1,720,977 research outputs found
Fluctuating hydrodynamics model for homogeneous and heterogeneous vapor bubble nucleation
Full text linkAt the molecular scale, even in conditions of thermodynamic equilibrium, the fluids do not exhibit a deterministic behavior. Going down below the micrometer scale, the effects of thermal fluctuations play a dominant role in the dynamics of the system, calling for a suitable description of thermal fluctuations. These models not only play an important role in physics of fluids, but a deep understanding of these phenomena is necessary for the progress of some of the latest nanotechnology. For instance the modeling of thermal fluctuations is crucial in the design of flow micro-devices, in the study of biological systems, such as lipid membranes, in the theory of Brownian engines and in the development of artificial molecular motor prototypes. Another problem with a huge technological impact is the phenomenon of nucleation – the precursor of the phase transition in metastable systems – in this context related to bubble formation in liquid-vapor phase transition. Vapor bubbles form in liquids by two main mechanisms: boiling, by increasing the tempe- rature over the boiling threshold, and cavitation, by reducing the pressure below the vapor pressure threshold. The liquid can be held in these metastable states (overheating and tensile conditions, respectively) for a long time without forming bubbles. Bubble nucleation is indeed an activated process, requiring a significant amount of energy to overcome the free energy barrier and bring the liquid from the metastable conditions to the thermodynami- cally stable state where vapor is observed. Depending on the thermodynamic conditions, the nucleation time may be exceedingly long, the so-called "rare- event" issue. Nowadays molecular dynamics is the unique tool to investigate such thermally activated processes. However, its computational cost limits its application to small systems (less than few tenth of nanometers) and to very short times, preventing the study of hydrodynamic interactions. The latter effects are crucial to understand the cavitation phenomenon in its entirety, starting from the vapor embryos nucleation up to the macroscopic motion.
In this thesis a continuum diffuse interface model of the two-phase fluid has been embedded with thermal fluctuations in the context of the so-called Fluctuating Hydrodynamics (FH) and has been exploited to address cavitation. This model provides a set of partial stochastic differential equations, whose deterministic part is represented by the capillary Navier-Stokes equations and reproducing the Einstein-Boltzmann probability distribution for the macroscopic fields. This mesoscale approach enables the description of the liquid-vapor transition in extended systems and the evaluation of bub- ble nucleation rates in different metastable conditions by means of numerical simulations. Such model is expected to have a huge impact on the understanding of the nucleation dynamics since, by reducing the computational cost by orders of magnitude, it allows the unique possibility of investigating systems of realistic dimensions on macroscopic time scales. In addition, after the nucleating phase, the deterministic equations have been used to address the collapse of a cavitation nanobubble near a solid boundary, showing an unprecedented description of interfacial flows that naturally takes into account topology modification and phase changes (both vapor/liquid and vapor/supercritical fluid transformations)
Topological transitions in fluid lipid vesicles: activation energy and force fields
Full text linkLipid bilayer membranes are the fundamental biological barriers that permit
life.
The bilayer dynamics largely participates in orchestrating cellular workings
and is characterized by substantial stability together with extreme plasticity
that allows controlled morphological/topological changes.
Modeling and understanding the topological change of vesicle-like membrane at
the scale of a full cell has proved an elusive aim.
We propose and discuss here a continuum model able to encompass the
fusion/fission transition of a bilayer membrane at the scale of a Large
Unilamellar Vesicle and evaluate the minimal free energy path across the
transition, inspired by the idea that fusion/fission-inducing proteins have
evolved in Nature towards minimal work expenditure.
The results predict the correct height for the energetic barrier and provide
the force field that, by acting on the membrane, can induce the transition.
They are found in excellent agreement, in terms of intensity, scale, and
spatial localization with experimental data on typical protein systems at play
during the transition.
