Virtual microfluidic traps, filters, channels and pumps using Marangoni flows

This paper describes how Marangoni flows of various forms can be generated in thin liquid films for the purposes of microfluidic manipulation. Several microfluidic components, including traps, channels, filters and pumps, for manipulating aqueous droplets suspended in a film of oil on blank, unpatterned substrates are demonstrated. These are 'virtual' devices because they have no physical structure; they accomplish their function entirely by localized variations in surface tension (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 to 1000 mu m. A point source generates toroidal flows which can be used for droplet merging and mixing. Virtual channels and traps, emulated by linear and annular heat fluxes, respectively, demonstrate nearly 100% size selectivity for droplets ranging from 300 to 1000 mu m. A source of heat flux that is parallel to the surface and is triangular with a 10. taper serves as a linear pump, translating droplets of about the same size at speeds up to 200 mu m s(-1). The paper includes simulations that illuminate the working principle of the devices. Models show that Marangoni flows scale favorably to small length scales. By using microscale thermal devices delivering sharp temperature gradients, it is possible to generate mm s(-1) flow velocities with only small increases (< 1 degrees) in liquid temperature.