1. Dynamics of Dewetting and Self-Assembly of Pulsed Laser-Irradiated Metallic Liquids
Formation of organized nanostructures from unstable bilayers of thin metallic liquids
(with Sagar Yadavali and Ramki Kalyanaraman)
Copyright (2011) American Institute
of Physics. This article may be downloaded for personal use only.
Any
other use requires prior permission of the author and the American
Institute of Physics.
The above article appeared in Phys. Fluids 23, 122105 (2011); doi: 10.1063/1.3665618 and may be found at
http://link.aip.org/link/?PHF/23/122105.
Abstract
Dewetting of pulsed-laser irradiated, thin (<20 nm), optically reflective metallic bilayers on an
optically transparent substrate with a reflective support layer is studied within the lubrication
equations model. A steady-state bilayer film thickness (h) dependent temperature profile is derived
based on the mean substrate temperature estimated from the elaborate thermal model of transient
heating and melting/freezing. Large thermocapillary forces are observed along the plane of the
liquid-liquid and liquid-gas interfaces due to this h-dependent temperature, which, in turn, is
strongly influenced by the h-dependent laser light reflection and absorption. Consequently the
dewetting is a result of the competition between thermocapillary and intermolecular forces. A
linear analysis of the dewetting length scales established that the non-isothermal calculations better
predict the experimental results as compared to the isothermal case within the bounding Hamaker
coefficients. Subsequently, a computational non-linear dynamics study of the dewetting pathway
was performed for Ag/Co and Co/Ag bilayer systems to predict the morphology evolution. We
found that the systems evolve towards formation of different morphologies, including core-shell,
embedded, or stacked nanostructure morphologies. C 2011 American Institute of Physics.
Thickness-dependent spontaneous dewetting morphology of ultrathin Ag films, Nanotechnology 21, (2010) 155601
(with
H. Krishna, R. Sachan, J. Strader, C. Favazza, and
Ramki Kalyanaraman; due to copyright restrictions, fulltext is
available on request only)
Abstract
In this paper the lubrication-type dynamical model is developed of a molten, pulsed laser-irradiated metallic film.
The heat transfer problem that incorporates the absorbed heat from a single beam or interfering beams is solved
analytically. Using this temperature field, we derive the 3D long-wave evolution PDE for the film height.
To get insights into dynamics of dewetting, we study the 2D version of the evolution equation by means of a linear
stability analysis and by numerical simulations. The stabilizing and destabilizing effects of various system parameters,
such as the peak laser beam intensity, the film optical thickness, the Biot and Marangoni numbers, etc. are elucidated.
It is observed that the film stability is promoted for such parameters variations that increase the heat absorption in the
film. In the numerical simulations the impacts of different irradiation modes are investigated. In particular, we obtain
that in the interference heating mode the spatially periodic irradiation results in a spatially periodic film rupture with the
same, or nearly equal period. The 2D model qualitatively reproduces, for the first time, the results of the experimental
observations of a film stability and spatial ordering of a re-solidified nanostructures.
Thermocapillary effects in driven dewetting and self-assembly of pulsed laser-irradiated metallic films (with Agegnehu Atena), Phys. Rev. B 80, 075402 (2009)
--- selected for the August 17, 2009 issue of Virtual Journal of Nanoscale Science & Technology
Right panel: Irradiation of a thin metallic film by pulsed laser
interference irradiation, PLII (which creates a laterally non-uniform
temperature distribution) allows for the lateral control, through the thermocapillary flow, of the
number and the locations of dewetting
areas. For comparison, a film dewets uniformly in the absence of an external drive (left panel). --- From [1].
PLII-driven self-assembly is studied experimentally by Ramki Kalyanaraman and his group at UTK.
2. Dynamics of Instabilities in Liquid Films
Long-wave Marangoni convection in a thin film heated from below, Phys. Rev. E 85, 016328 (2012)
(with Sergey Shklyaev and Alexei Alabuzhev)
Abstract
We consider long-wave Marangoni convection in a liquid layer atop a substrate of low thermal conductivity,
heated from below.We demonstrate that the critical perturbations are materialized at the wave number K ∼
√Bi, where Bi is the Biot number which characterizes the weak heat flux from the free surface. In addition to the
conventional monotonic mode, a novel oscillatory mode is found. Applying the K ∼
√Bi scaling, we derive a new set of amplitude equations. Pattern selection on square and hexagonal lattices shows
that supercritical branching is possible. A large variety of stable patterns is found for both modes of instability.
Finite-amplitude one-dimensional solutions of the set, corresponding to either steady or traveling rolls, are studied
numerically; a complicated sequence of bifurcations is found in the former case. The emergence of an oscillatory mode
in the case of heating from below and stable patterns with finite-amplitude surface deformation are shown in this
system for the first time.
Oscillatory and monotonic modes of longwave Marangoni convection
in a thin film
, Phys. Rev. E 82, 025302 (2010)
(with Sergey Shklyaev and Alexei Alabuzhev)
Abstract
We study longwave Marangoni convection in a layer heated from below. Using
the scaling k = O(\sqrt{Bi}), where k is the
wavenumber and Bi
is the Biot number, we derive a set of amplitude equations. Analysis of this
set shows presence of monotonic
and oscillatory modes of instability.
Oscillatory mode has not been previously found for such direction of heating.
Studies of weakly
nonlinear dynamics demonstrate that stable steady and
oscillatory patterns can be found near the stability threshold.
Influences of a longitudinal and tilted vibration on stability and dewetting of a liquid film (with Sergey Shklyaev and Alexei Alabuzhev), Phys. Rev. E 79, 051603 (2009)
Abstract
We consider the dynamics of a thin liquid film in the attractive substrate potential and under the action of a
longitudinal or a tilted vibration. Using a multiscale technique we split the film motion into the oscillatory and
the averaged parts. The frequency of the vibration is assumed high enough for the inertial effects to become
essential for the oscillatory motion. Applying the lubrication approximation for the averaged motion we obtain
the amplitude equation, which includes contributions from gravity, van der Waals attraction, surface tension,
and the vibration. We show that the longitudinal vibration leads to destabilization of the initially planar film.
Stable solutions corresponding to the deflected free surface are possible in this case. Linear analysis in the case
of tilted vibration shows that either stabilization or destabilization is possible. Stabilization of the dewetting
film by mechanical action i.e., the vibration, was first reported by us [Phys. Rev. E 77, 036320 (2008)]. This
effect may be important for applications. Also, it is shown that the tilted vibration causes the averaged
longitudinal fluid flow, which can be used to transport microparticles.
Enhanced stability of a dewetting thin liquid film in a single-frequency vibration field (with Sergey Shklyaev and Alexei Alabuzhev), Phys. Rev. E 77, 036320 (2008)
Abstract
Dynamics of a thin dewetting liquid film on a vertically oscillating substrate is considered. We assume
moderate vibration frequency and large (compared to the mean film thickness) vibration amplitude. Using the
lubrication approximation and the averaging method, we formulate the coupled sets of equations governing the
pulsatile and the averaged fluid flows in the film, and then derive the nonlinear amplitude equation for the
averaged film thickness. We show that there exists a window in the frequency-amplitude domain where the
parametric and shear-flow instabilities of the pulsatile flow do not emerge. As a consequence, in this window
the averaged description is reasonable and the amplitude equation holds. The linear and nonlinear analyses of
the amplitude equation and the numerical computations show that such vibration stabilizes the film against
dewetting and rupture.
