Application note
Oxygen transport across slippery and curved gas-liquid interfaces
using phosphorescence lifetime imaging
e transport phenomena at interfaces oen
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. e So Matter Group at the University of Twente (e
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.
1
Application note
e 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.
e LIFA-X, consisting of a LED light source
and a Lambert Instruments intensified CCD
camera (TRiCAM1 ), 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 microchan-
nels and deoxygenated RTDP aqueous solution was the working liquid flowing past the
transversely aligned oxygen bubbles (figures 1c
and 2).
e lifetime of RTDP across the liquid side
microchannel height was measured at different axial locations (figure 2). e 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.
e 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 spaceaveraged total flux of oxygen absorption was
calculated from these local flux profiles.
Figure 2: Successive lifetime fields in axial position x. Quantitative visualization of the increasing boundary layer thickness along downstream flow (45 μl/min). e flow direction
is from le to right. e color bar refers to the lifetime, which is given in nanoseconds. e
dashed arrows indicate the axial positions at which the local oxygen concentration profiles
across the microchannel height are obtained.
1
e experiment was performed with a LI2 CAM, the predecessor of the current TRiCAM
2
Application note
e experimental results obtained by
FLIM revealed an additional mass transfer resistance to gas dissolution on slippery microbubbles at short contact times of gas and liquid.
is 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. e results indi-
cate 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, So Matter, 2013, 9, 11098-11106
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