(518b) Three-Dimensional Surfactant-Covered Flows of Thin Liquid Films on Rotating Cylinders | AIChE

(518b) Three-Dimensional Surfactant-Covered Flows of Thin Liquid Films on Rotating Cylinders


Kumar, S. - Presenter, University of Minnesota-Twin Cities
Li, W., University of Minnesota
The coating of discrete objects is an important but poorly understood
step in the manufacturing of a broad variety of products. An important model problem
is the flow of a thin liquid film on a rotating cylinder, where instabilities can arise
and compromise coating uniformity. In this work, we use lubrication theory and flow
visualization experiments to study the influence of surfactant on these flows. Two
coupled evolution equations describing the variation of film thickness and concentration
of insoluble surfactant as a function of time, the angular coordinate, and the
axial coordinate are solved numerically. The results show that surface-tension forces
arising from both axial and angular variations in the angular curvature drive flows
in the axial direction that tend to smooth out free-surface perturbations and lead
to a stable speed window in which axial perturbations do not grow. The presence
of surfactant leads to Marangoni stresses that can cause the stable speed window to
disappear by driving flow that opposes the stabilizing flow. In addition, Marangoni
stresses tend to reduce the spacing between droplets that form at low rotation rates,
and reduce the growth rate of rings that form at high rotation rates. Flow visualization
experiments yield observations that are qualitatively consistent with predictions
from linear stability analysis and the simulation results. The visualizations also indicate
that surfactants tend to suppress dripping, slow the development of free-surface
perturbations, and reduce the shifting and merging of rings and droplets, allowing
more time for solidifying coatings in practical applications.