Gas-perfused porous surfaces: Actuating droplets without levitating them

Porous surfaces are widely used for wetting phenomena control exploiting either the gas entrapped on the porous network, or their ability to be infused, or impregnated by lubricants. Still gas entraps (pockets) tend to collapse after prolonged exposure to liquid environments, or at high liquid pressures, while lubricants tend to cloak the droplets and gradually dry out. In our study we combine the two aforementioned approaches and we use porous media and have them perfused with gas to dynamically actuate, and manipulate droplets on their surfaces. By adjusting the backpressure, i.e. the pressure from the rear side of the porous medium, the droplet may be actuated, and its downward velocity may be controlled without completely levitate it. This entails low values of backpressure, in the order of few mbar, depending on the porous network characteristics. In this work we are going to present the basic principles of this approach, and demonstrate it in various applications including droplet impingement, valving in digital microfluidics, and droplet logic operations. The mechanisms of actuation have been studied by means of simulations encompassing the momentum conservation and the continuity equations along with the Cahn−Hilliard phasefield equations in a 2D computational domain. The droplet actuation mechanism involves depinning of the receding contact line and movement by means of forward wave propagation reaching the front of the droplet, yielding to a forward skipping of the droplet. New experimental results with highly viscous liquids and on non-symmetric surfaces will be shown

In-between full levitation and stable Cassie-Baxter: A range of interesting wetting states enabled by gas perfusion through porous media

Actuation of droplets and manipulation of their mobility on surfaces is very crucial for a wide range of applications related to interfacial phenomena1-15. In treating such challenges various methods have been proposed and demonstrated using respective trigger signals to interact with the liquid phase, the solid phase or the ambient, including electrical magnetic, thermal, acoustical or combinations. For porous hydrophobic surfaces in particular droplet actuation may be enabled also by gas perfusion through the porous body. This was mainly achieved by applying the adequate gas flow rate in order to depin the initially quiescent droplet from the porous surface resting on the solid faction (partially wetting, Cassie-Baxter state), to a fully levitated state on which the droplet move frictionless (non-wetting, Leidenfrost-like state). This actuation required high flow rates and therefore high amount of energy. In this work we explore the states in-between these two extremes and prove that actuation and mobility manipulation may be delivered at ultra-low gas flow rates, corresponding to pressures up to few mbar and accordingly to ultra-low energy consumption. The actuation mechanism was followed employing the continuity equation and the equations of momentum transfer that are coupled with the Volume of Fluid (VOF) method, to track the shape of the droplet in both 2D and 3D calculations. Applications for water droplets on plane surfaces, confined surfaces (fluidics) as well as for viscous fluids will be provided

A new microfluidic pressure-controlled Field Effect Transistor (pFET) in digital fluidic switch operation mode

Lab-on-Chip is currently considered the technology with the potential to revolutionize future biochemical analysis providing miniaturized, low-reagent volume microchips as an alternative to traditional benchtop analysis. Automated control of droplet flow is currently a key objective in microfluidics research, aiming for droplet logic microfluidic circuits. To this end, microfluidic research has been following the electronics paradigm, with several digital fluidic components being demonstrated towards the realization of digital fluidic circuits for automated liquid control and delivery. In this work, we introduce a new concept of microfluidic pressure controlled field-effect transistors (pFETs), towards droplet logic operations. Using a fluidic with porous and hydrophobic walls, the inherently pinned plug depins by pressure application through the porous wall (backpressure), thus enabling the actuation and the downward transportation of the plug due the action of gravity. This concept resembles the logic operation of a metal–oxide–semiconductor field-effect transistor (MOSFET). The pFET operating parameters are thus defined in a manner analogous to MOSFET digital switches and their dependence on the channel width is studied also for the first time. The successful operation of pFET devices for droplet logic operation is verified in continuous ON/OFF cycles, achieving OFF-ON and ON-OFF switching times under 1 s (0.864 s and 0.841 s respectively) and therefore promising rapid liquid switching times, comparable to electronic circuit ones

Estimating the trace of the matrix inverse by interpolating from the diagonal of an approximate inverse

A number of applications require the computation of the trace of a matrix that is implicitly available through a function. A common example of a function is the inverse of a large, sparse matrix, which is the focus of this paper. When the evaluation of the function is expensive, the task is computationally challenging because the standard approach is based on a Monte Carlo method which converges slowly. We present a different approach that exploits the pattern correlation, if present, between the diagonal of the inverse of the matrix and the diagonal of some approximate inverse that can be computed inexpensively. We leverage various sampling and fitting techniques to fit the diagonal of the approximation to the diagonal of the inverse. Depending on the quality of the approximate inverse, our method may serve as a standalone kernel for providing a fast trace estimate with a small number of samples. Furthermore, the method can be used as a variance reduction method for Monte Carlo in some cases. This is decided dynamically by our algorithm. An extensive set of experiments with various technique combinations on several matrices from some real applications demonstrate the potential of our method. (C) 2016 Published by Elsevier Inc

Nanocrystalline La0.8Sr0.2MnyFe1-yO3-δ perovskites and their oxygen deficiency correlation with their oxygen permeation and CO oxidation properties

