| Recently, planar microfluidics have attracted intensive research interest due to their advantages compared with channel microfluidics. However, planar microfluidics usually suffer from the strong contact angle hysteresis(CAH) for droplet transport, and has a good application prospect in drug screening, biology and other fields. In this thesis, we propose a large-area, inexpensive and facile method to fabricate ultra-slippery polydimethylsiloxane(PDMS) using the microbead assembly on a simple adhesive tape, and demonstrated it as the novel platform for droplet manipulation on planar microfluidics. The effects of the sizes of microbead and the lubricant viscosity on the performances of the ultra-slippery surface are then discussed with the experiments. Finaly to achieve the generation, transport and other typical manipulations of single droplet or droplet arrays using the facile method of gravity driving, we futher pattened microstructure by tailoring the adhesive tape.We proposed a inexpensive method to produce large-area ultra-slippery PDMS to overcome the CAH bottleneck. The microfabrication was based on soft lithography and the two-dimensional assembly of microspheres, by using the adhesion between the adhesive tape and the compact polystyrene(PS) microbeads. Monolayer PS microbeads have been readily achieved as the template for bowl-like PDMS, which can lock lubricant oils as the ultra-slippery surface. To be specific, adhesive tapes were used as the substrates to stick monolayer microbeads. An adhesive tape was firstly fixed on a glass slide, then they were immersed in the aqueous solution suspending plenty of PS microbeads. After water was evaporated or manually extracted, monolayer and compactly aligned PS microbeads were adhered to the tape when shaking off the top microbeads. They were used as the template for PDMS using the soft lithography with the aid of methyltrichlorosilane coating for demolding and these microbeads were dissolved by toluene. Finally, various lubricant oils immiscible with water were spin-coated on the porous microstructures so that ultra-slippery PDMS surface for water was obtained. Immiscible with water, less volatile and non-toxic lubricating oil was selected as a lubricant to infiltrate the bowl-like microstructure and an oil film layer is formed on the surface, thus the contact angle hysteresis and roll angle of the droplet on the PDMS surface was greatly reduced. This kind of method to fabricate the ultra-slippery surface based on PDMS was convenient and quick. We can choose different microbeads to fabricate the film, thus we can improve the porous fraction of the bowl-like microstructure.We found that small sizes of microbeads led to more compact monolayer microbead alignment, which can reach the maximum porous fraction(Fp) 85%. PDMS with different PS template sizes have comparable low critical sliding angle(CSA)<5° for water, and CSAs can be further enhanced by increasing Fp. We also compared the average water moving velocities at 15° sliding angle depending on different PS sizes using the same lubricant(phenyl silicone oil), same spin-coating time(800rpm, 15s), and found that they were declined with the increasing microbead size or reducing Fp. We also compared various CSAs and water moving velocities on different lubricant viscosity for 20μm microbeads and similar oil surface tension. Higher viscosity led to slightly lower SCA and higher moving velocity.Take the advantage of patterned bowl-like microstructure and oil film to produce droplet arrays, we can realize the droplet generation, transport and reaction in a same microfluidic chip using only gravity. It was demonstrated that patterned ultra-slippery surfaces can greatly increase the flexibility of droplet manipulation, and simplify the droplet manipulation on the chip. |
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