Coating is an essential step in adjusting the surface properties of materials. Superhydrophobic coatings with contact angles greater than 150° and roll-off angles below 10° for water have been developed, based on low-energy surfaces and roughness on the nano- and micrometer scales. However, these surfaces are still wetted by organic liquids such as surfactant-based solutions, alcohols, or alkanes. Coatings that are simultaneously superhydrophobic and superoleophobic are rare. We designed an easily fabricated, transparent, and oil-rebounding superamphiphobic coating. A porous deposit of candle soot was coated with a 25-nanometer-thick silica shell. The black coating became transparent after calcination at 600°C. After silanization, the coating was superamphiphobic and remained so even after its top layer was damaged by sand impingement.

Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating

We proposed a novel separation method, which is the first report using ion concentration polarization (ICP) to separate particles continuously. We analyzed the electrical forces that cause the repulsion of particles in the depletion region formed by ICP. Using the electrical repulsion, micro- and nano-sized particles were separated based on their electrophoretic mobilities. Because the separation of particles was performed using a strong electric field in the depletion region without the use of internal electrodes, it offers the advantages of simple, low-cost device fabrication and bubble-free operation compared with conventional continuous electrophoretic separation methods, such as miniaturizing free-flow electrophoresis (μ-FFE). This separation device is expected to be a useful tool for separating various biochemical samples, including cells, proteins, DNAs and even ions.

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Correction: Corrigendum: Ion concentration polarization-based continuous separation device using electrical repulsion in the depletion region

Ion concentration polarization (ICP) is a fundamental electrokinetic process that occurs near a perm-selective membrane under dc bias. Overall process highly depends on the current transportation mechanisms such as electro-convection, surface conduction and diffusioosmosis and the fundamental characteristics can be significantly altered by external parameters, once the permselectivity was fixed. In this work, a new ICP device with a bifurcated current path as for the enhancement of the surface conduction was fabricated using a polymeric nanoporous material. It was protruded to the middle of a microchannel, while the material was exactly aligned at the interface between two microchannels in a conventional ICP device. Rigorous experiments revealed out that the propagation of ICP layer was initiated from the different locations of the protruded membrane according to the dominant current path which was determined by a bulk electrolyte concentration. Since the enhancement of surface conduction maintained the stability of ICP process, a strong electrokinetic flow associated with the amplified electric field inside ICP layer was significantly suppressed over the protruded membrane even at condensed limit. As a practical example of utilizing the protruded device, we successfully demonstrated a non-destructive micro/nanofluidic preconcentrator of fragile cellular species (i.e. red blood cells).

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Ion Concentration Polarization by Bifurcated Current Path.

In this work, the first-ever membrane-based single unit operation that enables gravity driven, on-demand separation of various oil-water mixtures is developed. Using this methodology, the on-demand separation of free oil and water, oil-in-water emulsions, and water-in-oil emulsions is demonstrated, with ≥99.9% separation efficiency. A scaled-up apparatus to separate larger quantities (several liters) of oil-water emulsions is also developed.

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On-Demand Separation of Oil-Water Mixtures

Superhydrophobic surfaces are advantageous for a cost-effective maintenance of a variety of surfaces. The combination of micro and nano-sized roughness increases the contact angle of water such that water droplets cannot adhere but roll off. Therefore, superhydrophobic coatings are self-cleaning and anticorrosive. If the superhydrophobic surface were even transparent, the range of possible applications could be expanded to glass-based substrates such as goggles or windshields and, equally important, prevent an efficiency degradation of solar cells by pollution accumulation. Moreover mechanical robustness is also particularly critical because the dual scale roughness can easily be destroyed irreversibly leading to a rapid decrease of the contact angle and an increase of contact angle hysteresis. We use porous silica capsules as key components to build lotus leaf-like superhydrophobic surfaces. The latter are highly transparent as well as mechanically and thermally stable, see Fig. 1, left. When used as transparent coatings for organic solar cells they leave their performance unaffected, see Fig. 1, right.

Transparent, Thermally Stable and Mechanically Robust Superhydrophobic Surfaces Made from Porous Silica Capsules

We present a visualization of the predicted instability in ionic conduction from a binary electrolyte into a charge selective solid. This instability develops when a voltage greater than critical is applied to a thin layer of copper sulfate flanked by a copper anode and a cation selective membrane. The current-voltage dependence exhibits a saturation at the limiting current. With a further increase of voltage, the current increases, marking the transition to the overlimiting conductance. This transition is mediated by the appearing vortical flow that increases with the applied voltage.

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Direct Observation of a Nonequilibrium Electro-Osmotic Instability

We demonstrate that the equilibrium shape of the composite interface between superhydrophobic surfaces and drops in the superhydrophobic Cassie state under electrowetting is determined by the balance of the Maxwell stress and the Laplace pressure. Energy barriers due to pinning of contact lines at the edges of the hydrophobic pillars control the transition from the Cassie to the Wenzel state. Barriers due to the narrow gap between adjacent pillars control the lateral propagation of the Wenzel state. We demonstrate how reversible switching between the two wetting states can be achieved locally using suitable surface and electrode geometries

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Electrical switching of wetting states on superhydrophobic surfaces: a route towards reversible Cassie-to-Wenzel transitions.

We study possible mechanisms of the transition from the Cassie state to the Wenzel state on superhydrophobic surfaces under the influence of electric fields as a function of the aspect ratio and the wettability of the surface. A simple analytical model for axisymmetric cavities and small deflections of the liquid menisci within the cavities reveals the existence of a novel electric-field–driven instability of the liquid surface. Fully self-consistent calculations of both electric-field distribution and surface profiles show that this instability evolves from a global one towards a local Taylor cone-like instability for increasing aspect ratio of the cavities. A two-dimensional map is derived indicating the prevalence of the interfacial instability as compared to the depinning scenario of the three-phase contact line, which is well known from ordinary superhydrophobic surfaces

https://iopscience.iop.org/article/10.1209/0295-5075/93/56001/pdf

G. Manukyan

Papers

Direct Observation of a Nonequilibrium Electro-Osmotic Instability

Electrical switching of wetting states on superhydrophobic surfaces: a route towards reversible Cassie-to-Wenzel transitions.

Electric-field–driven instabilities on superhydrophobic surfaces

Electric field driven instabilities on superhydrophobic surfaces