The Solute-Exclusion Zone: A Promising Application for Mirofluidics
While unique phenomena exist at fluid-solid phase intersections, many interfacial phenomena manifest solely on limited scales-i.e., the nm-μm ranges-which stifles their application potential. Here, we constructed microfluidic chips that utilize the unique long-distance interface effects of the Solute-Exclusion Zone (EZ) phenomenon to mix, separate, and guide samples in desired directions within microfluidic channels. On our "EZ Chip", we utilized the interfacial force generated by EZs to
... rt specimens across streamlines without the need of an off-chip power source. The advantages of easyintegration, low fabrication cost, and no off-chip energy input make the EZ suitable for independent, portable lab-on-chip system applications. 1467 oscillatory salvation force at liquid-solid interfaces [3, 4] ; and interfacial forces were applied for nanoparticle assembly  . Moreover, structural water molecules may account for protein folding and may be involved in signal transductions in aquatic solutions [2, 6, 7] . Though hydrogen-bond water networks are widely recognized and appreciated on sub-micron scales, interfacial forces have not been fully explored for distances greater than one micrometer [2, 4] . Solute-exclusion zones (EZs) were initially observed in the vicinity of hydrophilic surfaces  . The discovery that water can form several-hundred-micron-thick, ordered layers when adjacent to hydrophilic surfaces-to effectively exclude colloidal particles and other solutes hundreds of microns from its surface-opened up promising interfacial water applications within the liquid-solid phase  . Indeed, the concept of EZs prompted on-going investigation into novel applications of interfacial water. Within the EZ, microspheres were excluded ~600 μm from polymer surfaces at various velocities, depending on microsphere surface modifications  ; macromolecules, such as pH indicator dyes, were excluded as well  . The interfacial forces generated from EZs have been well documented [8, 11] near diverse interfaces with various solvent parameters, such as ion concentrations and pH, where similar phenomenon have been reported [9, 12] . Molecular dynamic simulations predicted the formation of a pseudo-crystal water lattice at polymer-water interfaces  . Additionally, the hydration shell assembly and interfacial water clusters were proposed to interpret the formation of the exclusion force [2, 14] . The energy stored within an EZ was investigated recently as well [15, 16] , and these studies confirmed the robust observations of EZ formation in the vicinity of various surfaces and interfaces. While the notable length scales of EZs range several hundred micrometers, herein, we applied EZ characteristics to perform basic manipulations, such as sorting, separation, and guided movement within microfluidic chip.