Rectification and separation of mixtures of active and passive particles driven by temperature difference.

Transport and separation of binary mixtures of active and passive particles are investigated in the presence of temperature differences. It is found that temperature differences can strongly affect the rectification and separation of the mixtures. For active particles, there exists an optimal temperature difference at which the rectified efficiency is maximal. Passive particles are not propelled and move by collisions with active particles, so the response to temperature differences is more complicated. By changing the system parameters, active particles can change their directions, while passive particles always move in the same direction. The simulation results show that the separation of mixtures is sensitive to the system parameters, such as the angular velocity, the temperature difference, and the polar alignment. The mixed particles can be completely separated under certain conditions.

Mixing and demixing of binary mixtures of polar chiral active particles.

We study a binary mixture of polar chiral (counterclockwise or clockwise) active particles in a two-dimensional box with periodic boundary conditions. Besides the excluded volume interactions between particles, the particles are also subjected to the polar velocity alignment. From the extensive Brownian dynamics simulations, it is found that the particle configuration (mixing or demixing) is determined by the competition between the chirality difference and the polar velocity alignment. When the chirality difference competes with the polar velocity alignment, the clockwise particles aggregate in one cluster and the counterclockwise particles aggregate in the other cluster; thus, the particles are demixed and can be separated. However, when the chirality difference or the polar velocity alignment is dominant, the particles are mixed. Our findings could be used for the experimental pursuit of the separation of binary mixtures of chiral active particles.

Giant negative mobility of inertial particles caused by the periodic potential in steady laminar flows.

Transport of an inertial particle advected by a two-dimensional steady laminar flow is numerically investigated in the presence of a constant force and a periodic potential. Within particular parameter regimes, this system exhibits absolute negative mobility, which means that the particle can travel in a direction opposite to the constant force. It is found that the profile of the periodic potential plays an important role in the nonlinear response regime. Absolute negative mobility can be drastically enhanced by applying appropriate periodic potential, the parameter regime for this phenomenon becomes larger and the amplitude of negative mobility grows exceedingly large (giant negative mobility). In addition, giant positive mobility is also observed in the presence of appropriate periodic potential.

Transport of particles driven by the traveling obstacle arrays.

Transport of three types of particles (passive particles, active particles without polar interaction, and active particles with polar interaction) is numerically investigated in the presence of traveling obstacle arrays. The transport behaviors are different for different types of particles. For passive particles, there exists an optimal traveling speed (or the translational diffusion) at which the average velocity of particles takes its maximum value. For active particles without polar interaction, the average velocity of particles is a peaked function of the obstacle traveling speed. The average velocity decreases monotonically with increase of the rotational diffusion for large driving speed, while it is a peaked function of the rotational diffusion for small driving speed. For active particles with polar interaction, interestingly, within particular parameter regimes, active particles can move in the opposite direction to the obstacles. The average velocity of particles can change its direction by changing the system parameters (the obstacles driving speed, the polar interaction strength, and the rotational diffusion).

Chirality separation of mixed chiral microswimmers in a periodic channel.

Dynamics and separation of mixed chiral microswimmers are numerically investigated in a channel with regular arrays of rigid half-circle obstacles. For zero shear flow, transport behaviors are the same for different chiral particles: the average velocity decreases with increase of the rotational diffusion coefficient, the direction of the transport can be reversed by tuning the angular velocity, and there exists an optimal value of the packing fraction at which the average velocity takes its maximal value. However, when the shear flow is considered, different chiral particles show different behaviors. By suitably tailoring parameters, particles with different chiralities can move in different directions and can be separated. In addition, we also proposed a space separation method by introducing a constant load, where counterclockwise and clockwise particles stay in different regions of the channel.

Entropic ratchet transport of interacting active Brownian particles.

Directed transport of interacting active (self-propelled) Brownian particles is numerically investigated in confined geometries (entropic barriers). The self-propelled velocity can break thermodynamical equilibrium and induce the directed transport. It is found that the interaction between active particles can greatly affect the ratchet transport. For attractive particles, on increasing the interaction strength, the average velocity first decreases to its minima, then increases, and finally decreases to zero. For repulsive particles, when the interaction is very weak, there exists a critical interaction at which the average velocity is minimal, nearly tends to zero, however, for the strong interaction, the average velocity is independent of the interaction.

Nonlinear thermal conductance in single-wall carbon nanotubes: negative differential thermal resistance.

Nonlinear thermal conductance in single-wall carbon nanotubes (CNTs) is investigated by applying the large temperature difference. From the classical molecular dynamics simulations, we observe the occurrence of negative differential thermal resistance (NDTR) in CNTs, namely, there exist a region of temperature difference, where the heat flux decreases as the applied temperature difference increases. Double NDTRs even occur in some CNTs with special diameters. When the length of CNTs increases, NDTR regime reduces and vanishes in the thermodynamic limit. CNTs with NDTR may be the good candidate materials for designing thermal devices such as thermal transistors, thermal logic gates, and thermal memory.

