90 research outputs found
Lattice Boltzmann Simulations of Droplet formation in confined Channels with Thermocapillary flows
Based on mesoscale lattice Boltzmann simulations with the "Shan-Chen" model,
we explore the influence of thermocapillarity on the break-up properties of
fluid threads in a microfluidic T-junction, where a dispersed phase is injected
perpendicularly into a main channel containing a continuous phase, and the
latter induces periodic break-up of droplets due to the cross-flowing.
Temperature effects are investigated by switching on/off both positive/negative
temperature gradients along the main channel direction, thus promoting a
different thread dynamics with anticipated/delayed break-up. Numerical
simulations are performed at changing the flow-rates of both the continuous and
dispersed phases, as well as the relative importance of viscous forces, surface
tension forces and thermocapillary stresses. The range of parameters is broad
enough to characterize the effects of thermocapillarity on different mechanisms
of break-up in the confined T-junction, including the so-called "squeezing" and
"dripping" regimes, previously identified in the literature. Some simple
scaling arguments are proposed to rationalize the observed behaviour, and to
provide quantitative guidelines on how to predict the droplet size after
break-up.Comment: 18 pages, 9 figure
Droplet Transport System And Methods
Embodiments of droplet transport systems and methods are disclosed for levitating and transporting single or encapsulated droplets using thermocapillary convection. One method embodiment, among others comprises providing a droplet of a first liquid; and applying thermocapillary convection to the droplet to levitate and move the droplet.Georgia Tech Research Corporatio
Azimuthal instability of the radial thermocapillary flow around a hot bead trapped at the water-air interface
We investigate the radial thermocapillary flow driven by a laser-heated
microbead in partial wetting at the water-air interface. Particular attention
is paid to the evolution of the convective flow patterns surrounding the hot
sphere as the latter is increasingly heated. The flow morphology is nearly
axisymmetric at low laser power P. Increasing P leads to symmetry breaking with
the onset of counter-rotating vortex pairs. The boundary condition at the
interface, close to no-slip in the low-P regime, turns about stress-free
between the vortex pairs in the high-P regime. These observations strongly
support the view that surface-active impurities are inevitably adsorbed on the
water surface where they form an elastic layer. The onset of vortex pairs is
the signature of a hydrodynamic instability in the layer response to the
centrifugal forced flow. Interestingly, our study paves the way for the design
of active colloids able to achieve high-speed self-propulsion via vortex pair
generation at a liquid interface
Influence of Mechanical and Thermal Boundary Conditions on Stabilizing/Destabilizing Mechanisms in Evaporating Liquid Films
Liquid films, evaporating or non-evaporating, are ubiquitous in nature and technology. The dynamics of evaporating liquid films is a study applicable in several industries such as water recovery, heat exchangers, crystal growth, drug design etc. The theory describing the dynamics of liquid films crosses several fields such as engineering, mathematics, material science, biophysics and volcanology to name a few.
Interfacial instabilities typically manifest by the undulation of an interface from a presumed flat state or by the onset of a secondary flow state from a primary quiescent state or both. To study the instabilities affecting liquid films, an evaporating/non-evaporating Newtonian liquid film is subject to a perturbation. Numerical analysis is conducted on configurations of such liquid films being heated on solid surfaces in order to examine the various stabilizing and destabilizing mechanisms that can cause the formation of different convective structures. These convective structures have implications towards heat transfer that occurs via this process. Certain aspects of this research topic have not received attention, as will be obvious from the literature review.
Static, horizontal liquid films on solid surfaces are examined for their resistance to long wave type instabilities via linear stability analysis, method of normal modes and finite difference methods. The spatiotemporal evolution equation, available in literature, describing the time evolution of a liquid film heated on a solid surface, is utilized to analyze various stabilizing/destabilizing mechanisms affecting evaporating and non-evaporating liquid films. The impact of these mechanisms on the film stability and structure for both buoyant and non-buoyant films will be examined by the variation of mechanical and thermal boundary conditions.
