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175
Artificial Brownian motors: Controlling transport on the nanoscale
 REV MOD. PHYS
, 2009
"... In systems possessing spatial or dynamical symmetry breaking, Brownian motion combined with unbiased external input signals, deterministic and random alike, can assist directed motion of particles at submicron scales. In such cases, one speaks of “Brownian motors. ” In this review the constructive r ..."
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In systems possessing spatial or dynamical symmetry breaking, Brownian motion combined with unbiased external input signals, deterministic and random alike, can assist directed motion of particles at submicron scales. In such cases, one speaks of “Brownian motors. ” In this review the constructive role of Brownian motion is exemplified for various physical and technological setups, which are inspired by the cellular molecular machinery: the working principles and characteristics of stylized devices are discussed to show how fluctuations, either thermal or extrinsic, can be used to control diffusive particle transport. Recent experimental demonstrations of this concept are surveyed with particular attention to transport in artificial, i.e., nonbiological, nanopores, lithographic tracks, and optical traps, where singleparticle currents were first measured. Much emphasis is given to two and threedimensional devices containing many interacting particles of one or more species; for this class of artificial motors, noise rectification results also from the interplay of particle Brownian motion and geometric constraints. Recently, selective control and optimization of the transport of interacting colloidal particles and magnetic vortices have been successfully achieved, thus leading to the new generation of microfluidic and superconducting devices presented here. The field has recently been enriched with impressive experimental achievements in building artificial Brownian motor devices that even operate within the quantum domain by harvesting quantum Brownian motion. Sundry akin topics include activities aimed at noiseassisted shuttling other degrees of freedom such as charge, spin, or even heat and the assembly of chemical synthetic molecular motors. This review ends with a perspective for future pathways and potential new applications.
Evaluation of ion binding to DNA duplexes using a sizemodified Poisson–Boltzmann theory
 Biophys. J
, 2007
"... ABSTRACT PoissonBoltzmann (PB) theory is among the most widely applied electrostatic theories in biological and chemical science. Despite its reasonable success in explaining a wide variety of phenomena, it fails to incorporate two basic physical effects, ion size and ionion correlations, into its ..."
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ABSTRACT PoissonBoltzmann (PB) theory is among the most widely applied electrostatic theories in biological and chemical science. Despite its reasonable success in explaining a wide variety of phenomena, it fails to incorporate two basic physical effects, ion size and ionion correlations, into its theoretical treatment. Recent experimental work has shown significant deviations from PB theory in competitive monovalent and divalent ion binding to a DNA duplex. The experimental data for monovalent binding are consistent with a hypothesis that attributes these deviations to counterion size. To model the observed differences, we have generalized an existing sizemodified PoissonBoltzmann (SMPB) theory and developed a new numerical implementation that solves the generalized theory around complex, atomistic representations of biological molecules. The results of our analysis show that good agreement to data at monovalent ion concentrations up to;150 mM can be attained by adjusting the ionsize parameters in the new sizemodified theory. SMPB calculations employing calibrated ionsize parameters predict experimental observations for other nucleic acid structures and salt conditions, demonstrating that the theory is predictive. We are, however, unable to model the observed deviations in the divalent competition data with a theory that only accounts for size but neglects ionion correlations, highlighting the need for theoretical descriptions that further incorporate ionion correlations. The accompanying numerical solver has been released publicly, providing the general scientific community the ability to compute SMPB solutions around a variety of different biological structures with only modest computational resources.
Towards an understanding of inducedcharge electrokinetics at large applied voltages in concentrated solutions
 ADV. COLLOID INTERFACE SCI
, 2009
"... The venerable theory of electrokinetic phenomena rests on the hypothesis of a dilute solution of pointlike ions in quasiequilibrium with a weakly charged surface, whose potential relative to the bulk is of order the thermal voltage (kT/e ≈ 25 mV at room temperature). In nonlinear electrokinetic ph ..."
