered polymers to a surface, can exhibit rapid and sharp conformational transitions due to specific stimuli, which offer intriguing possibilities for surface-based sensing of the stimuli. The key toward unlocking these possibilities is the development of methods to readily transduce signals from polymer conformational changes. Herein, we report on single-fluorophore integrated ultrathin (<40 nm) polymer brush surfaces that exhibit changing fluorescence properties based on polymer conformation. The basis of our methods is the change in occupied volume as the polymer brush undergoes a collapse transition, which enhances the effective concentration and aggregation of the integrated fluorophores, leading to a self-quenching of the fluorophoresâ fluorescence and thereby reduced fluorescence lifetimes. By using fluorescence lifetime imaging microscopy, we reveal spatial details on polymer brush conformational transitions across complex interfaces, including at the airâwaterâsolid interface and at the interface of immiscible liquids that solvate the surface. Furthermore, our method identifies the swelling of polymer brushes from outside of a direct droplet (i.e., the polymer phase with vapor above), which is controlled by humidity. These solvation-sensitive surfaces offer a strong potential for surface-based sensing of stimuli-induced phase transitions of polymer brushes with spatially resolved output in high resolution
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