V. Lobaskin - Academia.edu (original) (raw)
Papers by V. Lobaskin
Journal of Physics: Condensed Matter, 2011
Optical tweezers are experimental tools with extraordinary resolution in positioning (±1 nm) a mi... more Optical tweezers are experimental tools with extraordinary resolution in positioning (±1 nm) a micron-sized colloid and in the measurement of forces (±50 fN) acting on it-without any mechanical contact. This enables one to carry out a multitude of novel experiments in nano-and microfluidics, of which the following will be presented in this review: (i) forces within single pairs of colloids in media of varying concentration and valency of the surrounding ionic solution, (ii) measurements of the electrophoretic mobility of single colloids in different solvents (concentration, valency of the ionic solution and pH), (iii) similar experiments as in (i) with DNA-grafted colloids, (iv) the nonlinear response of single DNA-grafted colloids in shear flow and (v) the drag force on single colloids pulled through a polymer solution. The experiments will be described in detail and their analysis discussed.
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2013
Optical Tweezers are employed to study the electrophoretic and the electroosmotic motion of a sin... more Optical Tweezers are employed to study the electrophoretic and the electroosmotic motion of a single colloid immersed in electrolyte solutions of ion concentrations between 10 −5 and 1 mol/l and of different valencies (KCl, CaCl 2 , LaCl 3 ). The measured particle mobility in monovalent salt is found to be in agreement with computations combining primitive model molecular dynamics simulations of the ionic double layer with the standard electrokinetic model. Mobility reversal of a single colloid-for the first time-is observed in the presence of trivalent ions (LaCl 3 ) at ionic strengths larger than 10 −2 mol/l. In this case, our numerical model is in a quantitative agreement with the experiment only when ion specific attractive forces are added to the primitive model, demonstrating that at low colloidal charge densities, ion correlation effects alone do not suffice to produce mobility reversal.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2014
We study the orientational ordering in systems of self-propelled particles with selective interac... more We study the orientational ordering in systems of self-propelled particles with selective interactions. To introduce the selectivity we augment the standard Vicsek model with a bounded-confidence collision rule: a given particle only aligns to neighbors who have directions quite similar to its own. Neighbors whose directions deviate more than a fixed restriction angle α are ignored. The collective dynamics of this system is studied by agent-based simulations and kinetic mean-field theory. We demonstrate that the reduction of the restriction angle leads to a critical noise amplitude decreasing monotonically with that angle, turning into a power law with exponent 3/2 for small angles. Moreover, for small system sizes we show that upon decreasing the restriction angle, the kind of the transition to polar collective motion changes from continuous to discontinuous. Thus, an apparent tricritical point with different scaling laws is identified and calculated analytically. We investigate th...
Physical Review Letters, 2007
We study the electrophoretic mobility of spherical charged colloids in a low-salt suspension as a... more We study the electrophoretic mobility of spherical charged colloids in a low-salt suspension as a function of the colloidal concentration. Using an effective particle charge and a reduced screening parameter, we map the data for systems with different particle charges and sizes, including numerical simulation data with full electrostatics and hydrodynamics and experimental data for latex dispersions, on a single master curve. We observe two different volume fraction-dependent regimes for the electrophoretic mobility that can be explained in terms of the static properties of the ionic double layer. 82.70.Dd,66.20.+d Many important properties of colloidal dispersions are directly or indirectly determined by the electric charge of the colloidal particles. Phase stability is provided by the repulsive interaction between like charges, while the details of the static and dynamic behavior are the result of the interplay between electrostatic interactions between macroions, counterions, and added salt ions, the dielectric response of the solvent, and the solvent hydrodynamics. Depending on whether the concentration of the background electrolyte (relative to that of the "native" counterions) is large or small, one must expect quite different behavior both with respect to statics and dynamics. The salt-dominated regime (high salt concentration) has been studied extensively both for static [1] and dynamic properties . In this case, the large reservoir of salt ions results in a strong Debye screening of the electrostatic interactions between the macroions, such that the net charge density is essentially zero throughout the dispersion except for narrow ionic atmospheres around the colloids. An external electric field will therefore exert forces only within these layers, such that hydrodynamic interactions are also strongly screened in a system subjected to electrophoresis . Therefore, the problems both of ion cloud structure and of electrophoresis can be treated within a single-macroion framework , and the dependence of the electrophoretic mobility µ = v/E (v denoting the colloid drift velocity and E the driving electric field) on the macroion volume fraction Φ is weak.
