Jiang-Xing Chen

Interaction of a Chemically Propelled Nanomotor with a Chemical Wave

Self-propulsion on small scales is an ubiquitous phenomenon in biology. Bacteria and other microorganisms swim in order to obtain food or to respond to stimuli, molecular motors like ATP synthase are the engines that power swimming motion, and kinesin and other molecular motors are essential for active transport in the cell, participate in the DNA replication process and perform a variety of other essential tasks. More recently, there has been considerable interest in synthetic nanoand micrometer-scale self-propelled objects. 6] Chemists have fabricated a variety of nanomachines that either swim by unsymmetrical motions driven by external fields or utilize chemical reactions to effect directed motion. Theoretical models have been constructed to describe chemically powered nanomotors. Several supramolecular entities, such as pseudorotaxanes, rotaxanes, and catenanes, that can be used as molecular switches, molecular brakes, and rachets have been developed. Synthetic nanomotors with no moving parts and use chemical energy for their directed motion provide some of the simplest examples of self-propulsion on nanoand micrometer scales. These include bimetallic nanorods, Pt–silica sphere dimers, and Janus particles. There has also been a significant effort aimed at controlling the motion and transport of nanoscale objects by magnetic fields, microchannel networks, chemical sensing, and other means. The interest in these synthetic nanomachines stems from their potential applications: targeted drug delivery, pick up and delivery of cargo, motion-based biosensing, nanoscale assembly, targeted synthesis, nanoand microfluidics, collective oscillations, nanoactuators, etc. 23–30] It is well known that chemical systems which are displaced far from equilibrium may exhibit temporal oscillations and chaos or may self-organize to form spatially inhomogeneous patterns such as chemical waves in solution or on catalytic surfaces, Turing patterns, etc. Questions that naturally arise are, how do nanomotors move and respond to chemically active environments, and can the inhomogeneity in the environment be used to influence the dynamics of motors and possibly provide a way to control their motions? Here, we consider how a sphere dimer motor moves in a chemically active medium and interacts with a chemical wave. We find that a chemical wave is able to reflect a dimer motor (Figure 1) and suggest that this effect can provide a possible

Mesoscopic dynamics of diffusion-influenced enzyme kinetics

A particle-based mesoscopic model for enzyme kinetics is constructed and used to investigate the influence of diffusion on the reactive dynamics. Enzymes and enzyme-substrate complexes are modeled as finite-size soft spherical particles, while substrate, product, and solvent molecules are point particles. The system is evolved using a hybrid molecular dynamics-multiparticle collision dynamics scheme. Both the nonreactive and reactive dynamics are constructed to satisfy mass, momentum, and energy conservation laws, and reversible reaction steps satisfy detailed balance. Hydrodynamic interactions among the enzymes and complexes are automatically accounted for in the dynamics. Diffusion manifests itself in various ways, notably in power-law behavior in the evolution of the species concentrations. In accord with earlier investigations, regimes where the product production rate exhibits either monotonic or nonmonotonic behavior as a function of time are found. In addition, the species concentrations display both t(-1/2) and t(-3/2) power-law behavior, depending on the dynamical regime under investigation. For high enzyme volume fractions, cooperative effects influence the enzyme kinetics. The time dependent rate coefficient determined from the mass action rate law is computed and shown to depend on the enzyme concentration. Lifetime distributions of substrate molecules newly released in complex dissociation events are determined and shown to have either a power-law form for rebinding to the same enzyme from which they were released or an exponential form for rebinding to different enzymes. The model can be used and extended to explore a variety of issues related concentration effects and diffusion on enzyme kinetics.

Termination of pinned spirals by local stimuli

The termination of pinned spirals on a defect by means of local stimuli is studied. On a completely unexcitable defect, the elimination process is discussed and its corresponding mechanism is presented. Especially, the mechanism of unpinning spirals on a partially unexcitable defect, which has not been investigated so far, is explored. With fixed pacing frequency ω L , there exists a maximal radius R max above which the pinned spiral cannot be removed. It is found that the value of R max does not increase as ω L in a dynamical regime, forming a platform in the curves. Based on analyzing the dispersion relation on the spiral tip around the obstacle, the underlying mechanism is clarified. Also, it is found that when multiple spirals are pinned, the value of R max decreases on a partially unexcitable defect while the change is very slight on a completely unexcitable one.

