胶体阴影揭示隐藏的溶质输运

中文摘要 AI 辅助

向周围流动中释放化学物质的物体会留下一个羽流，该羽流编码了释放速率，但对于大多数溶质而言，该羽流是不可见的。经典输运理论通过自相似的浓度边界层来描述这一现象，但在低雷诺数强制流动中，该边界层的结构从未被实验解析：在这些条件下长达一个世纪的测量仅返回了积分传质速率，而非场本身。在此，我们证明该场及其携带的信息均可通过实验获取。在微流控通道内通过光图案化制备的水凝胶柱周围，释放的溶质通过扩散泳（即胶体颗粒沿化学梯度的漂移）驱赶悬浮的胶体颗粒，在下游刻蚀出一个无颗粒的“阴影”。利用荧光溶质，我们解析了柱体周围的浓度场，并恢复了对流质量传递的经典自相似结构。对于不可见的表面活性剂，阴影的宽度（在同一平台上与直接成像的溶质进行基准对比）可作为柱体无量纲传质速率的代理指标。最后，比较三角形柱体的两种取向检验了Brenner的一个经典预测：局部浓度场重新排列，但未解析出积分传质速率的取向依赖性。对于扩散泳响应已知的溶质-示踪剂对，胶体阴影因此将普通示踪颗粒转变为无法直接观测的化学交换的定量报告器。

英文摘要

An object releasing a chemical into a surrounding flow leaves behind a plume that encodes the rate of release, but for most solutes, that plume is invisible. Classical transport theory describes this configuration through a self-similar concentration boundary layer whose structure has never been resolved experimentally in forced flow at low Reynolds number: a century of measurements at these conditions has returned integrated transfer rates rather than the field itself. Here we show that both the field and the information it carries are experimentally accessible. Around hydrogel posts photopatterned inside a microfluidic channel, a released solute drives suspended colloidal particles away by diffusiophoresis, i.e., their drift along chemical gradients, carving a particle-free ``shadow'' downstream. Using a fluorescent solute, we resolve the concentration field around the post and recover the classical self-similar structure of convective mass transfer. For an invisible surfactant, the width of the shadow, benchmarked against a directly imaged solute on the same platform, then serves as a proxy for the dimensionless rate of mass transfer from the post. Finally, comparing two orientations of a triangular post tests a classical prediction by Brenner: the local concentration fields rearrange, yet no orientation dependence of the integrated transfer rate is resolved. For solute--tracer pairs whose diffusiophoretic response is known, colloidal shadows thus turn ordinary tracer particles into quantitative reporters of chemical exchange that cannot be observed directly.

发表机构

• Yale University（耶鲁大学）

机构由 AI 辅助整理，请以论文原文为准。