Physicists
have predicted that under the influence of sufficiently high electric fields,
liquid droplets of certain materials will undergo solidification, forming
crystallites at temperature and pressure conditions that correspond to liquid
droplets at field-free conditions. This electric-field-induced phase
transformation is termed electrocrystallization.
The study, performed by scientists at the Georgia Institute of Technology,
appears online and is scheduled as a feature and cover article in the 42nd
issue of Volume 115 of the Journal of Physical Chemistry C.
“We
show that with a strong electric field, you can induce a phase transition
without altering the thermodynamic parameters,” said Uzi Landman, Regents’ and
Institute Professor in the School of Physics, F.E. Callaway Chair and director
of the Center for Computational Materials Science (CCMS) at Georgia Tech.
In
these simulations, Landman and Senior Research Scientists David Luedtke and
Jianping Gao at the CCMS set out first to explore a phenomenon described by Sir
Geoffrey Ingram Taylor in 1964 in the course of his study of the effect of
lightning on raindrops, expressed as changes in the shape of liquid drops when
passing through an electric field. While liquid drops under field-free
conditions are spherical, they alter their shape in response to an applied
electric field to become needle-like liquid drops. Instead of the water
droplets used in the almost 50-year-old laboratory experiments of Taylor, the
Georgia Tech researchers focused their theoretical study on a 10 nanometer (nm)
diameter liquid droplet of formamide, which is a material made of small polar
molecules each characterized by a dipole moment that is more than twice as
large as that of a water molecule.
With
the use of molecular dynamics simulations developed at the CCMS, which allow
scientists to track the evolution of materials systems with ultra-high
resolution in space and time, the physicists explored the response of the
formamide nano-droplet to an applied electric field of variable strength.
Influenced by a field of less than 0.5V/nm, the spherical droplet elongated
only slightly. However, when the strength of the field was raised to a critical
value close to 0.5 V/nm, the simulated droplet was found to undergo a shape transition
resulting in a needle-like liquid droplet with its long axis – oriented along
the direction of the applied field – measuring about 12 times larger than the
perpendicular (cross-sectional) small axis of the needle-like droplet. The
value of the critical field found in the simulations agrees well with the
prediction obtained almost half a century ago by Taylor from general macroscopic
considerations.
Past
the shape transition further increase of the applied electric field yielded a
slow, gradual increase of the aspect ratio between the long and short axes of
the needle-like droplet, with the formamide molecules exhibiting liquid
diffusional motions.
“Here
came the Eureka moment,” said Landman. “When the field strength in the
simulations was ramped up even further, reaching a value close to 1.5V/nm, the
liquid needle underwent a solidification phase transition, exhibited by
freezing of the diffusional motion, and culminating in the formation of a
formamide single crystal characterized by a structure that differs from that of
the x-ray crystallographic one determined years ago under zero-field
conditions. Now, who ordered that?” he added.
Further
analysis has shown that the crystallization transition involved arrangement of
the molecules into a particular spatial ordered lattice, which optimizes the
interactions between the positive and negative ends of the dipoles of
neighboring molecules, resulting in minimization of the free energy of the
resulting rigid crystalline needle. When the electric field applied to the
droplet was subsequently decreased, the crystalline needle remelted and at
zero-field the liquid droplet reverted to a spherical shape. The field reversal
process was found to exhibit a hysteresis.
Analysis
of the microscopic structural changes that underlie the response of the droplet
to the applied field revealed that accompanying the shape transition at 0.5
V/nm is a sharp increase in the degree of reorientation of the molecular
electric dipoles, which after the transition lie preferentially along the
direction of the applied electric field and coincide with the long axis of the
needle-like liquid droplet. The directional dipole reorientation, which is
essentially complete subsequent to the higher field electrocrystallization
transition, breaks the symmetry and transforms the droplet into a field-induced
ferroelectric state where it possesses a large net electric dipole, in contrast
to its unpolarized state at zero–field conditions.
Along
with the large-scale atomistic computer simulations, researchers formulated and
evaluated an analytical free-energy model, which describes the balance between
the polarization, interfacial tension and dielectric saturation contributions.
This model was shown to yield results in agreement with the computer simulation
experiments, thus providing a theoretical framework for understanding the
response of dielectric droplets to applied fields.
“This
investigation unveiled fascinating properties of a large group of materials
under the influence of applied fields,” Landman said. “Here the field-induced
shape and crystallization transitions occurred because formamide, like water
and many other materials, is characterized by a relatively large electric
dipole moment. The study demonstrated the ability to employ external fields to
direct and control the shape, the aggregation phase (that is, solid or liquid)
and the properties of certain materials.”
Along
with the fundamental interest in understanding the microscopic origins of
materials behavior, this may lead to development of applications of
field-induced materials control in diverse areas, ranging from targeted drug delivery,
nanoencapsulation, printing of nanostructures and surface patterning, to
aerosol science, electrospray propulsion and environmental science.
This research was supported by a grant from the U.S. Air Force Office of Scientific Research.
