389 research outputs found
Fall and rise of small droplets on rough hydrophobic substrates
Liquid droplets on patterned hydrophobic substrates are typically observed
either in the Wenzel or the Cassie state. Here we show that for droplets of
comparable size to the roughness scale an additional local equilibrium state
exists, where the droplet is immersed in the texture, but not yet contacts the
bottom grooves. Upon evaporation, a droplet in this state enters the Cassie
state, leading to a qualitatively new self-cleaning mechanism. The effect is of
generic character and is expected to occur in any hydrophobic capillary wetting
situation where a spherical liquid reservoir is involved.Comment: 6 pages, 6 figures, version as published in EP
The relaxed-polar mechanism of locally optimal Cosserat rotations for an idealized nanoindentation and comparison with 3D-EBSD experiments
The rotation arises as the unique orthogonal
factor of the right polar decomposition of a given
invertible matrix . In the context of nonlinear elasticity
Grioli (1940) discovered a geometric variational characterization of as a unique energy-minimizing rotation. In preceding works, we have
analyzed a generalization of Grioli's variational approach with weights
(material parameters) and (Grioli: ). The
energy subject to minimization coincides with the Cosserat shear-stretch
contribution arising in any geometrically nonlinear, isotropic and quadratic
Cosserat continuum model formulated in the deformation gradient field and the microrotation field . The corresponding set of non-classical energy-minimizing
rotations represents a new relaxed-polar mechanism.
Our goal is to motivate this mechanism by presenting it in a relevant setting.
To this end, we explicitly construct a deformation mapping
which models an idealized nanoindentation and compare the corresponding optimal
rotation patterns with experimentally
obtained 3D-EBSD measurements of the disorientation angle of lattice rotations
due to a nanoindentation in solid copper. We observe that the non-classical
relaxed-polar mechanism can produce interesting counter-rotations. A possible
link between Cosserat theory and finite multiplicative plasticity theory on
small scales is also explored.Comment: 28 pages, 11 figure
Maintaining the equipartition theorem in small heterogeneous molecular dynamics ensembles
It has been reported recently that the equipartition theorem is violated in
molecular dynamics simulations with periodic boundary condition [Shirts et al,
J. Chem. Phys. 125, 164102 (2006)]. This effect is associated with the
conservation of the center of mass momentum. Here, we propose a fluctuating
center of mass molecular dynamics approach (FCMMD) to solve this problem. Using
the analogy to a system exchanging momentum with its surroundings, we work out
--and validate via simulations-- an expression for the rate at which
fluctuations shall be added to the system. The restoration of equipartition
within the FCMMD is then shown both at equilibrium as well as beyond
equilibrium in the linear response regime
Aging in Structural Changes of Amorphous Solids: A Study of First Passage Time and Persistence Time Distribution
The time distribution of relaxation events in an aging system is investigated
via molecular dynamics simulations. The focus is on the distribution functions
of the first passage time, , and the persistence time,
. In contrast to previous reports, both and are found to
evolve with time upon aging. The age dependence of the persistence time
distribution is shown to be sensitive to the details of the algorithm used to
extract it from particle trajectories. By updating the reference point in event
detection algorithm and accounting for the event specific aging time, we
uncover age dependence of , hidden to previous studies. Moreover, the
apparent age-dependence of in continuous time random walk with an age
independent is shown to result from an implicit synchronization of
all the random walkers at the starting time
Heterogeneous shear in hard sphere glasses
There is growing evidence that the flow of driven amorphous solids is not
homogeneous, even if the macroscopic stress is constant across the system. Via
event driven molecular dynamics simulations of a hard sphere glass, we provide
the first direct evidence for a correlation between the fluctuations of the
local volume-fraction and the fluctuations of the local shear rate. Higher
shear rates do preferentially occur at regions of lower density and vice versa.
The temporal behavior of fluctuations is governed by a characteristic time
scale, which, when measured in units of strain, is independent of shear rate in
the investigated range. Interestingly, the correlation volume is also roughly
constant for the same range of shear rates. A possible connection between these
two observations is discussed.Comment: 5 pages, 4 figures, accepted at Phys. Rev. Let
Single particle fluctuations and directional correlations in driven hard sphere glasses
Via event driven molecular dynamics simulations and experiments, we study the
packing fraction and shear-rate dependence of single particle fluctuations and
dynamic correlations in hard sphere glasses under shear. At packing fractions
above the glass transition, correlations increase as shear rate decreases: the
exponential tail in the distribution of single particle jumps broadens and
dynamic four-point correlations increase. Interestingly, however, upon
decreasing the packing fraction, a broadening of the exponential tail is also
observed, while dynamic heterogeneity is shown to decrease. An explanation for
this behavior is proposed in terms of a competition between shear and thermal
fluctuations. Building upon our previous studies [Chikkadi et al, Europhys.