Liberation of a pinned spiral wave by a rotating electric pulse

Spiral waves may be pinned to anatomical heterogeneities in the cardiac tissue, which leads to monomorphic ventricular tachycardia. Wave emission from heterogeneities (WEH) induced by electric pulses in one direction (EP) is a promising method for liberating such waves by using heterogeneities as internal virtual pacing sites. Here, based on the WEH effect, a new mechanism of liberation by means of a rotating electric pulse (REP) is proposed in a generic model of excitable media. Compared with the EP, the REP has the advantage of opening wider time window to liberate pinned spiral. The influences of rotating direction and frequency of the REP, and the radius of the obstacles on this new mechanism are studied. We believe this strategy may improve manipulations with pinned spiral waves in heart experiments.

Influences of periodic mechanical deformation on pinned spiral waves

In a generic model of excitable media, we study the behavior of spiral waves interacting with obstacles and their dynamics under the influences of simple periodic mechanical deformation (PMD). Depending on the characteristics of the obstacles, i.e., size and excitability, the rotation of a pinned spiral wave shows different scenarios, e.g., embedding into or anchoring on an obstacle. Three different drift phenomena induced by PMD are observed: scattering on small partial-excitable obstacles, meander-induced unpinning on big partial-excitable obstacles, and drifting around small unexcitable obstacles. Their underlying mechanisms are discussed. The dependence of the threshold amplitude of PMD on the characteristics of the obstacles to successfully remove pinned spiral waves on big partial-excitable obstacles is studied.

Emitting waves from heterogeneity by a rotating electric field.

In a generic model of excitable media, we simulate wave emission from a heterogeneity (WEH) induced by an electric field. Based on the WEH effect, a rotating electric field is proposed to terminate existed spatiotemporal turbulence. Compared with the effects resulted by a periodic pulsed electric field, the rotating electric field displays several improvements, such as lower required intensity, emitting waves on smaller obstacles, and shorter suppression time. Furthermore, due to rotation of the electric field, it can automatically source waves from the boundary of an obstacle with small curvature.

Control of spiral breakup by an alternating advective field

The control of spiral breakup due to Doppler instability is investigated. It is found that applying an alternating advective field with suitable amplitude and period can prevent the breakup of spiral waves. Further numerical simulations show that the growing meandering behavior of a spiral tip caused by decreasing the excitability of the medium can be efficiently suppressed by the alternating advective field, which inhibits the breakup of spiral waves eventually.

Dynamical phase of driven colloidal systems with short-range attraction and long-range repulsion.

We study the nonequilibrium dynamics of colloidal system with short-range depletion attraction and screened electrostatic repulsion on a disordered substrate. We find a growth-melting process of the clusters as the temperature is increased. By strengthening the screened electrostatic repulsion, a depinning transition from moving cluster to plastic flow is observed, which is characterized by a peak in threshold depinning force. The corresponding phase diagram is then mapped out. Due to the influences of disorder from substrate, the clusters are polarized by the strong external force, accompanied by the appearance of interesting orientational order parallel to the force and translational order perpendicular to the force. Under the condition of strong external force, the influences of density of pins and temperature are also studied.

Influences of periodic mechanical deformation on spiral breakup in excitable media.

Influences of periodic mechanical deformation (PMD) on spiral breakup that results from Doppler instability in excitable media are investigated. We present a new effect: a high degree of homogeneous PMD is favored to prevent the low-excitability-induced breakup of spiral waves. The frequency and amplitude of PMD are also significant for achieving this purpose. The underlying mechanism of successful control is also discussed, which is believed to be related to the increase of the minimum temporal period of the meandering spiral when the suitable PMD is applied.

Synchronization of a spiral by a circularly polarized electric field in reaction-diffusion systems

Synchronization of a spiral by a circularly polarized electric field (CPEF) in reaction-diffusion systems is investigated since they both possess rotation symmetry. It is found that spirals in different regimes (including rigidly rotating, meandering, and drifting spirals) can be forced to be rigidly rotating ones by CPEFs. Moreover, the rotational frequency of the entrained spiral is found to be synchronized with the frequency of the electric field in a ratio of 1:1.