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The venerable theory of electrokinetic phenomena rests on the hypothesis of a dilute solution of pointlike ions in quasiequilibrium with a weakly charged surface, whose potential relative to the bulk is of order the thermal voltage (kT/e ≈ 25 mV at room temperature). In nonlinear electrokinetic phenomena, such as AC or inducedcharge electroosmosis (ACEO, ICEO) and inducedcharge electrophoresis (ICEP), several Volts ≈ 100 kT/e are applied to polarizable surfaces in microscopic geometries, and the resulting electric fields and induced surface charges are large enough to violate the assumptions of the classical theory. In this article, we review the experimental and theoretical literatures, highlight discrepancies between theory and experiment, introduce possible modifications of the theory, and analyze their consequences. We argue that, in response to a large applied voltage, the “compact layer ” and “shear plane ” effectively advance into the liquid, due to the crowding of counterions. Using simple continuum models, we predict two general trends at large voltages: (i) ionic crowding against a blocking surface expands the diffuse double layer and thus decreases its differential capacitance, and (ii) a chargeinduced viscosity increase near the surface reduces the electroosmotic mobility; each trend is enhanced by dielectric saturation. The first effect is able to predict highfrequency flow reversal in ACEO
Optical manipulation of nanoparticles: a review
 J. Nanophoton
, 2008
"... Abstract. Optical trapping is an established field for movement of micronsize objects and cells. However, trapping of metal nanoparticles, nanowires, nanorods and molecules has received little attention. Nanoparticles are more challenging to optically trap and they offer ample new phenomena to exp ..."
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Abstract. Optical trapping is an established field for movement of micronsize objects and cells. However, trapping of metal nanoparticles, nanowires, nanorods and molecules has received little attention. Nanoparticles are more challenging to optically trap and they offer ample new phenomena to explore, for example the plasmon resonance. Resonance and size effects have an impact upon trapping forces that causes nanoparticle trapping to differ from micromanipulation of larger micronsized objects. There are numerous theoretical approaches to calculate optical forces exerted on trapped nanoparticles. Their combination and comparison gives the reader deeper understanding of the physical processes in an optical trap. A close look into the key experiments to date demonstrates the feasibility of trapping and provides a grasp of the enormous possibilities that remain to be explored. When constructing a singlebeam optical trap, particular emphasis has to be placed on the choice of imaging for the trapping and confinement of nanoparticles.
Autonomous microfluidics with stimuliresponsive hydrogels
 Soft Matter
, 2007
"... There has been increasing interest in integrated microfluidic systems because performing biological and chemical laboratory tasks on a single chip is appealing. One straightforward approach to constructing these ‘lab on chips ’ is to fabricate individual components and to assemble them for desired f ..."
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Cited by 10 (0 self)
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There has been increasing interest in integrated microfluidic systems because performing biological and chemical laboratory tasks on a single chip is appealing. One straightforward approach to constructing these ‘lab on chips ’ is to fabricate individual components and to assemble them for desired functionalities. As the functionalities of the microfluidic systems become increasingly complicated, more functional components and relevant controls need to be integrated on a miniaturized chip, especially when a closed loop is needed for autonomous functionality. Instead, an emerging approach is to incorporate stimuliresponsive hydrogels directly into microfluidics to reduce the system complexity. Due to the hydrogels ’ ability of transducing stimuli into mechanical actions in response to their surrounding aqueous environment, hydrogelbased microfluidic elements can act as both sensors and actuators simultaneously, alleviating the requirement of most controls and even power sources. This provides microfluidic systems with autonomous functionalities. In this article, we will focus on a few autonomous microfluidic devices including valves, flow sorters, pH regulators, pumps, mixers, drugdelivery devices, fluidic cooling devices, and liquid microlenses.
STOCHASTIC EULERIAN LAGRANGIAN METHODS FOR FLUIDSTRUCTURE INTERACTIONS WITH THERMAL FLUCTUATIONS
, 2010
"... Abstract. We present approaches for the study of fluidstructure interactions subject to thermal fluctuations. A mixed mechanical description is utilized combining Eulerian and Lagrangian reference frames. We establish general conditions for operators coupling these descriptions. Stochastic driving ..."
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Abstract. We present approaches for the study of fluidstructure interactions subject to thermal fluctuations. A mixed mechanical description is utilized combining Eulerian and Lagrangian reference frames. We establish general conditions for operators coupling these descriptions. Stochastic driving fields for the formalism are derived using principles from statistical mechanics. The stochastic differential equations of the formalism are found to exhibit significant stiffness in some physical regimes. To cope with this issue, we derive reduced stochastic differential equations for several physical regimes. We also present stochastic numerical methods for each regime to approximate the fluidstructure dynamics and to generate efficiently the required stochastic driving fields. To validate the methodology in each regime, we perform analysis of the invariant probability distribution of the stochastic dynamics of the fluidstructure formalism. We compare this analysis with results from statistical mechanics. To further demonstrate the applicability of the methodology, we perform computational studies for spherical particles having translational and rotational degrees of freedom. We compare these studies with results from fluid mechanics. The presented approach provides for fluidstructure systems a set of rather general computational methods for treating consistently structure mechanics, hydrodynamic coupling, and thermal fluctuations.
