约束条件下胶球间排空力的研究

用MonteCarlo方法对处于两平行硬板约束下三个浓度的大小胶球系统进行了模拟,通过对大胶球表面小胶球密度的统计,由密度积分公式获得了大胶球所受的排空力.研究结果显示,因为平行硬板的存在或当改变两平行硬板的距离时,同浓度下,排空力在硬板距离小的时候最明显;三个浓度中,浓度高的,排空力受硬板距离影响最大;有硬板约束比无该约束的时候,排空力效果更显著.

Effects of Geometrical Confinement on Depletion Force in Colloidal System

The depletion force on a large hard sphere in a solvent of small hard spheres under geometrical confinement is investigated by using local density integration method through Monte Carlo simulations. The model considered here is a rectangular box with two boundless hard plates placed in a direction. Small hard spheres are randomly distributed in the box to form a hard sphere fluid. The number of small spheres is determined by the given volume fraction. The size ratio of the large- to small-sphere is 5. Three systems maintained at bulk volume fractions 0.116, 0.229, and 0.341 are studied. The effects of geometrical confinement are taken into account through changing the distance of the two plates. To get rid of the finite size effect, the sizes of the box in other two directions are enlarged in a way when the distance between the two plates is decreased. The configurations of the small spheres are sampled according to the Metropolis algorithm with the two large spheres fixed at a separation. Each small sphere is chosen and relocated using a trial displacement. The new position is accepted so long as it does not result in an overlap with the large hard spheres, the other small spheres or the plates. To take the geometrical confinements into account, the fixed boundary condition is used corresponding to the two plates. Meanwhile the magnitude of the maximum random displacement is adjusted so that the overall acceptance ratio is about 0.3-0.5. The numerical results show that the depletion force is affected by the geometrical confinements. Furthermore, the nearer the two plates are to each other, the larger the effects from the geometrical confinement will be.