Hydrodynamically enforced entropic Brownian pump.

Transport of overdamped Brownian particles in a finite hydrodynamical channel is investigated in the presence of the ac driving force and the pressure-driven flow. The system is bounded by two particle reservoirs. With the help of the Fick-Jacobs method, we obtain the directed current of Brownian particles and the pumping capacity of the system. The directed transport is determined by the competitions among the asymmetry of the channel, the ac driving force, the pressure-driven flow, and the concentration difference. Their interplays can exhibit the peculiar properties. Remarkably, the particles can be pumped through the channel from the lower concentration reservoir to the higher concentration one, or from the lower pressure side to the higher pressure one. In addition, due to the existence of the pressure drop, ac driving force still plays the significant role on directed transport even in a completely symmetric channel. Our results could be implemented in constrained structures with narrow channels or pores where the particles are suspended in a solvent.

In silico synergism and antagonism of an anti-tumour system intervened by coupling immunotherapy and chemotherapy: a mathematical modelling approach.

Based on the logistic growth law for a tumour derived from enzymatic dynamics, we address from a physical point of view the phenomena of synergism, additivity and antagonism in an avascular anti-tumour system regulated externally by dual coupling periodic interventions, and propose a theoretical model to simulate the combinational administration of chemotherapy and immunotherapy. The in silico results of our modelling approach reveal that the tumour population density of an anti-tumour system, which is subject to the combinational attack of chemotherapeutical as well as immune intervention, depends on four parameters as below: the therapy intensities D, the coupling intensity I, the coupling coherence R and the phase-shifts Φ between two combinational interventions. In relation to the intensity and nature (synergism, additivity and antagonism) of coupling as well as the phase-shift between two therapeutic interventions, the administration sequence of two periodic interventions makes a difference to the curative efficacy of an anti-tumour system. The isobologram established from our model maintains a considerable consistency with that of the well-established Loewe Additivity model (Tallarida, Pharmacology 319(1):1-7, 2006). Our study discloses the general dynamic feature of an anti-tumour system regulated by two periodic coupling interventions, and the results may serve as a supplement to previous models of drug administration in combination and provide a type of heuristic approach for preclinical pharmacokinetic investigation.

Rectified brownian transport in corrugated channels: Fractional brownian motion and Lévy flights.

We study fractional brownian motion and Lévy flights in periodic corrugated channels without any external driving forces. From numerical simulations, we find that both fractional gaussian noise and Lévy-stable noise in asymmetric corrugated channels can break thermodynamical equilibrium and induce directed transport. The rectified mechanisms for fractional brownian motion and Lévy flights are different. The former is caused by non-uniform spectral distribution (low or high frequencies) of fractional gaussian noise, while the latter is due to the nonthermal character (occasional long jumps) of the Lévy-stable noise. For fractional brownian motion, average velocity increases with the Hurst exponent for the persistent case, while for the antipersistent case there exists an optimal value of Hurst exponent at which average velocity takes its maximal value. For Lévy flights, the group velocity decreases monotonically as the Lévy index increases. In addition, for both cases, the optimized periodicity and radius at the bottleneck can facilitate the directed transport. Our results could be implemented in constrained structures with narrow channels and pores where the particles undergo anomalous diffusion.

Negative differential thermal resistance induced by ballistic transport.

Using nonequilibrium molecular-dynamics simulations, we study the temperature dependence of the negative differential thermal resistance that appears in two-segment Frenkel-Kontorova lattices. We apply the theoretical method based on Landauer equation to obtain the relationship between the heat current and the temperature, which states a fundamental interpretation about the underlying physical mechanism of the negative differential thermal resistance. The temperature profiles and transport coefficients are demonstrated to explain the crossover from diffusive to ballistic transport. The finite-size effect is also discussed.

Scaling and the thermal conductivity of the Frenkel-Kontorova model.

We have studied the dependence of the thermal conductivity kappa on the strength of the interparticle potential lambda and the strength of the external potential beta in the Frenkel-Kontorova model. We found that the functional relation can be expressed in a scaling form, kappa proportional, variantlambda;{32}/beta;{2} . This result is first obtained by nonequilibrium molecular dynamics. It is then confirmed by two analytical methods, the self-consistent phonon theory and the self-consistent stochastic reservoirs method. The thermal conductivity kappa is therefore a decreasing functon of beta and an increasing function of lambda.

[Quantitative evaluation of CT-MRI images of various tumors with expansive or infiltrative growth pattern].

