Extremely Long-Range Light-Driven Repulsion of Porous Microparticles

The repulsive surface forces, such as electrostatic or steric, acting between particles explain why they remain well separated in aqueous electrolyte solutions and are responsible for the stability of colloidal dispersions. However, the effective range of these interactions is always well below hundreds of nanometers and typically can be controlled by advanced manipulations such as tuning the electrolyte concentration or modifying the particle surface or, in some more specific cases, via subjecting the suspension to an external electric or magnetic field. Here we employ solutions with small additives of a photosensitive ionic surfactant to investigate if a repulsive interaction of microsized particles sedimented at the solid surface can be remotely controlled simply by illuminating it with an appropriate wavelength. We show that interactions of conventional impermeable particles remain practically unaffected by light, but, in contrast, for porous particles, we observe a long-range repulsion, several orders of magnitude longer than any conceivable equilibrium surface force. This repulsion emerges due to the diffusio-osmotic flow generated near the porous particles that in this scenario are playing a role of micropumps. The diffusio-osmotic repulsion of porous particles can be used for a remote control of their two-dimensional assemblies at the solid wall, and in particular, we demonstrate that by simply using two different illumination wavelengths it is possible to reversibly switch the state of porous particle dispersion from densely packed surface aggregates to a periodic lattice of particles separated by distances on the order of tens of micrometers.粒子间的表面斥力（如静电斥力与空间位阻斥力）可解释其为何能在电解质水溶液中保持良好分散，并赋予胶体分散体系稳定性。然而，此类相互作用的有效作用范围始终远小于数百纳米，且通常可通过多种精准调控手段实现调节：例如调整电解质浓度、修饰粒子表面；在部分特定场景下，也可通过对悬浮体系施加外电场或外磁场来完成调控。本研究采用添加少量光敏离子型表面活性剂的溶液体系，探究沉降于固体表面的微米级粒子间的斥力相互作用，能否通过特定波长光照实现远程调控。研究结果显示，传统非渗透性粒子的相互作用几乎不受光照影响；与之相对，多孔粒子则呈现出长程斥力效应，其作用范围比任何可设想的平衡表面力都要长出数个数量级。该斥力源于多孔粒子表面产生的扩散渗透流——在此场景中，多孔粒子可充当微型泵。多孔粒子的扩散渗透斥力可用于实现其在固壁表面二维组装结构的远程调控；具体而言，本研究证实，仅需使用两种不同波长的光照，即可实现多孔粒子分散体系状态的可逆切换：从紧密堆积的表面聚集体，转变为粒子间距达数十微米量级的周期性晶格阵列。