Novel Behaviors Induced By Grafted Chains In The Active Bath

In the 20 th century,many new types of materials have been applied and extensively studied,including liquid crystals,colloids and polymers and so on.The development of physical understanding of these materials had to the birth of the concept--“soft matter”.Soft matters are different from traditional “hard” materials or simple liquids in several aspects.1)Unlike crystal solids,they are “soft”,either mechanically or very sensitive to external changes;small disturbances can lead to significant changes in the structure or properties.2)In contrast to simple liquids,they normally have more complex compositions and/or molecular architectures;the rich internal degrees of freedom lead to the self-assembly of versatile and hierarchical ordered structures.3)Entropy is important in the variation of structures;thermodynamically,the changes of energy for altering the structures or physical properties are of the order of kT.Soft matter is now a popular and cutting-edge interdisciplinary research area,which integrates the researchers from different backgrounds,such as physics,chemistry,materials science,biology,engineering and math.In recent years,active soft matter has become one of the rapid developing research subjects.Active soft matter raises new challenges for the non-equilibrium statistical physics and has potential applications in artificial intelligent microdevices.Grafting chains to a surface is a widely used strategy in chemistry,engineering and biomedicine for surface modification and functionalization.The properties and the dynamical assembling behaviors of the chain-grafted surface and its contact with different substances such colloidal or nanoparticles and bio-macromolecules have attracted abiding interest.On the other hand,active matter widely exists in the biological world.They exhibit novel nonequilibrium dynamical behaviors,e.g.giant density fluctuations,dynamical patterns,collective transportation and abnormal rheology.Recent research found that polymer chains or chain-like objects immersed in the bath of active agents show anomalous static and dynamic behaviors.In this thesis,we summarize our study on the novel phenomena when objects with grafted chains are in contact with self-propelled particles.The first chapter of this dissertation is introduced which expounds the above content of the research background in detail.In the second chapter,we introduced the simulation study of the molecular dynamics method used in the related concept,and especially,this article mainly involves polymer dynamics and active brown dynamics are described deeply.In the third chapter,we simulated the chain-brushes grafted on the plat immersed in the bath of active particles.The mixed system of chain/active particles is a sampling of biological model to study what happened when many small organisms meet hairy surface,this system has been artificial synthesized in the experiment.In the research on this system,we pay more attention to which conformation the grafting chain brush can form and how the surface of the brush impact on the movement and distribution of contacted active particles.We found that the self-propelling force can make more active particles moving into the inside layer of brush,and when the active force is large enough,the particle density in the bulk of the brush saturates and counterintuitively becomes eventually larger than the particle density outside the brush.At the same time,the grafting chain also shows a variety of conformation because of the collision between active particles and chain beads.In chapter 4,we studied a chain-grafted colloid surrounded with active particles in a two-dimensional system.We found that under certain conditions,the grafted chains will spontaneously product persistent asymmetric configuration for a long time due to the co-work with nearby trapped active particles,and this symmetric breaking lead to the directional rotation of the hairy colloid.In previous studies,in the bath of bacteria,the space asymmetry of micro-rotor has been fixed originally,to the contrary,we do not intentionally introduce inherent space asymmetric structure in our system.In this chapter,we focus on how the symmetry breaking of this model produced and maintained.The discovery of this chapter offers possibilities for making soft,flexible micro devices.The Chapter 5,we do the further research on the spontaneous unidirectional rotation of hairy colloid by changing the density of the active particles,the grafting density of chains and the chain rigidity based on the previous chapter.We found that the increased density of the active particles in a certain range will help enhance the rotor rotational speed,but when the density is large enough to form clusters around the colloid,the rotation of the colloid is restrained by cluster.In this section,we added bending potential between the bonded chain beads to characterize the semi-flexible of the chains grafting on the surface of the colloid.The studies show that the rigidity of the grafted chains can help form unidirectional rotation of the rotor when the grafting density is lower,and the rigidity is stronger,the angular speed of the hairy colloid is more stable.In Chapter 6,we summarize the main work of this dissertation and look forward to the future work.