3,555 research outputs found
Phase-field modeling droplet dynamics with soluble surfactants
Using lattice Boltzmann approach, a phase-field model is proposed for simulating droplet motion with soluble surfactants. The model can recover the Langmuir and Frumkin adsorption isotherms in equilibrium. From the equilibrium equation of state, we can determine the interfacial tension lowering scale according to the interface surfactant concentration. The model is able to capture short-time and long-time adsorption dynamics of surfactants. We apply the model to examine the effect of soluble surfactants on droplet deformation, breakup and coalescence. The increase of surfactant concentration and attractive lateral interaction can enhance droplet deformation, promote droplet breakup, and inhibit droplet coalescence. We also demonstrate that the Marangoni stresses can reduce the interface mobility and slow down the film drainage process, thus acting as an additional repulsive force to prevent the droplet coalescence
Modelling of surfactant-driven front instabilities in spreading bacterial colonies
The spreading of bacterial colonies at solid-air interfaces is determined by
the physico-chemical properties of the involved interfaces. The production of
surfactant molecules by bacteria is a widespread strategy that allows the
colony to efficiently expand over the substrate. On the one hand, surfactant
molecules lower the surface tension of the colony, effectively increasing the
wettability of the substrate, which facilitates spreading. On the other hand,
gradients in the surface concentration of surfactant molecules result in
Marangoni flows that drive spreading. These flows may cause an instability of
the circular colony shape and the subsequent formation of fingers. In this
work, we study the effect of bacterial surfactant production and substrate
wettability on colony growth and shape within the framework of a hydrodynamic
thin film model. We show that variations in the wettability and surfactant
production are sufficient to reproduce four different types of colony growth,
which have been described in the literature, namely, arrested and continuous
spreading of circular colonies, slightly modulated front lines and the
formation of pronounced fingers
Computational analysis of single rising bubbles influenced by soluble surfactant
This paper presents novel insights about the influence of soluble surfactants
on bubble flows obtained by Direct Numerical Simulation (DNS). Surfactants are
amphiphilic compounds which accumulate at fluid interfaces and significantly
modify the respective interfacial properties, influencing also the overall
dynamics of the flow. With the aid of DNS local quantities like the surfactant
distribution on the bubble surface can be accessed for a better understanding
of the physical phenomena occurring close to the interface. The core part of
the physical model consists in the description of the surfactant transport in
the bulk and on the deformable interface. The solution procedure is based on an
Arbitrary Lagrangian-Eulerian (ALE) Interface-Tracking method. The existing
methodology was enhanced to describe a wider range of physical phenomena. A
subgrid-scale (SGS) model is employed in the cases where a fully resolved DNS
for the species transport is not feasible due to high mesh resolution
requirements and, therefore, high computational costs. After an exhaustive
validation of the latest numerical developments, the DNS of single rising
bubbles in contaminated solutions is compared to experimental results. The full
velocity transients of the rising bubbles, especially the contaminated ones,
are correctly reproduced by the DNS. The simulation results are then studied to
gain a better understanding of the local bubble dynamics under the effect of
soluble surfactant. One of the main insights is that the quasi-steady state of
the rise velocity is reached without ad- and desorption being necessarily in
local equilibrium
Phase field modelling of surfactants in multi-phase flow
A diffuse interface model for surfactants in multi-phase flow with three or
more fluids is derived. A system of Cahn-Hilliard equations is coupled with a
Navier-Stokes system and an advection-diffusion equation for the surfactant
ensuring thermodynamic consistency. By an asymptotic analysis the model can be
related to a moving boundary problem in the sharp interface limit, which is
derived from first principles. Results from numerical simulations support the
theoretical findings. The main novelties are centred around the conditions in
the triple junctions where three fluids meet. Specifically the case of local
chemical equilibrium with respect to the surfactant is considered, which allows
for interfacial surfactant flow through the triple junctions
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