The model may pave the way for the development of more complete models of the
process where the dynamics is coupled to the fluid internal and external
environment and to other applications where the topological changes do either
occur or should be prevented
The diffuse interface description of fluid lipid membranes captures key features of the hemifusion pathway and lateral stress profile
Full text linkTopological transitions of lipid membranes are ubiquitous in key biological processes for cell life, like neurotransmission, fertilization, morphogenesis, and viral infections. Despite this, they are not well understood due to their multiscale nature, which limits the use of molecular models and calls for a mesoscopic approach such as the celebrated Canham-Helfrich one. Unfortunately, such a model cannot handle topological transitions, hiding the crucial involved forces and the appearance of the experimentally observed hemifused intermediates. In this work, we describe the membrane as a diffuse interface preserving the Canham-Helfrich elasticity. We show that pivotal features of the hemifusion pathway are captured by this mesoscopic approach, e.g. a (meta)stable hemifusion state and the fusogenic behavior of negative monolayer spontaneous curvatures. The membrane lateral stress profile is calculated as a function of the elastic rigidities, yielding a coarse-grained version of molecular models findings. Insights into the fusogenic mechanism are reported and discussed
Diffuse interface modeling of laser-induced nano/micro cavitation bubbles
Full text linkIn the present work, a diffuse interface model has been used to numerically investigate the laser-induced cavitation of nano/microbubbles. The mesoscale approach is able to describe the cavitation process in its entirety, starting from the vapor bubble formation due to the focused laser energy deposition, up to its macroscopic motion.In particular, the simulations show a complete and detailed description of the bubble formation and the subsequent breakdown wave emission with a precise estimation of the energy partition between the shockwave radiating in the liquid and the internal energy of the bubble. The scaling of the ratio between the energy stored in the bubble at its maximum radius and the one deposited by the laser is found in agreement with experimental observation on macroscopic bubbles
Thermally activated vapor bubble nucleation: The Landau-Lifshitz--Van der Waals approach
Full text linkVapor bubbles are formed in liquids by two mechanisms: evaporation (temperature above the boiling threshold) and cavitation (pressure below the vapor pressure). The liquid resists in these metastable (overheating and tensile, respectively) states for a long time since bubble nucleation is an activated process that needs to surmount the free energy barrier separating the liquid and the vapor states. The bubble nucleation rate is difficult to assess and, typically, only for extremely small systems treated at an atomistic level of detail. In this work a powerful approach, based on a continuum diffuse interface modeling of the two-phase fluid embedded with thermal fluctuations (fluctuating hydrodynamics), is exploited to study the nucleation process in homogeneous conditions, evaluating the bubble nucleation rates and following the long-term dynamics of the metastable system, up to the bubble coalescence and expansion stages. In comparison with more classical approaches, this methodology allows us on the one hand to deal with much larger systems observed for a much longer time than possible with even the most advanced atomistic models. On the other, it extends continuum formulations to thermally activated processes, impossible to deal with in a purely determinist setting
Dynamics of a vapor nanobubble collapsing near a solid boundary
Full text linkIn the present paper a diffuse interface approach is used to address the collapse of a sub-micron vapor bubble near solid boundaries. This formulation enables an unprecedented description of interfacial flows that naturally takes into account topology modification and phase changes (both vapor/liquid and vapor/supercritical fluid transformations). Results from numerical simulations are exploited to discuss the complex sequence of events associated with the bubble collapse near a wall, encompassing shock-wave emissions in the liquid and reflections from the wall, their successive interaction with the expanding bubble, the ensuing asymmetry of the bubble and the eventual jetting phase
Shock-induced collapse of a vapor nanobubble near solid boundaries
Full text linkThe
collapse
of
a
nano-bubble
near
a
solid
wall
is
addressed
here
exploiting
a
phase
field
model
recently
used
to
describe
the
process
in
free
space.
Bubble
collapse
is
triggered
by
a
normal
shock
wave
in
the
liquid.
The
dynamics
is
explored
for
different
bubble
wall
normal
distances
and
triggering
shock
inten-
sities.
Overall
the
dynamics
is
characterized
by
a
sequence
of
collapses
and
rebounds
of
the
pure
vapor
bubble
accompanied
by
the
emission
of
shock
waves
in
the
liquid.
The
shocks
are
reflected
by
the
wall
to
impinge
back
on
the
re-expanding
bubble.
The
presence
of
the
wall
and
the
impinging
shock
wave
break
the
symmetry
of
the
system,
leading,
for
su
ffi
ciently
strong
intensity
of
the
incoming
shock
wave,
to
the
poration
of
the
bubble
and
the
formation
of
an
annular
structure
and
a
liquid
jet.
Intense
peaks
of
pres-
sure
and
temperatures
are
found
also
at
the
wall,
confirming
that
the
strong
localized
loading
combined
with
the
jet
impinging
the
wall
is
a
potential
source
of
substrate
damage
induced
by
the
cavitation
Fluctuating hydrodynamics as a tool to investigate nucleation of cavitation bubbles
Full text linkVapor bubbles can be formed in liquids by increasing the temperature over the boiling threshold (evaporation) or by reducing the pressure below its vapor pressure threshold (cavitation). The liquid can be held in these tensile conditions (metastable states) for a long time without any bubble formation. The bubble nucleation is indeed an activated process, in fact a given amount of energy is needed to bring the liquid from that local stable condition into a more stable one, where a vapor bubble is formed. Crucial question in this field is how to correctly estimate the bubble nucleation rate, i.e. the amount of vapor bubbles formed in a given time and in a given volume of liquid, in different thermodynamic conditions. Several theoretical models have been proposed, ranging from classical nucleation theory, to density functional theory. These theories can give good estimate of the energy barriers but lack of a precise estimate of the nucleation rate, especially in complex systems. Molecular dynamics simulations can give more precise results, but the computational cost of this technique makes it unfeasible to be applied on systems larger than few tenth of nanometers. In this work the approach of fluctuating hydrodynamics has been embedded into a continuum diffuse interface modeling of the two-phase fluid. The resulting model provides a complete description of both the thermodynamic and fluid dynamic fields enabling the description of vapor-liquid phase change through stochastic fluctuations. The continuum model has been exploited to investigate the bubble nucleation rate in different metastable conditions. Such an approach has a huge impact since it reduces the computational cost and allows to investigate longer time scales and larger spatial scales with respect to more conventional techniques
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