Journal of Physics: Condensed Matter, 2008
We consider electrophoresis of a single charged colloidal particle in a finite box with periodic ... more We consider electrophoresis of a single charged colloidal particle in a finite box with periodic boundary conditions, where added counterions and salt ions ensure charge neutrality. A systematic rescaling of the electrokinetic equations allows us to identify a minimum set of suitable dimensionless parameters, which, within this theoretical framework, determine the reduced electrophoretic mobility. It turns out that the salt-free case can, on the Mean Field level, be described in terms of just three parameters. A fourth parameter, which had previously been identified on the basis of straightforward dimensional analysis, can only be important beyond Mean Field. More complicated behavior is expected to arise when further ionic species are added. However, for a certain parameter regime, we can demonstrate that the salt-free case can be mapped onto a corresponding system containing additional salt. The Green-Kubo formula for the electrophoretic mobility is derived, and its usefulness demonstrated by simulation data. Finally, we report on finite-element solutions of the electrokinetic equations, using the commercial software package COMSOL.
The European Physical Journal Special Topics, 2013
ABSTRACT Motivated by both still open fundamental theoretical questions as well as novel applicat... more ABSTRACT Motivated by both still open fundamental theoretical questions as well as novel applications, the electro-kinetics of highly charged model colloids have attracted considerable interest in the last few years. The present paper reviews corresponding new developments and trends emerging mainly from novel instrumentations for both strongly interacting and extremely dilute systems. We also highlight recent uses of local electric fields and electro-kinetic currents to realize complex micro-swimmers or prescribe crystal micro-structures.
Computer Physics Communications, 1999
Computer simulation of highly asymmetric electrolytes with charge asymmetry 60" 1 and 60" 2 V. Lo... more Computer simulation of highly asymmetric electrolytes with charge asymmetry 60" 1 and 60" 2 V. Lobaskin 1, R Linse 2
We derive the single molecule equation of state (force-extension relation) for DNA molecules bear... more We derive the single molecule equation of state (force-extension relation) for DNA molecules bearing sliding loops and deflection defects. Analytical results are obtained in the large force limit by employing an analogy with instantons in quantum mechanical tunneling problems. The results reveal a remarkable feature of sliding loops--an apparent strong reduction of the persistence length. We generalize these results to several other experimentally interesting situations ranging from rigid DNA-protein loops to the problem of anchoring deflections in atomic force microscopy stretching of semiflexible polymers. Expressions relating the force-extension measurements to the underlying loop or boundary deflection geometry are provided and applied to the case of the gal repressor dimer protein. The theoretical predictions are complemented and quantitatively confirmed by molecular dynamics simulations.
Journal of Physics: Condensed Matter, 2011
Optical tweezers are experimental tools with extraordinary resolution in positioning (±1 nm) a mi... more Optical tweezers are experimental tools with extraordinary resolution in positioning (±1 nm) a micron-sized colloid and in the measurement of forces (±50 fN) acting on it-without any mechanical contact. This enables one to carry out a multitude of novel experiments in nano-and microfluidics, of which the following will be presented in this review: (i) forces within single pairs of colloids in media of varying concentration and valency of the surrounding ionic solution, (ii) measurements of the electrophoretic mobility of single colloids in different solvents (concentration, valency of the ionic solution and pH), (iii) similar experiments as in (i) with DNA-grafted colloids, (iv) the nonlinear response of single DNA-grafted colloids in shear flow and (v) the drag force on single colloids pulled through a polymer solution. The experiments will be described in detail and their analysis discussed.
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2013
Optical Tweezers are employed to study the electrophoretic and the electroosmotic motion of a sin... more Optical Tweezers are employed to study the electrophoretic and the electroosmotic motion of a single colloid immersed in electrolyte solutions of ion concentrations between 10 −5 and 1 mol/l and of different valencies (KCl, CaCl 2 , LaCl 3 ). The measured particle mobility in monovalent salt is found to be in agreement with computations combining primitive model molecular dynamics simulations of the ionic double layer with the standard electrokinetic model. Mobility reversal of a single colloid-for the first time-is observed in the presence of trivalent ions (LaCl 3 ) at ionic strengths larger than 10 −2 mol/l. In this case, our numerical model is in a quantitative agreement with the experiment only when ion specific attractive forces are added to the primitive model, demonstrating that at low colloidal charge densities, ion correlation effects alone do not suffice to produce mobility reversal.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2014
We study the orientational ordering in systems of self-propelled particles with selective interac... more We study the orientational ordering in systems of self-propelled particles with selective interactions. To introduce the selectivity we augment the standard Vicsek model with a bounded-confidence collision rule: a given particle only aligns to neighbors who have directions quite similar to its own. Neighbors whose directions deviate more than a fixed restriction angle α are ignored. The collective dynamics of this system is studied by agent-based simulations and kinetic mean-field theory. We demonstrate that the reduction of the restriction angle leads to a critical noise amplitude decreasing monotonically with that angle, turning into a power law with exponent 3/2 for small angles. Moreover, for small system sizes we show that upon decreasing the restriction angle, the kind of the transition to polar collective motion changes from continuous to discontinuous. Thus, an apparent tricritical point with different scaling laws is identified and calculated analytically. We investigate th...