Lett. (2012)], we further address the issue of anisotropy of the dynamic
correlations.Comment: 8 pages, 10 figure
Shear stress in lattice Boltzmann simulations
A thorough study of shear stress within the lattice Boltzmann method is
provided. Via standard multiscale Chapman-Enskog expansion we investigate the
dependence of the error in shear stress on grid resolution showing that the
shear stress obtained by the lattice Boltzmann method is second order accurate.
This convergence, however, is usually spoiled by the boundary conditions. It is
also investigated which value of the relaxation parameter minimizes the error.
Furthermore, for simulations using velocity boundary conditions, an artificial
mass increase is often observed. This is a consequence of the compressibility
of the lattice Boltzmann fluid. We investigate this issue and derive an
analytic expression for the time-dependence of the fluid density in terms of
the Reynolds number, Mach number and a geometric factor for the case of a
Poiseuille flow through a rectangular channel in three dimensions. Comparison
of the analytic expression with results of lattice Boltzmann simulations shows
excellent agreement.Comment: 15 pages, 4 figures, 2 table
Wetting gradient induced separation of emulsions: A combined experimental and lattice Boltzmann computer simulation study
Guided motion of emulsions is studied via combined experimental and
theoretical investigations. The focus of the work is on basic issues related to
driving forces generated via a step-wise (abrupt) change in wetting properties
of the substrate along a given spatial direction. Experiments on binary
emulsions unambiguously show that selective wettability of the one of the fluid
components (water in our experiments) with respect to the two different parts
of the substrate is sufficient in order to drive the separation process. These
studies are accompanied by approximate analytic arguments as well as lattice
Boltzmann computer simulations, focusing on effects of a wetting gradient on
internal droplet dynamics as well as its relative strength compared to
volumetric forces driving the fluid flow. These theoretical investigations show
qualitatively different dependence of wetting gradient induced forces on
contact angle and liquid volume in the case of an open substrate as opposed to
a planar channel. In particular, for the parameter range of our experiments,
slit geometry is found to give rise to considerably higher separation forces as
compared to open substrate.Comment: 34 pages, 12 figure
On the spheroidized carbide dissolution and elemental partitioning in a high carbon bearing steel 100Cr6
We report on the characterization of high carbon bearing steel 100Cr6 using
electron microscopy and atom probe tomography in combination with
multi-component diffusion simulations (DICTRA). Scanning electron micrographs
show that around 14 vol.% spheroidized carbides are formed during soft
annealing and only 3 vol.% remain after dissolution into the austenitic matrix
by austenitization at 1123 K (850 {\deg}C) for 300 s. The spheroidized
particles are identified as (Fe, Cr)3C by transmission electron microscopy.
Atom probe analyses reveal the redistribution and partitioning behaviors of
elements, i.e. C, Si, Mn, Cr, Fe in both, the spheroidized carbides and the
bainitic matrix in the sample isothermally heat-treated at 773 K (500 {\deg}C)
after austenitization. A homogeneous distribution of C and gradual gradient of
Cr was detected within the spheroidized carbides. Due to its limited
diffusivity in (Fe, Cr)3C, Cr exhibits a maximum concentration at the surface
of spheroidized carbides (16 at.%) and decreases gradually from surface towards
the core down to a level of about 2 at.%. The atom probe results also indicate
that the partially dissolved spheroidized carbides during austenitization may
serve as nucleation sites for intermediate temperature cementite within
bainite, which results in a relatively softer surface and harder core in
spheroidized particles. This microstructure may contribute to the good wear
resistance and fatigue propertie
Computational Discovery of Energy-Efficient Heat Treatment for Microstructure Design using Deep Reinforcement Learning
Deep Reinforcement Learning (DRL) is employed to develop autonomously
optimized and custom-designed heat-treatment processes that are both,
microstructure-sensitive and energy efficient. Different from conventional
supervised machine learning, DRL does not rely on static neural network
training from data alone, but a learning agent autonomously develops optimal
solutions, based on reward and penalty elements, with reduced or no
supervision. In our approach, a temperature-dependent Allen-Cahn model for
phase transformation is used as the environment for the DRL agent, serving as
the model world in which it gains experience and takes autonomous decisions.
The agent of the DRL algorithm is controlling the temperature of the system, as
a model furnace for heat-treatment of alloys. Microstructure goals are defined
for the agent based on the desired microstructure of the phases. After
training, the agent can generate temperature-time profiles for a variety of
initial microstructure states to reach the final desired microstructure state.
The agent's performance and the physical meaning of the heat-treatment profiles
generated are investigated in detail. In particular, the agent is capable of
controlling the temperature to reach the desired microstructure starting from a
variety of initial conditions. This capability of the agent in handling a
variety of conditions paves the way for using such an approach also for
recycling-oriented heat treatment process design where the initial composition
can vary from batch to batch, due to impurity intrusion, and also for the
design of energy-efficient heat treatments. For testing this hypothesis, an
agent without penalty on the total consumed energy is compared with one that
considers energy costs. The energy cost penalty is imposed as an additional
criterion on the agent for finding the optimal temperature-time profile
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