OBJECTIVE: To investigate quantitatively the proliferation of tumors in characteristic of either expansive or infiltrative growth pattern. METHODS: The CT/MRI images of 34 tumor patients with 6 different pathological types were adopted to undergo quantitative evaluation of the growth pattern of these tumors. Three key parameters, fractal dimension (Df) of the border between the tumor and neighboring tissues, degree of heterogeneity (H), and clumsiness (C) inside the tumor, were computed numerically. RESULTS: The Df values of the infiltrative tumors were higher than those of the expansive tumors, thus revealing the more evident complexity and openness of the border of infiltrative tumor. Heterogeneity and clumsiness existed within both expansive and infiltrative tumors. The clumsiness derived from heterogeneity and they were correlated positively with each other. The growth coefficient of clumsiness of the expansive tumor was greater than that of the infiltrative tumor. CONCLUSION: The method to analyze the fractal dimension of tumor border, and degrees of heterogeneity and clumsiness within a tumor presented in this paper gives some enlightenment to the fundamental research in characterizing tumor growth and shows practical value in clinical diagnosis of tumor, and may be of benefit to the quantitative evaluation of radiological images and computer-aided diagnosis.

Effects of hydrodynamic interactions on rectified transport of self-propelled particles.

Directed transport of self-propelled particles is numerically investigated in a three-dimensional asymmetric potential. Beside the steric repulsive forces, hydrodynamic interactions between particles have been taken into account in an approximate way. From numerical simulations, we find that hydrodynamic interactions can strongly affect the rectified transport of self-propelled particles. Hydrodynamic interactions enhance the performance of the rectified transport when particles can easily pass across the barrier of the potential, and reduce the rectified transport when particles are mainly trapped in the potential well.

Pure multiplicative stochastic resonance of a theoretical anti-tumor model with seasonal modulability.

Pure multiplicative noise-induced stochastic resonance, which appears in an anti-tumor system modulated by a seasonal external field, is studied by using theoretical analyses of the generalized potential and numerical simulations. For optimally selected values of the multiplicative noise intensity stochastic resonance is observed, which is manifested by the quasisymmetry of two potential minima. Theoretical results and numerical simulations are in good quantitative agreement.

Spatiotemporal fluctuation-induced transition in a tumor model with immune surveillance.

We report on a simple model of spatially extended antitumor system with a fluctuation in growth rate, which can undergo a nonequilibrium phase transition. Three states as excited, subexcited and nonexcited states of a tumor are defined to describe its growth. The multiplicative noise is found to have opposite effects: The positive effect on a nonexcited tumor and the negative effect on an excited tumor.

Autonomous pump against concentration gradient.

Using non-equilibrium molecular dynamics and Monte Carlo methods, we have studied the molecular transport in asymmetric nanochannels. The efficiency of the molecular pump depends on the angle and apertures of the asymmetric channel, the environmental temperature and average concentration of the particles. The pumping effect can be explained as the competition between the molecular force field and the thermal disturbance. Our results provide a green approach for pumping fluid particles against the concentration gradient through asymmetric nanoscale thin films without any external forces. It indicates that pumping vacuum can be a spontaneous process.

Rectification and diffusion of self-propelled particles in a two-dimensional corrugated channel.

Rectification and diffusion of noninteracting self-propelled particles is numerically investigated in a two-dimensional corrugated channel. From numerical simulations, we obtain the average velocity and the effective diffusion coefficient. It is found that the self-propelled particles can be rectified by the self-propelled velocity. There exist optimal values of the parameters (the self-propelled velocity, the translational diffusion constant, and the height of the potential) at which the average velocity takes its maximal value. There exists an optimal translational diffusion at which the effective diffusion constant is maximal. The self-propelled velocity can strongly increase the effective diffusion, while the large rotational diffusion rate can strongly suppress the effective diffusion.

Thermal control in graphene nanoribbons: thermal valve, thermal switch and thermal amplifier.

We study the thermal transport in graphene nanoribbons by using nonequilibrium molecular dynamics simulations. It is reported that the three-terminal graphene nanoribbons can perform some functions of thermal devices such as thermal valve, thermal switch and thermal amplifier. Electronic devices have transformed almost all aspects of our lives. It has not escaped our attention that the graphene nanoribbons we have presented here may have similar surprising applications in devices that allow the flow of heat to be controlled in a short future.

Regulation of Turing patterns in a spatially extended chlorine-iodine-malonic-acid system with a local concentration-dependent diffusivity.

A chlorine-iodine-malonic-acid Turing system involving a local concentration-dependent diffusivity (LCDD) has fundamental significance for physical, chemical, and biological systems with inhomogeneous medium. We investigated such a system by both numerical computation and mathematical analysis. Our research reveals that a variable local diffusivity has an evident effect on regulating the Turing patterns for different modes. An intrinsic square-root law is given by λ ∼ (c(1)+c(2)k)(1/2), which relates the pattern wavelength (λ) with the LCDD coefficient (k). This law indicates that the system pattern has the properties of an equivalent Turing pattern. The current study confirms that, for the Turing system with LCDD, the system pattern form retains the basic characteristics of a traditional Turing pattern in a wide range of LCDD coefficients.