Computer simulations of single particles in external electric fields

Abstract

Applying electric fields is an attractive way to control and manipulate single particles or molecules, e.g., in lab-on-a-chip devices. However, the response of nanosize objects in electrolyte solution to external fields is far from trivial. It is the result of a variety of dynamical processes taking place in the ion cloud surrounding charged particles and in the bulk electrolyte, and it is governed by an intricate interplay of electrostatic and hydrodynamic interactions. Already systems composed of one single particle in electrolyte solution exhibit a complex dynamical behaviour. In this review, we discuss recent coarse-grained simulations that have been performed to obtain a molecular-level understanding of the dynamic and dielectric response of single particles and single macromolecules to external electric fields. We address both the response of charged particles to constant fields (DC fields), which can be characterized by an electrophoretic mobility, and the dielectric response of both uncharged and charged particles to alternating fields (AC fields), which is described by a complex polarizability. Furthermore, we give a brief survey of simulation algorithms and highlight some recent developments.

Overcharging and reentrant condensation of thermoresponsive ionic microgels

“…As the temperature is raised, the particle surface becomes better defined and the overall | μ | behavior resembles that of compact hard particles, being characterized by a pronounced maximum at C NaCl ≈ 30 mM. The presence of a maximum is predicted by standard electrokinetic models taking into account retardation forces due to double layer relaxation 61 , 62 around hard spheres for sufficiently short screening lengths ( R h / λ > 3, where λ is the Debye screening length). There are essentially four forces accounted for in these models that determine the steady velocity of a particle subject to an external electric field: (1) the electric force acting on the colloid; (2) a hydrodynamic drag force; (3) a further electrostatic contribution due to the ion cloud displacement with respect to the center of the colloid; (4) a relaxation force, hydrodynamic in origin, resulting from the ion motion altering the solvent flow velocity around the particle.…”

“…Indeed the former scales as λ ∼ C −1/2 N aCl , while the latter scale as λ 2 . [60][61][62] With decreasing salt concentration |µ| increases until the faster growing relaxation forces take over, determining the decrease of the mobility. This is the case of our microgels at T ≥ 30 • C, where mobility is non-monotonic and for which 44 R h /λ 123.…”

DNA dielectrophoresis: Theory and applications a review

“…Recent developments, however, show a trend in the theoretical studies of dielectric particle properties toward more complex particle shapes, such as rod‐like “polyelectrolytes” (see for instance ), which may even be allowed to bend in order to mimic simple‐model DNA . Moreover, advances in simulation techniques and increased computer power nowadays allow simulating electrokinetic phenomena to such resolution that the mechanisms behind the dielectric response of single polyelectrolyte macromolecules to external electric fields can be studied even down to the molecular level . For instance, in [, ] ion distributions around DNA fragments have been computed using different simulation approaches, ranging from Brownian dynamics to all atom techniques.…”

Electrophoresis in dilute polymer solutions

“…In recent years, the topic has been extended to consider the effects of boundaries (Yariv 2006), nonlinear particle velocity due to the higher-order terms in the weak field expansion (Schnitzer et al 2013), strong fields (Schnitzer & Yariv 2012b), oscillating electric fields (Mangelsdorf & White 1992;Sawatzky & Babchin 1993) and soft particles (Ohshima 2013). More numerical simulations of electrophoresis of a single particle can be found in a recent review by Zhou & Schmid (2015). The electrophoresis of a single particle is important because it serves as a basis for many electrokinetic phenomena, such as electroacoustics and electrorheology (Saville 1977).…”