Films evaporating in zero gravity are studied using the evolution equation. It is found that films that are stable to long wave type instabilities in terrestrial gravity are prone to destabilization via long wave instabilities in zero gravity
Phase change and complex phenomena in drops and bubbles of pure and binary fluids
Evaporation, wetting and multiphase flows of drops and bubbles are everyday life
phenomena with potential impact in many industrial, biological, medical or
engineering applications. The understanding and controlling of the physical and
chemical mechanisms governing these phenomena have become of paramount
importance. This thesis encompasses three topics: evaporation of sessile droplets of
polymer solutions, the role of thermocapillarity on self-rewetting fluid dynamics and
migration of bubbles in liquid flows.
Firstly, the evaporative behaviour of sessile droplets of aqueous polymer solutions and
the effect of different molecular weights on the drying process has been studied. Drop
shape analysis allowed monitoring the evolution of all stages during drying and
indicating the transitions between stages. The mechanisms taking place during the
crucial stages of pinning and depinning were illustrated, revealing the effects of
adhesion and contact line friction forces on the final morphology of the dried
polymeric deposits. Additionally, the effect of varying substrates from hydrophilic to
hydrophobic was examined demonstrating the importance of interfacial interaction
phenomena.
The initial spreading dynamics of binary alcohol mixtures (and pure liquids) deposited
on different substrates in partially wetting situations, under non-isothermal conditions
was systematically investigated. Moreover, the temporal and spatial thermal dynamics
within pure droplets and alcohol mixtures using IR thermography revealed the
existence of characteristic thermal patterns due to thermal and/or solutal instabilities.
The contribution of the Marangoni effect as an important heat transport mechanism
within the evaporating droplets was investigated.
The motion of buoyancy-driven bubbles in a vertical microchannel and the significant
role of thermocapillarity was reported in this series of experiments. The behaviour of
the bubbles in self-rewetting fluid flows departed considerably from that of pure
liquids flows. Furthermore, heat transfer coefficient calculations in the single and two
phase flows demonstrated that the presence of Marangoni (surface tension) stresses
resulted in the enhancement of the heat transfer distribution in the self-rewetting fluid
flows compared with the pure ones
Efficient simulation of complex capillary effects in advanced manufacturing processes using the finite volume method
The accurate representation of surface tension driven flows in multiphase systems is considered a challeng- ing problem to resolve numerically. Although there have been extensive works in the past that have presented approaches to resolve these so called Marangoni flows at the phase boundaries, the question of how to efficiently resolve the interface in a universal and conservative manner remains largely open in comparison. Such problems are of high practical relevance in many manufacturing processes, especially in the microfluidic regime where capillary effects dominate the local force equilibria. In this work, we present a freely available numerical solver based on the Finite Volume Method that is able to resolve arbitrarily complex, incompressible multiphase systems with the mentioned physics at phase boundaries. An efficient solution with respect to the number of degrees of freedom can be obtained by either using high order WENO stencils or by employing adaptive cell refinement. We demonstrate the capabilities of the solver by investigating a model benchmark case as well as a single track laser melting process that is highly relevant within laser additive manufacturing
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Influence of boundary slip on the optimal excitations in thermocapillary driven spreading
Thin liquid films driven to spread on homogeneous surfaces by thermocapillarity can undergo frontal breakup and parallel rivulet formation with well-defined wavelength. Previous modal analyses have relieved the well-known divergence in stress that occurs at a moving contact line by matching the front region to a precursor film. Because the linearized disturbance operator is non-normal, a generalized, nonmodal analysis is required to probe film stability at all times. The effect of the contact line model on nonmodal stability has not been previously investigated. This work examines the influence of boundary slip on thermocapillary driven spreading using a transient stability analysis, which recovers the conventional modal results in the long-time limit. In combination with earlier work on thermocapillary driven spreading, this study verifies that the dynamics and stability of this system are rather insensitive to the choice of contact line model and that the leading eigenvalue is physically determinant, thereby assuring results that agree with the eigenspectrum. Modal results for the flat precursor film model are reproduced with appropriate choice of slip coefficient and contact line slope
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