INTEC   05402
INSTITUTO DE DESARROLLO TECNOLOGICO PARA LA INDUSTRIA QUIMICA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Dip coating process with both soluble and insoluble surfactant. Flow pattern transition and inertial effects
Autor/es:
DIEGO M. CAMPANA; MARIA DELIA GIAVEDONI; FERNANDO A. SAITA
Lugar:
Praga
Reunión:
Congreso; 19th International Congress of Chemical and Process Engineering- CHISA 2010 ECCE7; 2010
Institución organizadora:
CSCHI
Resumen:
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Dip coating has been one
of the extensively studied coating processes since the pioneering
work carried out by Landau and Levich (1942). This process deposits a
thin uniform film on a solid by withdrawing it at a constant speed
from a bath containing the liquid to be coated. Landau and Levich
obtained an analytical expression predicting the thickness of the
film formed at very small substrate speeds; i.e. when viscous and
capillary forces balance each other. Since then, numerous works has
been performed to assess the influence on the film thickness of
forces that are usually present, but were not consider by Landau and
Levich (viz. gravity, inertia, elastic forces originated by surface
active agents, etc.).
The
effects produced by the presence of surfactants, particularly on the
thickness of the film coated, have been plentifully analyzed during
the last two decades. Experimental works carried out on fibers
(Ramdane and Quéré, 1997, Shen et al., 2002) as well as those
carried out on flat plates (Krechetnikov and Homsy, 2005) indicate
that thicker coating films are obtained due to their presence.
Theoretical asymptotic analysis (Park, 1991) and numerical solutions
of 2-D governing equations (Campana et al., 2009) also indicate that
the thickening factor ratio (i.e.
the ratio between the predicted film thickness and the Landau-Levich
result for a uniformly distributed surfactant) is always larger than
one. An exception is the numerical work of Krechetnikov and Homsy
(2006); these authors found, in contradiction with their own
experiments, thickening factors smaller than one. They assume that
their numerical results are correct and
explain the discrepancy stating that pure hydrodynamic models are not
enough to describe the dip coating process when elastic forces are
present.
In
this work we present film thickness predictions when dip coating
is implemented on a flat substrate in the presence of both soluble
and insoluble surfactants. Our numerical model solves the governing
equations of the 2-D problem simultaneously, i.e. velocities,
pressures, surfactant concentrations and the spatial location of the
points representing the discrete version of the free surface are all
determined at once. This is a key difference regarding the technique
employed by Krechetnikov and Homsy that uses an iterative procedure
to update the shape and location of the free surface. Our model was
validated with the asymptotic results obtained by Park (1991) and
also with the experiments performed by Krechetnikov and Homsy (2005).
Results
obtained show a wealth of information: they depict how the flow
patterns change when the surfactant solubility diminishes. For highly
soluble surfactants the streamlines present the usual single
stagnation point located on the free surface and close to the dynamic
meniscus. As the solubility decreases the stagnation point moves
along the free surface and away from the dynamic meniscus; at the
same time a second stagnation point arises in the bulk near the
dynamic meniscus together with a swirl located between the free
surface and the streamline that ends at the free surface. Finally,
when the solubility becomes zero the stagnation point at the free
surface disappears; at this instant the flow pattern shows the motion
of two separated liquid streams (one moving with the substrate and
the other one with the free surface) enclosing a recirculation-flow
region. The two streams merge at the stagnation point constituting
the final film to be deposited on the substrate.
We
also analyzed the influence of inertia on film thickness. Park
(1991) already showed that viscous forces prevail over the elastic
ones when Capillary number is increased (i.e. when the coating speed
is increased); thus, the thickening factor ratio decreases until the
Landau and Levich solution is recovered. Our predictions indicate
that inertia plays a role similar to viscous forces; however, since
inertia forces vary with speed in a quadratic way, the Landau and
Levich solution is reached much faster.
The
foregoing results indicate that a pure hydrodynamic model correctly
reproduces the features observed
in conventional deep coating; therefore, the present analysis is now
being extended to fiber coating.