Showing papers by "Friedrich Gunther Mugele published in 2013"

Salt Dependent Stability of Stearic Acid Langmuir–Blodgett Films Exposed to Aqueous Electrolytes

Abstract: We use contact angle goniometry, imaging ellipsometry, and atomic force microscopy to study the stability and wettability of Langmuir–Blodgett (LB) monolayers of stearic acid on silica substrates, upon drying and exposure to aqueous solutions of varying salinity. The influences of Ca2+ and Na+ ions are compared by varying their concentrations, both in the subphase before the LB transfer, and in the droplets to which the dried LB layers are exposed. Ca2+ ions in the subphase are found to enhance the stability, leading to contact angles up to 100°, as compared to less than 5° for Na+. Consistent with the macroscopic wettability, AFM images show almost intact films with few holes exposing bare substrate when prepared in the presence of Ca2+, while subphases containing Na+ result in large areas of bare substrate after exposure to aqueous drops. The observations on varying the composition of the droplets corroborate the stabilizing effect of Ca2+. We attribute these findings to the cation-bridging ability of Ca2+ ions, which can bind the negatively charged stearate groups to the negatively charged substrates. We discuss the relevance of our findings in the context of enhanced oil recovery

An Open FOAM-based electro-hydrodynamical model

Abstract: In this work we present an OpenFOAM-based electro-hydrodynamics model. We have incorporated Gauss’s Law and a free charge transport equation in an existing Volume-Of-Fluid model delivered with OpenFOAM. With the model it is possible to simulate the interaction of a fluid-fluid interface with an electric field, using perfect dielectric liquids as well as conductive liquids. Our implementation is validated using two approaches; first off, we simulate two vertically stacked liquids subjected to an electric field. The potential distribution is compared to the analytical result and a good comparison is found, using either a combination of two perfect dielectric liquids and a dielectric and a conductive liquid. For the latter case, we also find a good agreement with the theoretical relation for the surface charges. Secondly, we have simulated an artificial charge bump in a conductive liquid, and verify the charge transport equation by comparing the decay of the charge over time. Again, a good agreement with theory is found. Finally, we present a test case where we break up a droplet in an electric field. With that, we have shown to have a reliable EHD implementation in the OpenFOAM framework. For our future goal to simulate electrowetting phenomena, we require the electric field distribution in a coupled solid phase as well; we will use the current EHD model and extend it to a multi-region solver to include those effects.

Electrostatic potential wells for manipulations of drops in microchannels

Abstract: Precise control and manipulation of individual drops are crucial in many lab-on-a-chip applications. We present a novel hybrid concept for channel-based discrete microfluidics with integrated electrical actuation functionality. By combining the high throughput from channel-based microfluidics with individual drop control from electrical actuation we harvest the strengths of both worlds. The tunable strength of the electrostatic forces enables a wide range of drop manipulations, such as on-demand trapping and release, guiding, and sorting of drops. The conditions for trapping can be predicted using a simple model that balances the retaining electrostatic force with the hydrodynamic drag force.

An OpenFoam-based electro-hydordynamical model

Abstract: In this work we present an OpenFOAM-based electro-hydrodynamics model. We have incorporated Gauss’s Law and a free charge transport equation in an existing Volume-Of-Fluid model delivered with OpenFOAM. With the model it is possible to simulate the interaction of a fluid-fluid interface with an electric field, using perfect dielectric liquids as well as conductive liquids. Our implementation is validated using two approaches; first off, we simulate two vertically stacked liquids subjected to an electric field. The potential distribution is compared to the analytical result and a good comparison is found, using either a combination of two perfect dielectric liquids and a dielectric and a conductive liquid. For the latter case, we also find a good agreement with the theoretical relation for the surface charges. Secondly, we have simulated an artificial charge bump in a conductive liquid, and verify the charge transport equation by comparing the decay of the charge over time. Again, a good agreement with theory is found. Finally, we present a test case where we break up a droplet in an electric field. With that, we have shown to have a reliable EHD implementation in the OpenFOAM framework. For our future goal to simulate electrowetting phenomena, we require the electric field distribution in a coupled solid phase as well; we will use the current EHD model and extend it to a multi-region solver to include those effects.