TRANSPORT AT THE MICROSCOPIC INTERFACE – TRAM

Bubble mattress in a chip

Interfacial gas/liquid transport in microfluidic systems is generally improved by employing micro-structured surfaces. Since these surfaces are often characterised by effective wall slip, they can be used to reduce hydrodynamic friction drag. However, as analytical and numerical studies show, the extent of reduction depends on the microscopic geometry of the hybrid interface [1-3]. It is anticipated that the resulting convective flow at the slippery interface also can lead to an enhancement of interfacial mass transport.

To verify experimentally the dependency of effective slip on the bubble protrusion angle ϑ and the surface porosity, we fabricated novel microfluidic devices in which a stable and controllable bubble mattress could be established (Figure 1). Using µPIV we solved the flow field above the bubble mattress with high resolution, which were subsequently used to calculate the effective slip lengths.

Figure 1 A microfluidic device has been fabricated (A) that allows the establishment of a stable and slippery bubble mattress in a chip (B). By using μPIV we were able to validate experimentally how the effective slip length b depends on the bubble protrusion angle ϑ and the surface porosity (C).

Karatay, E., Haase, A.S., Visser, C.W., Sun, C., Lohse, D., Tsai, P.A. & R.G.H Lammertink. Control of slippage with tunable bubble mattresses. Proceedings of the National Academy of Sciences 110 (21): 8422–8426 (2013). http://dx.doi.org/10.1073/pnas.1304403110

Enhancing mass transport with a bubble mattress

To investigate how a partially slippery bubble mattress can enhance interfacial mass transport, we performed a 2-dimensional numerical study using COMSOL Multiphysics (Figure 2). It follows that with a bubble mattress, compared to a non-slippery flat wall, interfacial solute transport can be enhanced up to 20%, while simultaneously the hydrodynamic drag is reduced with 10%. A theoretical analysis, based on the local Péclet number, reveals that the effect of a bubble mattress on momentum and mass transport strongly depends on the bubble size. When the bubbles are smaller than 10 µm, the bubble mattress can be considered as a non-slippery and solute-saturated wall.

Figure 2 To investigate the effect of a slippery bubble mattress on interfacial mass transport, a 2-dimensional numerical model is built that resembles the bubble mattress in Figure 1.

Running research

Current research involves various directions. It is predicted that high-shear gas/liquid contacting can increase effective slip by almost two orders of magnitude as a result of film formation (Figure 3A) [4]. We would like to show this experimentally. Also liquid/liquid contacting has our interest (B), in particular the mutual influence flow patterns in both phases can have on each other and thereby on the effective slip. We also aim to study shear stresses in at the interface in situ by making use of stress-sensitive markers (C). This will be beneficial to investigate the effect of a Marangoni stress at bubble surfaces.

Figure 3 Film formation as a result of high-shear flow (A). Figure from [4]. Liquid/liquid contacting (B). Maragoni stress is expected to counteract wall slippage, which may be exploited for further control of effective slip (C). Figure adapted from [5, 6].

[1]

Davis, A.M.J. & E. Lauga. Physics of Fluids 21: 011701 (2009).

[2]

Steinberger, A., Cottin-Bizonne C., Kleimann P. & E. Charlaix. Nature Materials 2: 237-240 (2003).

[3]

Hyväluoma, J. & J. Harting. Physical Review Letters 100: 246001 (2008).

[4]

Gao, P. & J.J. Feng. Physics of Fluids 21: 102102 (2009).

[5]

Tasoglu S., Demirci U. & M. Muradoglu. Physics of Fluids 20: 040805 (2008).

[6]

Hosny, N.A. et al. Proceedings of the National Academy of Sciences 110 (23): 9225-9230 (2013).