2,838 research outputs found
Droplets on liquids and their long way into equilibrium
The morphological paths towards equilibrium droplets during the late stages
of the dewetting process of a liquid film from a liquid substrate is
investigated experimentally and theoretically. As liquids, short chained
polystyrene (PS) and polymethyl-methacrylate (PMMA) are used, which can be
considered as Newontian liquids well above their glass transition temperatures.
Careful imaging of the PS/air interface of the droplets during equilibration by
\emph{in situ} scanning force microscopy and the PS/PMMA interface after
removal of the PS droplets reveal a surprisingly deep penetration of the PS
droplets into the PMMA layer. Droplets of sufficiently small volumes develop
the typical lens shape and were used to extract the ratio of the PS/air and
PS/PMMA surface tensions and the contact angles by comparison to theoretical
exact equilibrium solutions of the liquid/liquid system. Using these results in
our dynamical thin-film model we find that before the droplets reach their
equilibrium they undergo several intermediate stages each with a well-defined
signature in shape. Moreover, the intermediate droplet shapes are independent
of the details of the initial configuration, while the time scale they are
reached depend strongly on the droplet volume. This is shown by the numerical
solutions of the thin-film model and demonstrated by quantitative comparison to
experimental results
Influence of Slip on the Plateau-Rayleigh Instability on a Fibre
The Plateau-Rayleigh instability of a liquid column underlies a variety of
fascinating phenomena that can be observed in everyday life. In contrast to the
case of a free liquid cylinder, describing the evolution of a liquid layer on a
solid fibre requires consideration of the solid-liquid interface. In this
article, we revisit the Plateau-Rayleigh Instability of a liquid coating a
fibre by varying the hydrodynamic boundary condition at the fibre-liquid
interface, from no-slip to slip. While the wavelength is not sensitive to the
solid-liquid interface, we find that the growth rate of the undulations
strongly depends on the hydrodynamic boundary condition. The experiments are in
excellent agreement with a new thin film theory incorporating slip, thus
providing an original, quantitative and robust tool to measure slip lengths
Spray dryer modelling
Both spraying and drying are critical to spray dryer performance. Models are developed
which explain the very different performance of a spray dryer when large droplets of
film forming materials are created using a Rayleigh resonance atomiser. The droplet
diameter distribution from this "Acoustic Atomiser" is inadequately described by
previously reported spray size distribution functions, but well described by the Stable
distribution. The alpha parameter of this distribution was found to tend towards the
Gaussian limit for low viscosity fluids and the Lorentz limit with increasing viscosity,
consistent with behaviour as a simple and damped forced harmonic oscillator
respectively, and hence with the physics of the atomisation process. Droplet drying
kinetics dominate model predictions. A device using an ultramicrobalance to measure
droplet drying kinetics with unprecedented accuracy and range has been designed. A
scaling and residence time analysis model was able to account for experimental spray
dryer observations. Sprayability even of complex fluids is predicted adequately by the
Ohnesorge diagram, provided that extensional rather than shear viscosity is plotted. A
new determination of the transient apparent extensional viscosity from arbitrary CaBER
time-diameter curves has successfully been used for fluids too complex to analyse using
previously published rheological models
Stratospheric constituent measurements using UV solar occultation technique
The photochemistry of the stratospheric ozone layer was studied as the result of predictions that trace amounts of pollutants can significantly affect the layer. One of the key species in the determination of the effects of these pollutants is the OH radical. A balloon flight was made to determine whether data on atmospheric OH could be obtained from lower resolution solar spectra obtained from high altitude during sunset
Free surface films of binary liquid mixtures
Model-H is used to describe structures found in the phase separation in films of binary liquid mixture that have a
surface that is free to deform and also may energetically prefer one
of the components. The film rests on a solid smooth substrate that has no preference for any component. On the one hand the study
focuses on static aspects by investigating steady states that are
characterised by their concentration and film height profiles. A large variety of such states are systematically analysed by numerically constructing bifurcation diagrams in dependence of a number of control parameters. The numerical method used is based on minimising the free energy functional at given constraints within a finite element method for a variable domain shape. The structure of the bifurcation diagrams is related to the symmetry properties of the individual solutions on the various branches. On the other hand the full time dependent model-H is linearised about selected steady states, in particular, the laterally invariant, i.e.\ layered states. The resulting dispersion relations are discussed and related to the corresponding bifurcation points of the steady states. In general, the results do well agree and confirm each other.
