Flow induced two-dimensional $ζ$ potential on chemically uniform solid-liquid interface: A possible electrokinetic origin of streamwise vortices
The $ζ$ potential at solid-liquid interfaces governs electrokinetic phenomena across chemistry, biology, and engineering. Despite its centrality, local measurement of this potential has remained a formidable challenge, forcing the universal assumption of spatial uniformity on chemically homogeneous surfaces. Here we introduce Fluorescence Photobleaching Electrochemistry Analysis (FLEA)-- a non-invasive technique that achieves 180 nm lateral resolution -- to map the $ζ$ potential at unprecedented spatial detail. We discover that a perfectly uniform glass surface, under laminar flow, develops an intrinsic two-dimensional $ζ$ potential landscape: approximately symmetrical V-shaped distribution across the channel width that varies systematically with flow velocity, pH, and ionic strength. This two-dimensional electrochemical heterogeneity generates a rotational electric body force that spontaneously produces streamwise vortices-structures commonly observed in boundary layers. Our findings establish FLEA as a transformative tool for surface characterization, reveal a previously unrecognized origin of streamwise vorticity at interfaces from the aspect of electrokinetic.