  The motion of a long gas bubble in a confined capillary tube is ubiquitous in
a wide range of engineering and biological applications. While the
understanding of the deposited thin viscous film near the tube wall in
Newtonian fluids is well developed, the deposition dynamics in commonly
encountered non-Newtonian fluids remains much less studied. Here, we
investigate the dynamics of a confined bubble moving in shear-thinning fluids
with systematic experiments, varying the zero-shear-rate capillary number
$Ca_0$ in the range of $O(10^{-3}-10^2)$ considering the zero-shear-rate
viscosity. The thickness of the deposited liquid film, the bubble speed and the
bubble front/rear menisci are measured, which are further rationalized with the
recent theoretical studies based on appropriate rheological models. Compared
with Newtonian fluids, the film thickness decreases for both the carboxymethyl
cellulose and Carbopol solutions when the shear-thinning effect dominates. We
show that the film thickness follows the scaling law from
\citet{aussillous2000quick} with an effective capillary number $Ca_e$,
considering the characteristic shear rate in the film as proposed by
\citet{picchi2021motion}. $Ca_e$ is calculated by the Carreau number and the
power-law index from the Carreau-Yasuda rheological model. The shear-thinning
effect also influences the bubble speed and delays the transition to the
parabolic region in the bubble front and rear menisci. In particular, a high
degree of undulations on the bubble surface results in intricate rear viscosity
distribution for the rear meniscus and the deviation between the experiments
and theory may require a further investigation to resolve the axial velocity
field. Our study may advance the fundamental understandings and engineering
guidelines for coating processes involving thin-film flows and non-Newtonian
fluids.
