2,313 research outputs found
The Marangoni flow of soluble amphiphiles
Surfactant distribution heterogeneities at a fluid/fluid interface trigger
the Marangoni effect, i.e. a bulk flow due to a surface tension gradient. The
influence of surfactant solubility in the bulk on these flows remains
incompletely characterized. Here we study Marangoni flows sustained by
injection of hydrosoluble surfactants at the air/water interface. We show that
the flow extent increases with a decrease of the critical micelle
concentration, i.e. the concentration at which these surfactants self-assemble
in water. We document the universality of the surface velocity field and
predict scaling laws based on hydrodynamics and surfactant physicochemistry
that capture the flow features.Comment: 5 pages, 4 figures, submitte
Marangoni flow in freely suspended liquid films
We demonstrate controlled material transport driven by temperature gradients
in thin freely suspended smectic films. The films with submicrometer
thicknesses and lateral extensions of several millimeters were studied in
microgravity during suborbital rocket flights. In-plane temperature gradients
cause two specific Marangoni effects, directed flow and convection patterns. At
low gradients, practically thresholdless, flow transports material with a
normal (negative) temperature coefficient of the surface tension,
, from the hot to the cold film edge. That material accumulates
at the cold film border. In materials with positive temperature coefficient,
, the reverse transport from the cold to the hot edge is
observed. We present a model that describes the effect quantitatively.Comment: 5 pages, 5 figure
Collective dynamics of chemically active particles trapped at a fluid interface
Chemically active colloids generate changes in the chemical composition of
their surrounding solution and thereby induce flows in the ambient fluid which
affect their dynamical evolution. Here we study the many-body dynamics of a
monolayer of active particles trapped at a fluid-fluid interface. To this end
we consider a mean-field model which incorporates the direct pair interaction
(including also the capillary interaction which is caused specifically by the
interfacial trapping) as well as the effect of hydrodynamic interactions
(including the Marangoni flow induced by the response of the interface to the
chemical activity). The values of the relevant physical parameters for typical
experimental realizations of such systems are estimated and various scenarios,
which are predicted by our approach for the dynamics of the monolayer, are
discussed. In particular, we show that the chemically-induced Marangoni flow
can prevent the clustering instability driven by the capillary attraction.Comment: 8 pages, 2 figure
A bouncing oil droplet in a stratified liquid and its sudden death
Droplets can self-propel when immersed in another liquid in which a
concentration gradient is present. Here we report the experimental and
numerical study of a self-propelling oil droplet in a vertically stratified
ethanol/water mixture: At first, the droplet sinks slowly due to gravity, but
then, before having reached its density matched position, jumps up suddenly.
More remarkably, the droplet bounces repeatedly with an ever increasing jumping
distance, until all of a sudden it stops after about 30 min. We identify the
Marangoni stress at the droplet/liquid interface as responsible for the
jumping: its strength grows exponentially because it pulls down ethanol-rich
liquid, which in turn increases its strength even more. The jumping process can
repeat because gravity restores the system. Finally, the sudden death of the
jumping droplet is also explained. Our findings have demonstrated a type of
prominent droplet bouncing inside a continuous medium with no wall or sharp
interface.Comment: 6 pages, 4 figure
Phase coexistence in a monolayer of active particles induced by Marangoni flows
Thermally or chemically active colloids generate thermodynamic gradients in
the solution in which they are immersed and thereby induce hydrodynamic flows
that affect their dynamical evolution. Here we study a mean-field model for the
many-body dynamics of a monolayer of active particles located at a fluid-fluid
interface. In this case, the activity of the particles creates long-ranged
Marangoni flows due to the response of the interface, which compete with the
direct interaction between the particles. For the most interesting case of a
soft repulsion that models the electrostatic or magnetic interparticle
forces, we show that an "onion-like" density distribution will develop within
the monolayer. For a sufficiently large average density, two-dimensional phase
transitions (freezing from liquid to hexatic, and melting from solid to
hexatic) should be observable in a radially stratified structure. Furthermore,
the analysis allows us to conclude that, while the activity may be too weak to
allow direct detection of such induced Marangoni flows, it is relevant as a
collective effect in the emergence of the experimentally observable spatial
structure of phase coexistences noted above. Finally, the relevance of these
results for potential experimental realizations is critically discussed.Comment: 11 page
Patterns Formation in Drying Drops of Blood
The drying of a drop of human blood exhibits coupled physical mechanisms,
such as Marangoni flow, evaporation and wettability. The final stage of a whole
blood drop evaporation reveals regular patterns with a good reproducibility for
a healthy person. Other experiments on anaemic and hyperlipidemic people were
performed, and different patterns were revealed. The flow motion inside the
blood drop is observed and analyzed with the use of a digital camera: the
influence of the red blood cells (RBCs) motion is revealed at the drop
periphery as well as its consequences on the final stage of drying. The
mechanisms which lead to the final pattern of the dried blood drops are
presented and explained on the basis of fluid mechanics in conjunction with the
principles of haematology. The blood drop evaporation process is evidenced to
be driven only by Marangoni flow. The same axisymetric pattern formation is
observed, and can be forecast for different blood drop diameters. The
evaporation mass flux can be predicted with a good agreement, assuming only the
knowledge of the colloids mass concentration.Comment: 1 page + conference APS 2011 (1 movie for the gallery + 1 movie for
ArXiv
Evaporating pure, binary and ternary droplets: thermal effects and axial symmetry breaking
The Greek aperitif Ouzo is not only famous for its specific anise-flavored
taste, but also for its ability to turn from a transparent miscible liquid to a
milky-white colored emulsion when water is added. Recently, it has been shown
that this so-called Ouzo effect, i.e. the spontaneous emulsification of oil
microdroplets, can also be triggered by the preferential evaporation of ethanol
in an evaporating sessile Ouzo drop, leading to an amazingly rich drying
process with multiple phase transitions [H. Tan et al., Proc. Natl. Acad. Sci.
USA 113(31) (2016) 8642]. Due to the enhanced evaporation near the contact
line, the nucleation of oil droplets starts at the rim which results in an oil
ring encircling the drop. Furthermore, the oil droplets are advected through
the Ouzo drop by a fast solutal Marangoni flow. In this article, we investigate
the evaporation of mixture droplets in more detail, by successively increasing
the mixture complexity from pure water over a binary water-ethanol mixture to
the ternary Ouzo mixture (water, ethanol and anise oil). In particular,
axisymmetric and full three-dimensional finite element method simulations have
been performed on these droplets to discuss thermal effects and the complicated
flow in the droplet driven by an interplay of preferential evaporation,
evaporative cooling and solutal and thermal Marangoni flow. By using image
analysis techniques and micro-PIV measurements, we are able to compare the
numerically predicted volume evolutions and velocity fields with experimental
data. The Ouzo droplet is furthermore investigated by confocal microscopy. It
is shown that the oil ring predominantly emerges due to coalescence
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