Spatially Extended DNA Phase Separation in Viscosity-Stratified Miscible Interfaces: Application in Microliter...

Bridging interfacial hydrodynamics and molecular self-assembly promises breakthroughs in functional materials and sensors. A flowing fluid interface across a viscosity gradient (viscous-stratification region) drives complex hydrodynamic events─vortices, mixing in an otherwise laminar interface, with implications for active colloids, interfacial bioprocesses, tissue engineering, microfluidic diagnosis, etc. To date, the formation of liquid-liquid phase separation (LLPS)-driven membraneless biocondensates in the viscous-stratification zone has received little attention. We demonstrate LLPS-mediated DNA-spermine condensate formation within the hydrodynamically unstable region inside a microfluidic device under continuous flow, with varying solution viscosity (η). The viscosity (η) was varied by altering the amount and size of the crowding agent, polyethylene glycol. The width of the DNA-spermine condensate clustering zone increases (∼2.3-fold higher than in the aqueous buffered system alone) with increasing interfacial Δη. Coarsening and the skewness of the condensate size distribution undergo a distinct change at the interface with contrasting viscosities. Finally, we extended this approach to a noncontinuous fluidic system to demonstrate its potential as a microliter-scale viscometric technique (with millimeter-scale spatial sensitivity) using blood serum samples, relevant for pathological gamma-globulin analysis.