Virtual components for droplet control using marangoni flows: Size-selective filters, traps, channels, and pumps

This paper describes several microfluidic components, including channels, filters, traps, and pumps, for manipulating aqueous droplets suspended in a film of oil on blank, unpatterned substrates. These “virtual” devices have no physical structure; they accomplish their function entirely by localized Marangoni flows created in a non-contact manner by heat sources suspended just above the liquid surface. Various flow patterns can be engineered through the geometric design of the heat sources on size scales ranging from 10–1,000 μm. Channels and circular traps, emulated by linear and annular heat fluxes respectively, demonstrate nearly 100% selectivity for droplets ranging from 300–1,000 μm. A pump, emulated by a triangular heat flux with a 10° taper, translates droplets of about the same range at speeds up to 200 μm/s.