The transport phenomena at interfaces often determine or limit the overall performance of processes. Direct investigations of interfacial transport of momentum, mass and heat at the interfaces in micron scale are highly appreciable for further optimization of various micro and macroscale technologies. The Soft Matter Group at the University of Twente (The Netherlands), lead by Prof. Dr. Rob Lammertink, aims at gaining a better understanding of transport phenomena near boundaries, so that various processes such as desalination, separation of species and (photo)catalytic reactions can be improved.

Microfluidics offer an ideal platform allowing for the integration of 'controllable' surfaces and direct measurements of transport phenomena near them. Elif Karatay used a microfluidic bubble mattress during her PhD studies at the University of Twente, fabricating one of the microchannel walls as a superhydrophobic surface consisting of alternating solid walls and micro-bubbles (figure 1). She experimentally measured and numerically estimated the dynamic mass transfer of gas absorption at stable gas-liquid interfaces for short contacting times.

Figure 1. Microfluidic bubble mattress (a). Numerical simulations of dissolved oxygen in water with identical settings in FLIM experiments (b), the color bar shows the oxygen concentration. Lifetime field resolved by FLIM superimposed on the bright-field microscopy image showing bubbles protruding into the water (c), the color bar indicates fluorescence lifetime in nanoseconds.

Figure 1. Microfluidic bubble mattress (a). Numerical simulations of dissolved oxygen in water with identical settings in FLIM experiments (b), the color bar shows the oxygen concentration. Lifetime field resolved by FLIM superimposed on the bright-field microscopy image showing bubbles protruding into the water (c), the color bar indicates fluorescence lifetime in nanoseconds.

The rate of gas absorption into water was studied by in situ measurements of dissolved oxygen concentration profiles in aqueous solutions flowing over oxygen bubbles by frequency-domain fluorescence-lifetime imaging (FD-FLIM) microscopy.  FLIM was used to image the oxygen concentration using an oxygen sensitive luminescent dye, ruthenium tris(2,20 -dipyridyl) dichloride hexahydrate (RTDP), obeying a mono-decay function as quenched by oxygen. For the FLIM experiments, an eXtended Lambert Instruments FLIM Attachment (LIFA-X) system was used on a Zeiss Axio Observer inverted microscope. The LIFA-X, consisting of a LED light source and a Lambert Instruments intensified CCD camera (TRiCAM), was operated in gated mode to obtain phosphorescence lifetimes.

For calibration measurements, lifetimes of the RTDP were measured in oxygen-free (N2 saturated), aerated and oxygen-saturated aqueous solutions, where the micro-bubbles were established by nitrogen, air and oxygen gases, respectively. During the mass transfer experiments, oxygen gas micro-bubbles were established at the boundary of the microchannels and deoxygenated RTDP aqueous solution was the working liquid flowing past the transversely aligned oxygen bubbles (figures 1c and 2).

The lifetime of RTDP across the liquid side microchannel height was measured at different axial locations (figure 2). The bubble interface profiles and locations were experimentally determined by locating the minimum lifetime data measured near the hybrid wall. Figure 2} shows the successive lifetime fields measured with FLIM at different axial locations along the same microchannel embedded with micro-bubbles where the increasing boundary layer thickness along the downstream flow can be observed. Here, in figure 2c, the thickness of the diffusion boundary layer is 23% of the microchannel height H, and does not extend further into the microchannel due to a relatively large Reynolds number Re of 7.5.

Figure 2. Successive lifetime fields in axial position x. Quantitative visualization of the increasing boundary layer thickness along downstream flow (45 μl/min). The flow direction is from left to right. The color bar refers to the lifetime, which is given in nanoseconds. The dashed arrows indicate the axial positions at which the local oxygen concentration profiles across the microchannel height are obtained.

Figure 2. Successive lifetime fields in axial position x. Quantitative visualization of the increasing boundary layer thickness along downstream flow (45 μl/min). The flow direction is from left to right. The color bar refers to the lifetime, which is given in nanoseconds. The dashed arrows indicate the axial positions at which the local oxygen concentration profiles across the microchannel height are obtained.

The local profiles of oxygen flux from the gaseous phase to the liquid phase were obtained using the local concentration gradients measured by FLIM. Furthermore the space-averaged total flux of oxygen absorption was calculated from these local flux profiles.

The experimental results obtained by FLIM revealed an additional mass transfer resistance to gas dissolution on slippery micro-bubbles at short contact times of gas and liquid. This mass transfer resistance results in slower gas absorption than predicted by the conventionally accepted equilibrium interface model, Henry’s Law. Whereas the experimental results are in good agreement with the numerical results obtained from simulations considering non-equilibrium conditions. The results indicate that the phase equilibrium state may not be established at short contacting times.

Acknowledgements

Graphs courtesy of Dr. Elif Karatay, Stanford University, USA

References

E. Karatay, A.P. Tsai and R.G.H. Lammertink, Rate of gas absorption on a slippery bubble mattress, Soft Matter, 2013, 9, 11098-11106