粗糙和光滑椭球胶体的受限扩散

Experimental study of confined diffusion of rough and smooth ellipsoidal colloids

复杂受限环境中的扩散研究在凝聚态物理领域中备受关注.胶体体系的出现,为定量研究微观粒子的受限扩散提供了卓越的实验模型系统.当胶体粒子的形状由球形变为椭球形时,体系展现出各向异性的扩散动力学特性.近年来,研究者们发现粗糙表面能够诱发球形胶体体系异常的旋转动力学.然而,由于实验体系的局限性,粗糙表面对椭球形胶体粒子的受限扩散的影响依然知之甚少.本文建立了胶体受限扩散的模型体系,由粗糙圆球胶体构成受限环境,研究了粗糙和光滑椭球在其中的受限扩散.当圆球的堆积分数较低时,粗糙表面未发挥作用,因此光滑和粗糙椭球的平动和转动扩散相近.随着圆球堆积分数的增高,粗糙表面之间发生互锁,导致粗糙椭球的平动扩散明显慢于光滑椭球;随着堆积分数的进一步增高,由于粗糙表面产生的空间位阻效应,粗糙椭球的转动扩散也显著慢于光滑椭球.该工作表明粗糙表面会改变椭球的受限扩散,为揭示复杂环境中具有粗糙表面物质的扩散规律提供了实验依据.

The study of diffusion in complex confined environments has received great attention in the field of condensed matter physics.The emergence of colloidal systems provides an excellent experimental model system for quantitatively studying the confined diffusion of microscopic particles.When colloidal particles change from spherical to ellipsoidal shape,the system presents anisotropic diffusion dynamics.Recent studies have found that rough surfaces,another important physical parameter of colloids,can lead to unusual rotational dynamics in spherical colloidal systems.However,due to the lack of a suitable experimental system,little is known about the effect of rough surfaces on the confined diffusion of ellipsoidal colloidal particles.In this work,rough colloidal spheres,rough colloidal ellipsoids,and smooth colloidal ellipsoids are prepared,and then monolayer colloidal samples are prepared to study the confined diffusions of these two types of ellipsoids in dense packing of the rough sphere colloids.By calculating the mean square displacement,intermediate self-scattering function,and orientation correlation function of the ellipsoids,we quantitatively characterize the diffusion dynamics of rough and smooth ellipsoids in varying concentrations of rough spheres.The results indicate that the translational diffusion and rotational diffusion of rough ellipsoids and smooth ellipsoids slow down as the concentration of rough spheres increases.This is due to the confinement of the ellipsoid by the surrounding spheres.At low stacking fractions of spheres,smooth and rough ellipsoids show similar translational diffusion and rotational diffusion.However,as the stacking fraction of spheres increases,there is a significant difference in advection diffusion between rough ellipsoids and smooth ellipsoids.The advection diffusion of rough ellipsoids is significantly slower than that of smooth ellipsoids.This is because the rough surface strongly inhibits rotation,meaning that the rotational diffusion of the rough ellipsoids is significantly slower than that of the smooth ellipsoids.By extracting the diffusion coefficients for translation and rotation from the ellipsoid's longtime mean-square displacements,we find that at Φ=0.60 and 0.65,the diffusion coefficients of rough ellipsoids are smaller than those of smooth ellipsoids.The translational diffusion coefficient of the rough ellipsoids is notably smaller than that of the smooth ellipsoids.However,the rotation diffusion coefficient of the rough ellipsoids is not significantly different from that of the smooth ellipsoids.This suggests that the rough surface mainly affect translational diffusion,strongly suppressing the translational diffusion of the ellipsoids.By calculating the displacement probability distribution for ellipsoidal motion,we find that at Φ=0.65,the translational displacements of rough ellipsoids have a relatively narrow distribution.This suggests that the translational motion of particles is suppressed by the rough surface.However,the distributions of rotation displacement for smooth ellipsoids and rough ellipsoids are very similar,indicating that the rough surface has less influence on particle rotation.At Φ=0.74,the rough surface suppresses both the translation and the rotation of the ellipsoid,resulting in a narrower displacement distribution than in the case of smooth ellipsoid.These findings suggest that rough surfaces significantly impede ellipsoidal diffusion,leading the translational and rotational motions not to occur simultaneously.This study provides an in-depth understanding of the role of rough surfaces of colloidal particles in confined diffusion,as well as an experimental basis for explaining the diffusion laws of rough materials.