The Simulation On The Mechanical Response Of Soft Particles Under External Force Field

Lipids amphiphilic molecules will aggregate into bilayer membrane in aqueous solution. These membranes form closed vesicles at low lipids concentration to reduce the large energy along the edges. Other than liposome, vesicle, bacterial or viral capsids cell and other biologic vesicles, artificial vesicle such as block copolymer, surfactants have been assembled. This type of particle, such as vesicle and micelle, whose dimension is between nanometer and micrometer, differs from the traditional hard particle. So, in virtue of the definition of the "soft matter" raised by de Gennes, the name of "soft particle" has been brought forward to define the aforementioned particle. The importance of understanding the self-assembling behavior and mechanical properties of soft particles has long been recognized in many research areas such as colloidal science and biomedical application, as well as in material science (e.g.:the rubber particle toughening polymer material). For instance, if a microcapsule for drug delivery wants to go through the various types of blood vessel, it must possess the capacity of deforming to a certain extent, and simultaneously, its mechanical strength should reach some standards to avoid membrane rupture before the particle reaches to the focus.The elastic curvature model for lipid vesicles, which was proposed by Canham and W. Helfrich, explained the biconcave shape of red blood cells successfully. The success established the groundwork for the study of soft particle. There are many theories and experiment researches are based on the Helfrich model, a simplified model of biological membrane and the works provides us a lot of valuable information, helping us understand the complex biological system. The development of theories which ground on Helfrich model explained some experimental phenomena perfectly.On the other hand, several advanced techniques have made it possible to study the deformation behavior of a single soft particle and therefore its mechanical properties can be obtained. However, some parameters can't be obtained through experiment. Moreover, restricted by experimental condition, some parameters can't be designed according to researchers'demand, thus, further comprehension and judgment to some experimental phenomena have been spoiled. Theoretical modeling and simulation, however, doesn't involve such problems. As long as altering parameters in an appropriate range, systemic theoretical results can be got and furthermore the dominant influencing factor of the experimental phenomena could be obtained.The paper focuses on the modeling of soft particle and the simulation of the particle in the force field and morphological transformation. The main jobs of the current paper are as follows:(1) With the help of the FEM software, Surface Evolver, we got a spherical particle, carved up by triangulated-mesh. And this is the initial particle shape.(2) By discrete spatial variation method, we studied the deformation behavior of the model in the force field, stretching and compression were included. Stretching research includes stretching at a constant force, stretching at a constant speed; compression includes point load compression, parallel plates compression and colloidal probe compression. During the process of compression, the deformation, force, energy and volume of the particle were recorded simultaneously, by which we can attain some mechanical curves and some dependant relation with the measurable parameters in the experiment.(3) We investigated the influence of size, compression speed and membrane bending modulus to the deformation and the bursting force. These results obtained by our model were compared with some existing experiments and good agreements were showed.