具有表面弛豫的液体自扩散的Monte Carlo模拟

Monte Carlo Simulation of Liquid Self-diffusion with Surface Relaxation

研究多孔介质中液体的自扩散行为能获得介质的微观结构信息,有助于了解介质中液体的传输性质.以具有不同孔隙大小的无限长圆柱体模型中的液体为对象,采用Monte Carlo随机行走方法,模拟存在表面弛豫时液体的自扩散系数和核自旋磁化强度随时间的变化,导出将NMR弛豫参数和随机行走参数联系在一起的表达式.结果表明:在快扩散区,液体的核自旋磁化强度随弛豫时间呈单指数衰减,且自扩散系数在短时情况下独立于表面弛豫率;在慢扩散区,液体的核自旋磁化强度衰减和自扩散系数在短时情况下均与表面弛豫率无关.模拟结果与理论分析相吻合,可用于求解介质的表面积与体积之比及孔径大小等结构信息.

Time-dependent self-diffusion coefficients and magnetization attenuation of liquid with surface relaxation in porous media in an infinitely long cylinder are calculated with Monte Carlo random walk method. Relationship between parameters of NMR relaxation and random walk is obtained. In fast-diffusion area, magnetization of nuclear spins attenuates exponentially, and self-diffusion coefficients are independent of surface relaxivity within short diffusion time. In slow-diffusion area, both magnetization attenuation and self-diffusion coefficients are independent of surface relaxivity within short diffusion time. These results agree well with theoretical analysis. Surface-to-volume ratio and pore size can be determined.