The present paper aims at the investigation of the interrelation between the composition, microstructural properties and the oxygen deficiency of La0.8Sr0.2MnyFe1-yO3-δ (y=0-1) type perovskites, influencing their properties as membranes or oxidation catalysts

GLYCEROL STEAM REFORMING ON NICKEL LOADED APATITE-TYPE LANTHANUM SILICATES

In the present paper for the first time it is reported the application of apatite-type lanthanum silicates in the steam reforming of glycerol reaction. The La9.83Si5Fe0.75Al0.25O26±d, apatite oxide prepared by solid state synthesis was applied as a supporting material for a 8wt% Ni catalyst. Apatite oxide and catalyst (fresh, reduced and used) samples were characterized by means of the XRD, SEM, TEM techniques. Catalytic testing experiments were performed using a fixed bed reactor at temperatures ranging from 400 to 700 oC with a feed consisting of C3H8Ο3 (20% v/v.) and H2O in the liquid phase

Fabrication of ceramic hollow beads by phase inversion method

In this study ceramic hollow beads with a diameter of around 1-2mm but a uniform size were prepared by a phase inversion method and subsequent thermal treatment. During phase inversion a homogeneous polymer solution is transformed into a two-phase system in which a solidified polymer phase forms a porous structure, while a liquid phase, poor in polymer, fills the pores. In the current study a desired amount of ceramic powder e.g aluminium oxide (Al2O3), yttria-stabilized zirconia (8-YSZ) is added to a polymer solution, which consists of polymer polyethersulfone (PESf) in N-methyl-2-pyrrolidone (NMP). The ceramic/ polymer/ solvent system can be seen as a suspension. Once immersed into a non-solvent (water) for the polymer which is miscible with the solvent, solvent/non solvent exchange takes place, leading to the precipitation of the polymer phase. Ceramic particles are immobilized by the polymer once precipitation takes place. Thermal treatment removes the polymer, forming inorganic beads of porous ceramic structure1. The features of the as formed porous macrostructure of the beads can be largely determined by adjusting parameters of the phase inversion process. The samples were characterized using scanning electron microscope (SEM), mercury intrusion porosimetry and thermogravimetric analysis1-6. The as formed ceramic beads show high degree of porosity and a hierarchical microstructure7-8. This microstructure is maintained even after thermal treatment at elevated temperatures (>1200oC). Critical factors affecting the structure and the properties of the ceramic beads such as viscosity of the slurry, ceramic powder/polymer ratio and sintering temperature were studied

Droplet mobility manipulation on porous media using backpressure

Wetting phenomena on hydrophobic surfaces are strongly related to the volume and pressure of gas pockets residing at the solid–liquid interface. In this study, we explore the underlying mechanisms of droplet actuation and mobility manipulation when backpressure is applied through a porous medium under a sessile pinned droplet. Reversible transitions between the initially sticky state and the slippery states are thus incited by modulating the backpressure. The sliding angles of deionized (DI) water and ethanol in DI water droplets of various volumes are presented to quantify the effect of the backpressure on the droplet mobility. For a 50 μL water droplet, the sliding angle decreases from 45 to 0° when the backpressure increases to ca. 0.60 bar. Significantly smaller backpressure levels are required for lower surface energy liquids. We shed light on the droplet actuation and movement mechanisms by means of simulations encompassing the momentum conservation and the continuity equations along with the Cahn–Hilliard phase-field equations in a 2D computational domain. The droplet actuation mechanism entails depinning of the receding contact line and movement by means of forward wave propagation reaching the front of the droplet. Eventually, the droplet skips forward. The contact line depinning is also corroborated by analytical calculations based on the governing vertical force balance, properly modified to incorporate the effect of the backpressure. “This document is the Accepted Manuscript version of a Published Work that appeared in final form in LANGMUIR, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acs.langmuir.6b00900

A new microfluidic pressure-controlled Field Effect Transistor (pFET) in digital fluidic switch operation mode

Lab-on-Chip is currently considered the technology with the potential to revolutionize future biochemical analysis providing miniaturized, low-reagent volume microchips as an alternative to traditional benchtop analysis. Automated control of droplet flow is currently a key objective in microfluidics research, aiming for droplet logic microfluidic circuits. To this end, microfluidic research has been following the electronics paradigm, with several digital fluidic components being demonstrated towards the realization of digital fluidic circuits for automated liquid control and delivery. In this work, we introduce a new concept of microfluidic pressure controlled field-effect transistors (pFETs), towards droplet logic operations. Using a fluidic with porous and hydrophobic walls, the inherently pinned plug depins by pressure application through the porous wall (backpressure), thus enabling the actuation and the downward transportation of the plug due the action of gravity. This concept resembles the logic operation of a metal–oxide–semiconductor field-effect transistor (MOSFET). The pFET operating parameters are thus defined in a manner analogous to MOSFET digital switches and their dependence on the channel width is studied also for the first time. The successful operation of pFET devices for droplet logic operation is verified in continuous ON/OFF cycles, achieving OFF-ON and ON-OFF switching times under 1 s (0.864 s and 0.841 s respectively) and therefore promising rapid liquid switching times, comparable to electronic circuit ones