Guided motion of emulsions is studied via combined experimental and theoretical investigations.The focus of the work is on basic issues related to driving forces generated via a step-wise (abrupt) change in wetting properties of the substrate along a given spatial direction. Experiments on binary emulsions unambiguously show that selective wettability of the one of the fluid components (water in our experiments) with respect to the two different parts of the substrate is sufficient in order to drive the separation process. These studies are accompanied by approximate analytic arguments as well as lattice Boltzmann computer simulations, focusing on effects of a wetting gradient on internal droplet dynamics as well as its relative strength compared to volumetric forces driving the fluid flow. These theoretical investigations show qualitatively different dependence of wetting gradient induced forces on contact angle and liquid volume in the case of an open substrate as opposed to a planar channel. In particular, for the parameter range of our experiments, slit geometry is found to give rise to considerably higher separation forces as compared to open substrate.
A novel lab‐on‐a‐chip device based on a stack of four nanolayers for emulsion separation and simultaneous detection is introduced. Emulsions are separated on top of chemically patterned surfaces while the process is monitored using semiconductor sensors.
A novel single silicon thin film field-effect-transistor (FET) is developed for use as a sensor to monitor transport and chemical properties of liquids in microfluidic systems. The sensor elements which are compatible with existing (bio-)chemical sensor schemes based on ion-sensitive-field-effect-transistors (ISFET) can detect capillary filling speed and level in aqueous solutions. Using a transitor based detection scheme, this approach has the potential to enable high speed flow detection on large scales with high spatial resolution. The prototype devices presented in the present study have been fabricated by using a simple cost-efficient route for circuit board lithography. The thin film FET device characteristics are discussed and a theoretical model for liquid transport detection based on FETs is developed. Typical experimental data are also presented.
Emulsion separation is of high relevance for filtration applications, liquid-liquid-partitioning of biomolecules like proteins and recovery of products from droplet microreactors. Selective interaction of various components of an emulsion with substrates is used to design microfluidic flow chambers for efficient separation of emulsions into their individual components. Our lab-on-a-chip device consists of an emulsion separation cell with an integrated silicon sensor chip, the latter allowing the detection of liquid motion via the field-effect signal. Thus, within our lab-on-a-chip device, emulsions can be separated while the separation process is monitored simultaneously. For emulsion separation a surface energy step gradient, namely a sharp interface between the hydrophobic and hydrophilic parts of the separation chamber, is used. The key component of the lab-on-a-chip system is a multilayer and multifunctional nanofilm structure which not only provides the surface energy step gradient for emulsion separation but also constitutes the functional parts of the field-effect transistors. The proof-of-principle was performed using a model emulsion consisting of immiscible aqueous and organic solvent components. Droplet coalescence was identified as a key aspect influencing the separation process, with quite different effects during separation on open surfaces as compared to slit geometry. For a detailed description of this observation, an analytical model was derived and lattice Boltzmann computer simulations were performed. By use of grazing incidence small angle x-ray scattering (GISAXS) interfacial nanostructures during gold nanoparticle deposition in a flow field were probed to demonstrate the potential of GISAXS for in situ investigations during flow.
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