227 research outputs found
Controlling turbulent drag across electrolytes using electric fields
Reversible in operando control of friction is an unsolved challenge crucial
to industrial tribology. Recent studies show that at low sliding velocities,
this control can be achieved by applying an electric field across electrolyte
lubricants. However, the phenomenology at high sliding velocities is yet
unknown. In this paper, we investigate the hydrodynamic friction across
electrolytes under shear beyond the transition to turbulence. We develop a
novel, highly parallelised, numerical method for solving the coupled
Navier-Stokes Poisson-Nernest-Planck equation. Our results show that turbulent
drag cannot be controlled across dilute electrolyte using static electric
fields alone. The limitations of the Poisson-Nernst-Planck formalism hints at
ways in which turbulent drag could be controlled using electric fields.Comment: Accepted by the Faraday Discussions on Chemical Physics of
Electroactive Material
Degenerate Mobilities in Phase Field Models are Insufficient to Capture Surface Diffusion
Phase field models frequently provide insight to phase transitions, and are
robust numerical tools to solve free boundary problems corresponding to the
motion of interfaces. A body of prior literature suggests that interface motion
via surface diffusion is the long-time, sharp interface limit of microscopic
phase field models such as the Cahn-Hilliard equation with a degenerate
mobility function. Contrary to this conventional wisdom, we show that the
long-time behaviour of degenerate Cahn-Hilliard equation with a polynomial free
energy undergoes coarsening, reflecting the presence of bulk diffusion, rather
than pure surface diffusion. This reveals an important limitation of phase
field models that are frequently used to model surface diffusion
The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration
According to classical electrolyte theories interactions in dilute (low ion
density) electrolytes decay exponentially with distance, with the Debye
screening length the characteristic length-scale. This decay length decreases
monotonically with increasing ion concentration, due to effective screening of
charges over short distances. Thus within the Debye model no long-range forces
are expected in concentrated electrolytes. Here we reveal, using experimental
detection of the interaction between two planar charged surfaces across a wide
range of electrolytes, that beyond the dilute (Debye-Huuckel) regime the
screening length increases with increasing concentration. The screening lengths
for all electrolytes studied - including aqueous NaCl solutions, ionic liquids
diluted with propylene carbonate, and pure ionic liquids - collapse onto a
single curve when scaled by the dielectric constant. This non-monotonic
variation of the screening length with concentration, and its generality across
ionic liquids and aqueous salt solutions, demonstrates an important
characteristic of concentrated electrolytes of substantial relevance from
biology to energy storage.Comment: This document is the unedited authors' version of a Submitted Work
that was subsequently accepted for publication in the Journal of Physical
Chemistry Letters, copyright American Chemical Society, after peer review. To
access the final edited and published work see
http://pubsdc3.acs.org/articlesonrequest/AOR-EW6FuIC6wIh6D9qqEeH
Fluctuation Spectra and Force Generation in Non-equilibrium Systems
Many biological systems are appropriately viewed as passive inclusions
immersed in an active bath: from proteins on active membranes to microscopic
swimmers confined by boundaries. The non-equilibrium forces exerted by the
active bath on the inclusions or boundaries often regulate function, and such
forces may also be exploited in artificial active materials. Nonetheless, the
general phenomenology of these active forces remains elusive. We show that the
fluctuation spectrum of the active medium, the partitioning of energy as a
function of wavenumber, controls the phenomenology of force generation. We find
that for a narrow, unimodal spectrum, the force exerted by a non-equilibrium
system on two embedded walls depends on the width and the position of the peak
in the fluctuation spectrum, and oscillates between repulsion and attraction as
a function of wall separation. We examine two apparently disparate examples:
the Maritime Casimir effect and recent simulations of active Brownian
particles. A key implication of our work is that important non-equilibrium
interactions are encoded within the fluctuation spectrum. In this sense the
noise becomes the signal
Unravelling Nanoconfined Films of Ionic Liquids
The confinement of an ionic liquid between charged solid surfaces is treated
using an exactly solvable 1D Coulomb gas model. The theory highlights the
importance of two dimensionless parameters: the fugacity of the ionic liquid,
and the electrostatic interaction energy of ions at closest approach relative
to thermal energy, in determining how the disjoining pressure exerted on the
walls depends on the geometrical confinement. Our theory reveals that
thermodynamic fluctuations play a vital role in the "squeezing out" of charged
layers as the confinement is increased. The model shows good qualitative
agreement with previous experimental data, with all parameters independently
estimated without fitting
Scaling analysis of the screening length in concentrated electrolytes
The interaction between charged objects in an electrolyte solution is a
fundamental question in soft matter physics. It is well-known that the
electrostatic contribution to the interaction energy decays exponentially with
object separation. Recent measurements reveal that, contrary to the
conventional wisdom given by classic Poisson-Boltzmann theory, the decay length
increases with ion concentration for concentrated electrolytes and can be an
order of magnitude larger than the ion diameter in ionic liquids. We derive a
simple scaling theory that explains this anomalous dependence of the decay
length on ion concentration. Our theory successfully collapses the decay
lengths of a wide class of salts onto a single curve. A novel prediction of our
theory is that the decay length increases linearly with the Bjerrum length,
which we experimentally verify by surface force measurements. Moreover, we
quantitatively relate the measured decay length to classic measurements of the
activity coefficient in concentrated electrolytes, thus showing that the
measured decay length is indeed a bulk property of the concentrated electrolyte
as well as contributing a mechanistic insight into empirical activity
coefficients.Comment: To appear in Physical Review Letter
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