TETHER FORCE CONSTRAINTS IN STOKES FLOW BY THE IMMERSED BOUNDARY METHOD ON A PERIODIC DOMAIN
, 2009
"... The immersed boundary method is an algorithm for simulating the interaction of immersed elastic bodies or boundaries with a viscous incompressible fluid. The immersed elastic material is represented in the fluid equations by a system or field of applied forces. The particular case of Stokes flow wi ..."
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The immersed boundary method is an algorithm for simulating the interaction of immersed elastic bodies or boundaries with a viscous incompressible fluid. The immersed elastic material is represented in the fluid equations by a system or field of applied forces. The particular case of Stokes flow with applied forces on a periodic domain involves two related mathematical complications. One of these is that an arbitrary constant vector may be added to the fluid velocity, and the other is the constraint that the integral of the applied force must be zero. Typically, forces defined on a freely floating elastic immersed boundary or body satisfy this constraint, but there are many important classes of forces that do not. For example, the socalled tether forces that are used to prescribe the simulated configuration of an immersed boundary, possibly in a timedependent manner, typically do not sum to zero. Another type of force that does not have zero integral is a uniform force density that may be used to simulate an overall pressure gradient driving flow through a system. We present a method for periodic Stokes flow that when used with tether points, admits the use of all forces irrespective of their integral over the domain. A byproduct of this method is that the additive constant velocity associated with periodic Stokes flow is uniquely determined. Indeed, the additive constant is chosen at each time step so that the sum of the tether forces balances the sum of any other forces that may be applied.
Adynamic coupling model for hybrid atomistic–continuum computations.”
 Chemical Engineering Science.,
, 2007
"... Abstract A dynamic coupling model is developed for a hybrid atomisticcontinuum computation in microand nanofluidics. In the hybrid atomisticcontinuum computation, a molecular dynamics (MD) simulation is utilized in one region where the continuum assumption breaks down and the NavierStokes (NS) ..."
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Abstract A dynamic coupling model is developed for a hybrid atomisticcontinuum computation in microand nanofluidics. In the hybrid atomisticcontinuum computation, a molecular dynamics (MD) simulation is utilized in one region where the continuum assumption breaks down and the NavierStokes (NS) equations are used in another region where the continuum assumption holds. In the overlapping part of these two regions, a constrained particle dynamics is needed to couple the MD simulation and the NS equations. The currently existing coupling models for the constrained particle dynamics have a coupling parameter, which has to be empirically determined. In the present work, a novel dynamic coupling model is introduced where the coupling parameter can be calculated as the computation progresses rather than inputing a priori. The dynamic coupling model is based on the momentum constraint and exhibits a correct relaxation rate. The results from the hybrid simulation on the Couette flow and the Stokes flow are in good agreement with the data from the full MD simulation and the solutions of the NS equations, respectively.
Coalescence of Liquid Drops: Different Models Versus Experiment
, 2012
"... The process of coalescence of two identical liquid drops is simulated numerically in the framework of two essentially different mathematical models, and the results are compared with experimental data on the very early stages of the coalescence process reported recently. The first model tested is th ..."
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Cited by 7 (3 self)
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The process of coalescence of two identical liquid drops is simulated numerically in the framework of two essentially different mathematical models, and the results are compared with experimental data on the very early stages of the coalescence process reported recently. The first model tested is the ‘conventional ’ one, where it is assumed that coalescence as the formation of a single body of fluid occurs by an instant appearance of a liquid bridge smoothly connecting the two drops, and the subsequent process is the evolution of this single body of fluid driven by capillary forces. The second model under investigation considers coalescence as a process where a section of the free surface becomes trapped between the bulk phases as the drops are pressed against each other, and it is the gradual disappearance of this ‘internal interface ’ that leads to the formation of a single body of fluid and the conventional model taking over. Using the full numerical solution of the problem in the framework of each of the two models, we show that the recently reported electrical measurements probing the very early stages of the process are better described by the interface formation/disappearance model. New theoryguided experiments are suggested that would help to further elucidate the details of the coalescence phenomenon. As a byproduct of our research, the range of validity of different ‘scaling laws’ advanced as approximate solutions to the problem formulated using the conventional model is established.