Stokes efficiency in the overdamped Brownian ratchet

Lü Ming-Tao^{1}, Yan Ming-Yue^{1}, Ai Bao-Quan^{2}, Gao Tian-Fu^{1}, Zheng Zhi-Gang^{3}

1. College of Physical Science and Technology, Shenyang Normal University, Shenyang 110034, China;
2. School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, China;
3. College of Information Science and Engineering, Huaqiao University, Xiamen 361021, China

Molecular motors in life activities of cell are known to operate efficiently.They could convert molecular-scale chemical energy into macroscopic-scale mechanical work with high efficiency.In order to acquire the transport mechanism of the molecular motor,the Brownian ratchet has been proposed to explore the property of directed transport and energy conversion.There are different kinds of Brownian ratchet models like flashing ratchets,rocking ratchets,and time-asymmetric ratchets and so on.Through investigating the performance of Brownian ratchet moving in periodic potential,the directed transport of ratchet could be explained,and the effective usage of ratchet energy for directed transport could also be improved.Recently,optimizing the transport of Brownian ratchet has aroused the interest of researchers.It is found that the viscous resistance could reinforce the directed transport of the Brownian particle in damping liquid.Meanwhile,a large number of conclusions indicate that the transport of Brownian ratchets would be enhanced if the asymmetry of the potential is changed.Those results show that the influences of the external potential and the damping force on the particle flow cannot be neglected.Hence in this paper,the effects of the potential structure and the temperature of heat bath on transport are discussed.
Furthermore,how to use the ratchet energy effectively has been investigated in recent years.When the Brownian motor operates with load,the input energy is reduced.More importantly,the energy transformation efficiency defined as the ratio of the useful work done against the load to the input energy is assumed to be a zero value in the absence of load.With the help of stochastic energetic theory proposed by Sekimoto,the Stokes efficiency has been used to explore the performance of the Brownian ratchet.So far,the numerical solution has been used extensively in most theoretical investigations.Nevertheless,in our work,the Stokes efficiency is discussed analytically for explaining the mechanism of directed transport.We consider the transport performance of the Brownian ratchet described by the Fokker Planck equation which is corresponding to the Langevin equation under time-varying external force and thermal noise.Mainly, the effects of potential asymmetry,external force,height of the barrier,and intensity of the thermal noise on transport are discussed in detail.It is found that the transport direction of Brownian ratchet will be reversed under the condition of appropriate potential structures,and the probability current can reach a maximal value by changing the asymmetry of potential.It is worthwhile to point out that the performance of directed transport of the ratchet can be improved when an appropriate amplitude of the external force is applied.Meanwhile,there is an optimal value of the barrier height at which the Stokes efficiency reaches a maximal value and the directed transport of ratchet is enhanced.Through our conclusions,the ratchets of different structures could be designed for improving the transport property of Brownian motor.And the results have helpful theoretical guidance not only in the aspect of medical delivery but also in the control of nano-devices.

(Probability theory, stochastic processes, and statistics)

Fund:

Project supported by the National Natural Science Foundation of China (Grand Nos. 11475022, 11347003), the Scientific Research Funds of Huaqiao University, China and the Excellent Talents Program of Shenyang Normal University, China (Grand No. 91400114005), and the Teaching Research Program of Thermodynamics and Statistical Physics in the Institution of Higher Education, China.