The Structure And Dynamical Behavior Of A Polymer Chain In Active Brownian Particle Bath

Active matter refers to a variety of systems that the constituents locally convert energy into motion.The far-from-equilibrium feature of these systems leads to rich phenomena such as giant fluctuation,phase separation,and super-diffusion.Recently,the mixtures of active agents and passive agents have been intensively studied,such as stochastic thermodynamics of self-propelled particles(SPPs)and passive particles.In this dissertation,we bring a"polymer" chain into the non-equilibrium state via immersing them into active particles.We focus on the unfolding and folding of self-attracting chains in active Brownian particle baths,the dynamic unfolding behavior of AB-type block copolymers and the self-adaptive behavior of a nunchaku-shaped tracer in active particle baths.Our study plays an important role in understanding the formation mechanism of self-repairing polymer materials and designing self-repairing materials.The first chapter gives a detailed description of the research background.The second chapter presents the Brownian Dynamics simulation methods and how to characterize the dynamical property of the system.In chapter 3,we investigate carry out Brownian dynamics simulations to explore the behavior of a single self-attracting chain in a suspension of active Brownian particles(ABPs)As the propelling force increases,the globule-stretch(G-S)transition of the chain occurs due to the enhanced disturbance from the ABPs.Two distinct mechanisms of the transition in the limits of low and high rotational diffusion rates of ABPs have been observed:shear-induced stretching at a low rate and collision-induced melting at a high rate.The G-S and S-G(stretch-globule)curves form a hysteresis loop at the low rate,while they merge at the high rate.Besides,we find that two competing effects result in a non-monotonic dependence of the G-S transition on ABP density at the low rate.Our results suggest an alternative approach to manipulating the folding and unfolding of bio-polymers by utilizing active agents.In the chapter 4,we study the structural and dynamical behavior of an A-B di-block chain in the bath of ABPs by Brownian dynamics simulations in two dimensions.We are interested in the situation that the effective interaction between the A segments is attractive,while that between the B segments is repulsive.Therefore,in thermal(passive)equilibrium,the A block"folds" into a compact globule,while the B block is in the expanded coil state.Interestingly,we find that the A block could "unfold" sequentially like unknitting a sweater,driven by the surrounding ABPs when the propelling strength on them is beyond a certain value.This threshold value decreases and then levels off as the length of the B block increases.We also find a simple power-law relation between the unfolding time of the A block and the self-propelling strength and an exponential relation between the unfolding time and the length of the B block.Finally,we probe the translational and rotational diffusion of the chain and find that both of them show "super-diffusivity" in a large time window,especially when the self-propelling strength is small and the A block is in the folded state.Such super-diffusivity is due to the strong asymmetric distribution of ABPs around the chain.Our work provides new insights into the behavior of a polymer chain in the environment of active objects.In Chapter 5,we design a nunchakus-like tracer and investigate its self-adaptive behavior in the ABP bath via systematically tuning the activity and density of ABPs.Specifically,the nunchakus-like tracer will be a wedge-like shape in the ABP bath when the self-propelled force is large enough.We analyze the angle between two arms of the tracer and the velocity of the joint point of the tracer.The angle exhibits a non-monotonic behavior as a function of active force.However,it monotonically increases with density of ABPs.For the large self-propelled force,a simple linear relationship between the velocity and the self-propelled force is obtained.Furthermore,we find that the long-time super-diffusive behavior of the tracer on the dependence of the self-propelled force and the density of ABPs.Moreover,we find the tracer can flip at high density of ABPs.We give the conclusions and a perspective in the Chapter 6.