Virtual components for droplet control using Marangoni flows :size-selective filters, traps, channels, and pumps Advisor: Cheng-Hsien Liu Reporter: Y. S. Lin Date: 2007/6/20 Amar S.Basu, Seow Yuen Yee, and Yogesh B. Gianchandani University of Michigan, Ann Arbor, USA MEMS2007

Outline Introduction Size-selective virtual channel Single droplet trap Guidewire pump Conclusion

Introduction In droplet-based microfluidic systems, droplet motion is generally guided by microfabricated patterned surfaces Disadvantages - Droplet contact with solid surfaces and sample adsorption to channel walls or other surfaces - Actuators contact the liquid that has contamination concerns

Introduction Several virtual microfluidic components, including channels, filters, traps, and pumps on unpatterned substrates, accomplish their function entirely by localized Marangoni flows by heat sources suspended just above the liquid surface

Introduction Marangoni flow on a liquid surface driven by surface tension gradients Temperature gradient causes surface tension gradients and flow directed from high to low temperature MicroTAS05

Size-selective channel Two heated wires parallel to the liquid surface Recirculating flows occurring as a result of the Marangoni effect are shown with arrows

Size-selective channel 500um diameters droplets entering the channel while a smaller one is rejected S=600um, the minimum diameter for entry into the channel is 250um S=970um, the minimum diameter is 350um Nearly 100% exclusion of off-sized droplet is shown

Single droplet trap The single droplet trap is implemented using a ring- shaped annular heat source Metal pin (s=600um) that a 700um diameter droplet is actively pulled into the trap

Guidewire pump A triangular heat flux projected on the fluid surface pulls droplets in and along its longitudinal axis The droplet achieves a maximum velocity of 196 um/sec

Conclusion Several components for droplet manipulation were presented, all of which operate without physical structures solely by localized Marangoni flows Advantages -It is a non-contact method of actuation -Droplets do not make contact with solid surfaces -It does not require patterned substrates These virtual components are building blocks toward a microsystem for droplet-based assays

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Droplet mixing The four droplets (φ= um) merge together, one by one, eventually forming a single, φ=600 um droplet in 9 seconds. MicroTAS05

Surface tension & Marangoni effect t=t 0 (1-bT) t0 、 b ：隨液體而定之常數 T ：溫度 t ：表面張力 Ma=|dt/dT|α -1 μ -1 ΔTR 2 L -1 Ma ： Marangoni number 熱對流強度 dt/dT ：表面張力對溫度的變化率 α ：熱擴散係數 ΔT ：溫度差 μ ： 黏滯性 L ：流動區長度 R ：流動區半徑

Size-selective channel