68 research outputs found
Optimization of Patterned Surfaces for Improved Superhydrophobicity Through Cost-Effective Large-Scale Computations
The growing need for creating surfaces with specific wetting properties, such
as superhyrdophobic behavior, asks for novel methods for their efficient
design. In this work, a fast computational method for the evaluation of
patterned superhyrdophobic surfaces is introduced. The hydrophobicity of a
surface is quantified in energy terms through an objective function. The
increased computational cost led to the parallelization of the method with the
Message Passing Interface (MPI) communication protocol that enables
calculations on distributed memory systems allowing for parametric
investigations at acceptable time frames. The method is demonstrated for a
surface consisting of an array of pillars with inverted conical (frustum)
geometry. The parallel speedup achieved allows for low cost parametric
investigations on the effect of the fine features (curvature and slopes) of the
pillars on the superhydophobicity of the surface and consequently for the
optimization of superhyrdophobic surfaces.Comment: 18 pages, 18 figure
The normal field instability under side-wall effects: comparison of experiments and computations
We consider a single spike of ferrofluid, arising in a small cylindrical
container, when a vertically oriented magnetic field is applied. The height of
the spike as well as the surface topography is measured experimentally by two
different technologies and calculated numerically using the finite element
method. As a consequence of the finite size of the container, the numerics
uncovers an imperfect bifurcation to a single spike solution, which is forward.
This is in contrast to the standard transcritical bifurcation to hexagons,
common for rotational symmetric systems with broken up-down symmetry. The
numerical findings are corroborated in the experiments. The small hysteresis
observed is explained in terms of a hysteretic wetting of the side wall.Comment: accepted to New Journal of Physic
Double Rosensweig instability in a ferrofluid sandwich structure
We consider a horizontal ferrofluid layer sandwiched between two layers of
immiscible non-magnetic fluids. In a sufficiently strong vertical magnetic
field the flat interfaces between magnetic and non-magnetic fluids become
unstable to the formation of peaks. We theoretically investigate the interplay
between these two instabilities for different combinations of the parameters of
the fluids and analyze the evolving interfacial patterns. We also estimate the
critical magnetic field strength at which thin layers disintegrate into an
ordered array of individual drops
Stability analysis of polarized domains
Polarized ferrofluids, lipid monolayers and magnetic bubbles form domains
with deformable boundaries. Stability analysis of these domains depends on a
family of nontrivial integrals. We present a closed form evaluation of these
integrals as a combination of Legendre functions. This result allows exact and
explicit formulae for stability thresholds and growth rates of individual
modes. We also evaluate asymptotic behavior in several interesting limits.Comment: 12 pages, 3 figures, Late
Wave Number of Maximal Growth in Viscous Magnetic Fluids of Arbitrary Depth
An analytical method within the frame of linear stability theory is presented
for the normal field instability in magnetic fluids. It allows to calculate the
maximal growth rate and the corresponding wave number for any combination of
thickness and viscosity of the fluid. Applying this method to magnetic fluids
of finite depth, these results are quantitatively compared to the wave number
of the transient pattern observed experimentally after a jump--like increase of
the field. The wave number grows linearly with increasing induction where the
theoretical and the experimental data agree well. Thereby a long-standing
controversy about the behaviour of the wave number above the critical magnetic
field is tackled.Comment: 19 pages, 15 figures, RevTex; revised version with a new figure and
references added. submitted to Phys Rev
Particle breakage kinetics and mechanisms in attrition-enhanced deracemization
In this study, we report on experiments designed to deconvolute the particle breakage kinetics and mechanism from the parallel phenomena (growth-dissolution, agglomeration) in attrition-enhanced deracemization processes. Through such experiments, we derived the specific breakage rates and cumulative breakage distribution functions for three grinding methods typically used in deracemization experiments: (a) bead grinding, (b) ultrasound grinding, and (c) the combination of bead and ultrasound grinding. Subsequently, we tested these methods on their ability to induce deracemization. We show that in the conventional bead grinding process, breakage occurs mostly by fracture. This results in slow deracemization rates due to the delayed formation of submicron particles that are essential to the process. Conversely, ultrasound grinding very efficiently breaks particles by abrasion. This leads to fast generation of an abundance of submicron fragments resulting in fast deracemization. However, using ultrasound, large crystals fracture rates are an order of magnitude lower than those using bead grinding, which results in an insufficient size decrease of the large counter enantiomer crystals and eventually to incomplete deracemization. Remarkably, the simultaneous application of bead and ultrasound grinding leads, due to synergistic effects of both fracture and abrasion, to 2-fold higher total deracemization rates compared to bead grinding alone. The present work offers new insights into the key role of particle breakage in attrition-enhanced deracemization, together with a basis for decoupling the individual phenomena involved in the process
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