Physical Review Letters, 2007
We study the electrophoretic mobility of spherical charged colloids in a low-salt suspension as a... more We study the electrophoretic mobility of spherical charged colloids in a low-salt suspension as a function of the colloidal concentration. Using an effective particle charge and a reduced screening parameter, we map the data for systems with different particle charges and sizes, including numerical simulation data with full electrostatics and hydrodynamics and experimental data for latex dispersions, on a single master curve. We observe two different volume fraction-dependent regimes for the electrophoretic mobility that can be explained in terms of the static properties of the ionic double layer. 82.70.Dd,66.20.+d Many important properties of colloidal dispersions are directly or indirectly determined by the electric charge of the colloidal particles. Phase stability is provided by the repulsive interaction between like charges, while the details of the static and dynamic behavior are the result of the interplay between electrostatic interactions between macroions, counterions, and added salt ions, the dielectric response of the solvent, and the solvent hydrodynamics. Depending on whether the concentration of the background electrolyte (relative to that of the "native" counterions) is large or small, one must expect quite different behavior both with respect to statics and dynamics. The salt-dominated regime (high salt concentration) has been studied extensively both for static [1] and dynamic properties . In this case, the large reservoir of salt ions results in a strong Debye screening of the electrostatic interactions between the macroions, such that the net charge density is essentially zero throughout the dispersion except for narrow ionic atmospheres around the colloids. An external electric field will therefore exert forces only within these layers, such that hydrodynamic interactions are also strongly screened in a system subjected to electrophoresis . Therefore, the problems both of ion cloud structure and of electrophoresis can be treated within a single-macroion framework , and the dependence of the electrophoretic mobility µ = v/E (v denoting the colloid drift velocity and E the driving electric field) on the macroion volume fraction Φ is weak.
Journal of Physics: Condensed Matter, 2008
We consider electrophoresis of a single charged colloidal particle in a finite box with periodic ... more We consider electrophoresis of a single charged colloidal particle in a finite box with periodic boundary conditions, where added counterions and salt ions ensure charge neutrality. A systematic rescaling of the electrokinetic equations allows us to identify a minimum set of suitable dimensionless parameters, which, within this theoretical framework, determine the reduced electrophoretic mobility. It turns out that the salt-free case can, on the Mean Field level, be described in terms of just three parameters. A fourth parameter, which had previously been identified on the basis of straightforward dimensional analysis, can only be important beyond Mean Field. More complicated behavior is expected to arise when further ionic species are added. However, for a certain parameter regime, we can demonstrate that the salt-free case can be mapped onto a corresponding system containing additional salt. The Green-Kubo formula for the electrophoretic mobility is derived, and its usefulness demonstrated by simulation data. Finally, we report on finite-element solutions of the electrokinetic equations, using the commercial software package COMSOL.
The European Physical Journal Special Topics, 2013
ABSTRACT Motivated by both still open fundamental theoretical questions as well as novel applicat... more ABSTRACT Motivated by both still open fundamental theoretical questions as well as novel applications, the electro-kinetics of highly charged model colloids have attracted considerable interest in the last few years. The present paper reviews corresponding new developments and trends emerging mainly from novel instrumentations for both strongly interacting and extremely dilute systems. We also highlight recent uses of local electric fields and electro-kinetic currents to realize complex micro-swimmers or prescribe crystal micro-structures.
Computer Physics Communications, 1999
Computer simulation of highly asymmetric electrolytes with charge asymmetry 60" 1 and 60" 2 V. Lo... more Computer simulation of highly asymmetric electrolytes with charge asymmetry 60" 1 and 60" 2 V. Lobaskin 1, R Linse 2
We derive the single molecule equation of state (force-extension relation) for DNA molecules bear... more We derive the single molecule equation of state (force-extension relation) for DNA molecules bearing sliding loops and deflection defects. Analytical results are obtained in the large force limit by employing an analogy with instantons in quantum mechanical tunneling problems. The results reveal a remarkable feature of sliding loops--an apparent strong reduction of the persistence length. We generalize these results to several other experimentally interesting situations ranging from rigid DNA-protein loops to the problem of anchoring deflections in atomic force microscopy stretching of semiflexible polymers. Expressions relating the force-extension measurements to the underlying loop or boundary deflection geometry are provided and applied to the case of the gal repressor dimer protein. The theoretical predictions are complemented and quantitatively confirmed by molecular dynamics simulations.