3,896 research outputs found
Ion specificity and the theory of stability of colloidal suspensions
A theory is presented which allow us to accurately calculate the critical
coagulation concentration (CCC) of hydrophobic colloidal suspensions. For
positively charged particles the CCC's follow the Hofmeister (lyotropic)
series. For negatively charged particles the series is reversed. We find that
strongly polarizable chaotropic anions are driven towards the colloidal surface
by electrostatic and hydrophobic forces. Within approximately one ionic radius
from the surface, the chaotropic anions loose part of their hydration sheath
and become strongly adsorbed. The kosmotropic anions, on the other hand, are
repelled from the hydrophobic surface. The theory is quantitatively accurate
without any adjustable parameters. We speculate that the same mechanism is
responsible for the Hofmeister series that governs stability of protein
solutions.Comment: Phys. Rev. Lett. (in press
Determination of the zeta potential for highly charged colloidal suspensions
We compute the electrostatic potential at the surface, or zeta potential
, of a charged particle embedded in a colloidal suspension using a
hybrid mesoscopic model. We show that for weakly perturbing electric fields,
the value of obtained at steady state during electrophoresis is
statistically indistinguishable from in thermodynamic equilibrium. We
quantify the effect of counterions concentration on . We also evaluate
the relevance of the lattice resolution for the calculation of and
discuss how to identify the effective electrostatic radius.Comment: 8 pages, 3 figures with 2 panel
Aggregation kinetics of stiff polyelectrolytes in the presence of multivalent salt
Using molecular dynamics simulations, the kinetics of bundle formation for
stiff polyelectrolytes such as actin is studied in the solution of multivalent
salt. The dominant kinetic mode of aggregation is found to be the case of one
end of one rod meeting others at right angle due to electrostatic interactions.
The kinetic pathway to bundle formation involves a hierarchical structure of
small clusters forming initially and then feeding into larger clusters, which
is reminiscent of the flocculation dynamics of colloids. For the first few
cluster sizes, the Smoluchowski formula for the time evolution of the cluster
size gives a reasonable account for the results of our simulation without a
single fitting parameter. The description using Smoluchowski formula provides
evidence for the aggregation time scale to be controlled by diffusion, with no
appreciable energy barrier to overcome.Comment: 6 pages, 5 figures, Phys. Rev. E (Accepted
Effect of Salt Concentration on the Electrophoretic Speed of a Polyelectrolyte through a Nanopore
In a previous paper [S. Ghosal, Phys. Rev. E 74, 041901 (2006)] a
hydrodynamic model for determining the electrophoretic speed of a
polyelectrolyte through an axially symmetric slowly varying nanopore was
presented in the limit of a vanishingly small Debye length. Here the case of a
finite Debye layer thickness is considered while restricting the pore geometry
to that of a cylinder of length much larger than the diameter. Further, the
possibility of a uniform surface charge on the walls of the nanopore is taken
into account. It is thereby shown that the calculated transit times are
consistent with recent measurements in silicon nanopores.Comment: 4 pages, 2 figure
Manual Tracking Flight Control with Amplitude and Rate Constrained Dynamic Actuators
A new control methodology for manual flight control, viz., real-time tracking control, is developed. Amplitude and rate constrained dynamic actuators are considered. Optimal tracking control is made possible by the use of unique reference signal prediction strategies which extrapolate the reference signal over the optimization horizon. A receding horizon, linear-quadratic inner-loop controller is employed in conjunction with an outer-loop nonlinear element. The constraint effects mitigation strategy is to optimally track a modified reference signal which yields feasible actuator commands over the optimization horizon when the pilot demanded reference is too aggressive to be tracked by the inner-loop optimal control law. A discrete-time implementation yields computationally inexpensive, closed-form solutions which are implementable in real-time and which afford the optimal tracking of an exogenous, unknown a priori reference signal. The developed control algorithm is applied to an open-loop unstable aircraft model, with attention being given to the trade-offs associated with the conflicting objectives of aggressive tracking and saturation avoidance. One-step ahead constraint mitigation is shown to provide substantial improvement in the constrained system response, while slightly more complicated constraint mitigation strategies yield stronger stability properties
Local structure of percolating gels at very low volume fractions
The formation of colloidal gels is strongly dependent on the volume fraction
of the system and the strength of the interactions between the colloids. Here
we explore very dilute solutions by the means of numerical simulations, and
show that, in the absence of hydrodynamic interactions and for sufficiently
strong interactions, percolating colloidal gels can be realised at very low
values of the volume fraction. Characterising the structure of the network of
the arrested material we find that, when reducing the volume fraction, the gels
are dominated by low-energy local structures, analogous to the isolated
clusters of the interaction potential. Changing the strength of the interaction
allows us to tune the compactness of the gel as characterised by the fractal
dimension, with low interaction strength favouring more chain-like structures
Separation of suspended particles by arrays of obstacles in microfluidic devices
The stochastic transport of suspended particles through a periodic pattern of
obstacles in microfluidic devices is investigated by means of the Fokker-Planck
equation. Asymmetric arrays of obstacles have been shown to induce the
continuous separation of DNA molecules of different length. The analysis
presented here of the asymptotic distribution of particles in a unit cell of
these systems shows that separation is only possible in the presence of a
driving force with a non-vanishing normal component at the surface of the solid
obstacles. In addition, vector separation, in which different species move, in
average, in different directions within the device, is driven by differences on
the force acting on the various particles and not by differences in the
diffusion coefficient. Monte-Carlo simulations performed for different
particles and force fields agree with the numerical solutions of the
Fokker-Planck equation in the periodic system
Effective non-additive pair potential for lock-and-key interacting particles: the role of the limited valence
Theoretical studies of self-assembly processes and condensed phases in
colloidal systems are often based on effective inter-particle potentials. Here
we show that developing an effective potential for particles interacting with a
limited number of ``lock-and-key'' selective bonds (due to the specificity of
bio-molecular interactions) requires -- beside the non-sphericity of the
potential -- a (many body) constraint that prevent multiple bonding on the same
site. We show the importance of retaining both valence and bond-selectivity by
developing, as a case study, a simple effective potential describing the
interaction between colloidal particles coated by four single-strand DNA
chains.Comment: 4 pages, 5 figure
The Effects of Inter-particle Attractions on Colloidal Sedimentation
We use a mesoscopic simulation technique to study the effect of short-ranged
inter-particle attraction on the steady-state sedimentation of colloidal
suspensions. Attractions increase the average sedimentation velocity
compared to the pure hard-sphere case, and for strong enough attractions, a
non-monotonic dependence on the packing fraction with a maximum velocity
at intermediate is observed. Attractions also strongly enhance
hydrodynamic velocity fluctuations, which show a pronounced maximum size as a
function of . These results are linked to a complex interplay between
hydrodynamics and the formation and break-up of transient many-particle
clusters.Comment: 4 pages 4 figure
Systematic characterization of thermodynamic and dynamical phase behavior in systems with short-ranged attraction
In this paper we demonstrate the feasibility and utility of an augmented
version of the Gibbs ensemble Monte Carlo method for computing the phase
behavior of systems with strong, extremely short-ranged attractions. For
generic potential shapes, this approach allows for the investigation of
narrower attractive widths than those previously reported. Direct comparison to
previous self-consistent Ornstein-Zernike approximation calculations are made.
A preliminary investigation of out-of-equilibrium behavior is also performed.
Our results suggest that the recent observations of stable cluster phases in
systems without long-ranged repulsions are intimately related to gas-crystal
and metastable gas-liquid phase separation.Comment: 10 pages, 8 figure
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