The described analysis is performed for a number of important cases
whose comparison allows us to gain an advanced understanding of the
system behaviour: We distinguish the critical and off-critical case
that correspond to zero and non-zero mean concentration,
respectively. In the critical case the investigation focuses on (i)
flat films without surface bias, (ii) flat films with surface bias,
(iii) height-modulated films without surface bias, and (iv)
height-modulated films with surface bias. Each case is analysed for several mean film heights and (if applicable) energetic bias at the free surface using the lateral domain size as main control parameter. Linear stability analyses of layered films and symmetry considerations are used to understand the structures of the
determined bifurcation diagrams. For off-critical mixtures our study is more restricted. There we consider height-modulated films without and with surface bias for several mean film heights and (if applicable) energetic bias employing the mean concentration as main control parameter
Controlled topological transitions in thin film phase separation
In this paper the evolution of a binary mixture in a thin-film geometry with
a wall at the top and bottom is considered. By bringing the mixture into its
miscibility gap so that no spinodal decomposition occurs in the bulk, a slight
energetic bias of the walls towards each one of the constituents ensures the
nucleation of thin boundary layers that grow until the constituents have moved
into one of the two layers. These layers are separated by an interfacial region
where the composition changes rapidly. Conditions that ensure the separation
into two layers with a thin interfacial region are investigated based on a
phase-field model. Using matched asymptotic expansions a corresponding
sharp-interface problem for the location of the interface is established.
It is then argued that this newly created two-layer system is not at its
energetic minimum but destabilizes into a controlled self-replicating pattern
of trapezoidal vertical stripes by minimizing the interfacial energy between
the phases while conserving their area. A quantitative analysis of this
mechanism is carried out via a thin-film model for the free interfaces, which
is derived asymptotically from the sharp-interface model.Comment: Submitted 23/12/201
Liquid meniscus friction on a wet plate: Bubbles, lamellae and foams
Many microfluidics devices, coating processes or diphasic flows involve the
motion of a liquid meniscus on a wet wall. This motion induces a specific
viscous force, that exhibits a non-linear dependency in the meniscus velocity.
We propose a review of the theoretical and experimental work made on this
viscous force, for simple interfacial properties. The interface is indeed
assumed either perfectly compressible (mobile interface) or perfectly
incompressible (rigid interface). We show that, in the second case, the viscous
force exerted by the wall on the meniscus is a combination of two power laws,
scaling like and , with the capillary number. We
provide a prediction for the stress exerted on a foam sliding on a wet solid
and compare it with experimental data, for the incompressible case
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Experimental Study of Diesel-Fuel Droplet Impact on a Similarly Sized Polished Spherical Heated Solid Particle
The head-to-head impact of diesel-fuel droplets on a polished spherical brass target has been investigated experimentally. High-speed imaging was employed to visualize the impact process for wall surface temperatures and Weber and Reynolds numbers in the ranges of 140–340 °C, 30–850, and 210–1135, respectively. The thermohydrodynamic outcome regimes occurring for the aforementioned ranges of parameters were mapped on a We–T diagram. Seven clearly distinguishable postimpact outcome regimes were identified, which are conventionally called the coating, splash, rebound, breakup–rebound, splash–breakup–coating, breakup–coating, and splash–breakup–rebound regimes. In addition, the effects of the Weber number and surface temperature on the wettability dynamics were examined; the temporal variations of the dynamic contact angle, dimensionless spreading diameter, and liquid film thickness forming on the solid particle were measured and are reported
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