Max-Planck-Institut für Intelligente Systeme, Heisenbergstr. 3, D-70569 Stuttgart, Germany.
Soft Matter. 2018 Feb 21;14(8):1375-1388. doi: 10.1039/c7sm02347b.
The influence of a fluid-fluid interface on self-phoresis of chemically active, axially symmetric, spherical colloids is analyzed. Distinct from the studies of self-phoresis for colloids trapped at fluid interfaces or in the vicinity of hard walls, here we focus on the issue of self-phoresis close to a fluid-fluid interface. In order to provide physically intuitive results highlighting the role played by the interface, the analysis is carried out for the case that the symmetry axis of the colloid is normal to the interface; moreover, thermal fluctuations are not taken into account. Similarly to what has been observed near hard walls, we find that such colloids can be set into motion even if their whole surface is homogeneously active. This is due to the anisotropy along the direction normal to the interface owing to the partitioning by diffusion, among the coexisting fluid phases, of the product of the chemical reaction taking place at the colloid surface. Different from results corresponding to hard walls, in the case of a fluid interface the direction of motion, i.e., towards the interface or away from it, can be controlled by tuning the physical properties of one of the two fluid phases. This effect is analyzed qualitatively and quantitatively, both by resorting to a far-field approximation and via an exact, analytical calculation which provides the means for a critical assessment of the approximate analysis.
研究了具有轴对称球形胶体的化学活性自扩散，分析了流-流界面对流体质点自扩散的影响。与胶体被困在流界面或硬壁附近的自扩散研究不同，我们专注于靠近流-流界面的自扩散问题。为了提供突出界面作用的物理直观结果，我们分析了胶体对称轴垂直于界面的情况；此外，不考虑热涨落。与在硬壁附近观察到的情况类似，我们发现即使胶体的整个表面都是均匀活跃的，也可以使这样的胶体运动。这是由于扩散导致的界面两侧的各向异性，在胶体表面发生的化学反应产物在共存的流体相中进行分配。与硬壁对应的结果不同，在流体界面的情况下，通过调节两种流体相之一的物理性质，可以控制运动的方向，即朝向界面或远离界面。我们通过远场近似和精确的解析计算对这种效应进行了定性和定量的分析，这为近似分析提供了一个临界评估的手段。
