(160a) Layered Fluid-Fluid Interfaces Confined in Microfluidics

Fluid-fluid interactions are present in a vast number of multiphase processes in biological^1,2 and chemical fields^3,4. Understanding fluid-fluid mass transport at a interface can help: (i) optimize the manufacture of chemicals, commodities, and pharmaceuticals^5–8, (ii) monitor methane dissolved in groundwater caused by fracturing activities^9,10, (iii) study methane hydrate formation¹¹, and (iv) investigate methane release from melting permafrost ^12,13. Aside from the mentioned applications, preliminary studies of the methane/water, and non-polar/aqueous film thickness by direct probe has not yet been investigated on a flow microscale (scale of porous or grooved surfaces). In fluid-fluid mass transfer limited systems, the overall performance is closely related to how molecules are transported¹⁴, and how they are affected by attractive and repulsive forces and reactive states present near the interface^15,16. On the surroundings of an interface, intermolecular and viscous forces play an important role in defining the concentration profiles’ shape^17,18. At the molecular level, the shape is often a manifestation of the excess of free energy, leading to an increase or decrease of the local density^19,20. The molecular interactions are normally manifested macroscopically as the balance of forces tangent to the surface¹⁹.

The nature of these can be attractive or repulsive towards the interface. Understanding how these forces intervene in mass transfer is fundamental to better understand liquid-vapor processes. One of today’s challenges is related to scale-up molecular simulations (MD) to real world scenarios^21,22.

A microfluidic apparatus (semi-flow) associated with in situ Raman spectroscopy is reported for the investigation of toluene-, diethyl ether-, and xylenes- and methane-water interactions. The fluid/fluid phases interfaces exist in static contact by force balance; one is the moving phase across the microfluidic channel, other is trapped inside a µReservoir. The behavior of confined interfaces was characterized using high-resolution in situ1D, 2D, and 3D Raman spectroscopy.

It was found that rarefaction, mixture, thin film, and shockwave layers are present. For non-polar/aqueous interfaces, the Schlieren pattern was observed for all solvent pairs. The bulk-to-bulk region was characterized by water rarefaction (17.5 to 23.2 μm), mixing (3.4 to 6.4 μm), and hydrophobic shockwaves (34.9 to 38.8 μm). For gas/liquid interfaces, the apparent CH₄ and H₂O concentrations are reported for different Reynolds numbers (Re). The mixture interface is comprised different thicknesses, varying from 19 to 57 µm. The results indicate that the mixture layer thickness (δ) increases with Re. Remarkably, traditional transport film theory for unconfined interfaces does not explain this phenomena at confined interfaces. At the interface, it is expected molecular diffusivity enhancement, and/or an evaporation film.

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