Simulation Study On The Micelle Structure In Lid-driven Flow

Block copolymers can self-assembly into microphases with various structures, such as simple vesicles, sol-gel, giant micelles, multicompartment micelles and vesicles. These microstructures have been received extensively applications, for example, in the drug delivery and nanotechnology.Practically, the formation of microstructures depends on more factors, ranging from the structure of the block copolymer (including the chemical constitution, block sequences and their relative lengths, and the chain architecture) to property of the solution (such as concentration, temperature, PH condition), and to the processing techniques. It has been shown that external biasing fields, such as the temperature gradient, gravity field and flow field, play an significant role for the formation and evolution of microstructures from block copolymer. Hence a wide parameter space should be taken into account to understand the complex self-assembly behavior of the system.Molecular simulation is a technique in which classical Newtonian equations of motion are solved for each bead to evolve system to equilibrium state. As a time and cost efficient tool, molecular simulation method can not only explore the experiment from vast parameter space, but also give us the direct physical picture of interesting phenomena. Although all atomistic molecular dynamic simulation can precisely depict the phase behavior, its application on complex fluid system is limited in small spatial domain and shorter temporal and this method actually can not describe the behavior that is characterized at large domain and long temporal simulation.Researchers have proposed a variety of mesoscopic simulation method to study the complex fluid behaviors efficiently. By reducing local atomistic information, coarse-grained model is proposed to represent the essential interaction.Dissipative particle dynamics (DPD) method is a mesoscopic simulation technique to study the phase behavior at mesoscale. In DPD, each dissipative bead represents groups of solvent or several monomers and their interaction can be expressed by soft potential. Hence we can integrate the Newtonian motion equation at longer time step to accelerate the evolution of the system.Besides the conservation of total momentum, DPD technique can inherently present hydrodynamic interaction which is crucial for the phase transition in complex fluid system. By replacing classical DPD thermostat with the Lowe-Andersen counterpart, it can still present hydrodynamic interaction precisely but at the same time altering the viscosity in a wide range.Modified DPD method can show both the static and dynamic behavior and it is suitable to investigate the block copolymer self-assembly behavior in dilute solution.Our study includes two parts:1) By choosing suitable bounce back algorithm, we can avoid the unphysical phenomena of penetration behavior. Based on this algorithm, we built the system in an isolated simulation box. Confined lid-driven flow can be expressed by the motion of upper-wall beads toward one direction. Properties of wall, wetting or dewetting, can be adjusted by tuning the interaction strength between fluid components and wall. Replacing the classical DPD thermostat with Lowe-Andersen thermostat, one can investigate the influence of viscosity on the formation of microstructure and its evolution.2) The behavior of symmetric diblock copolymer A5B5 in dilute solution is studied. The effect of thermodynamic (including the interaction between components and their interaction strength with wall) and dynamic (viscosity and strength of lid-driven flow field) factors on microstructures is investigated systemically. Simulation results are shown as follows: Micelles from strong hydrophilic or hydrophobic block copolymers are less influenced by lid-driven flow field. Micelles with weak stability are inclined to fuse into larger ones under flow and they prefer to absorb on the surface of the wall. Viscosities of the system will also influence micelle structures. Lower viscosity will make micelles collide and fuse more easily to form larger ones. Higher viscosity can block their movement; hence more micelles with smaller sizes form in solution. The time to put system under lid-driven flow field can also have some effects on micelle structures. The formation of micelles is blocked if system is put under flow when simulation starts. Micelles with medium size can generate at late stages of simulation and they are reluctant to collide and fuse together. If system is put under flow after micelles form, flow will not destroy micelle structures, but promote their fusion.