Comment on "Drift without flux: Brownian walker with a space-dependent diffusion coefficient" by P. Lançon et al.
Comment on "Drift without flux: Brownian walker with a space-dependent diffusion coefficient" by P. Lançon et al.
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对 P. Lançon 等人的“无通量漂移：具有空间相关扩散系数的布朗步行器”的评论
DOI：
10.1209/epl/i2003-00494-8
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发表时间：
2003
期刊：
影响因子：
1.8
通讯作者：
S. Bhattacharya
中科院分区：
文献类型：
--
作者：
J. Blawzdziewicz;S. Bhattacharya
在最近的一项研究中，lanon等人研究了布朗粒子在流体中被限制在两个几乎平行的壁面之间的空间中的动力学。利用计算机视频显微镜，他们观察到单个粒子沿壁间间隙宽度增加的方向漂移。该效应采用步长与位置相关的布朗-沃克模型来解释，该模型在连续统极限下描述了具有空间变化扩散系数D的系统中的布朗运动。在实验系统中，扩散系数D的变化是由于颗粒流体动力迁移率与间隙宽度h的依赖。在这里，我们表明扩散系数的变化并不一定会导致颗粒在变化宽度的孔中漂移。在这样的系统中，漂移也可能是由熵的贡献引起的，熵的贡献对应于粒子穿过间隙所能达到的状态数的横向位置依赖。即使在没有扩散常数梯度的情况下，lanon等人没有考虑到的这一贡献也是非零的。对于无约束系统，粒子沿扩散系数增加方向的漂移可以直接由Smoluchowski方程推导。
In a recent study, Lançon et al. [1] investigated the dynamics of Brownian particles in a fluid confined to a space between two nearly parallel walls. Using computerized video microscopy, they observed that individual particles drift in the direction of increasing width of the gap between the walls. The effect was interpreted using a Brownian-walker model with a positiondependent step length, which, in the continuum limit, describes Brownian motion in a system with a spatially varying diffusion coefficient D. In the experimental system the diffusivity D varies due to the dependence of the particle hydrodynamic mobility on the gapwidth h. Here we show that the variation of the diffusion coefficient is not necessary to produce particle drift in a pore with a varying width. In such systems, the drift may also result from the entropic contribution that corresponds to the lateral-position dependence of the number of states accessible to a particle across the gap. This contribution, not considered by Lançon et al., is nonzero even in the absence of a diffusion-constant gradient. For an unconstrained system, particle drift in the direction of increasing diffusion coefficient can be derived directly from the Smoluchowski equation,