Strain-Induced Surface Microstructure In PDMS Soft/Stiff Film System And Its Application In Microfluidics

Microfluidic devices are of great significance in the fields of particle/cell manipulation,pharmacy and biomedicine,and can be applied to control fluid flow and process microparticles.Among these,microvalve systems are popular with researchers for their ability to precisely control particles and flow rates.However,conventional microvalve systems require the aid of large and expensive instruments,which greatly limits the miniaturization and integration of microfluidic devices.This thesis,therefore,proposes a simple and effective strategy to fabricate strain-tunable cracked and pleated microvalves with dimensions that respond to the applied tensile strain.Microfluidic devices with strain-tunable crack and wrinkle microvalves are subsequently demonstrated for use in microsphere screening and programmable microfluidic logic devices.The research for this paper is as follows:(1)Construction of cracks and wrinkles microstructures.The PDMS soft/stiff film system was prepared by UV ozone treatment combining with stretching/pre-stretching methods.Two microstructures,cracks,and wrinkles,were fabricated based on the mechanical deformation behavior of the elastic material and applied to a flexible microfluidic chip.The two microstructures have the advantages of a simple preparation process,low cost,and easy fabrication.The effects of UV ozone treatment duration and tensile strain on the size of the two microstructures were investigated.The excellent stretchability of PDMS allows the soft/stiff film system to be stretched to 100%,forming uniform,controlled crack,and wrinkle microstructures.(2)Design and realization of microparticle screening and liquid flow control.The flexible microfluidic chips are capable of two different types of microvalves showing different trends under the modulation of strain.In the released state,the crack microvalves close and with the application of strain,the crack microvalves open and the liquid flows,while the wrinkle microvalves show the opposite trend.In this paper,the burst pressure and flow rate of the crack microvalves are characterized in simulations,and the controllability of the burst pressure and flow rate is confirmed by experiments and simulations.(3)The reversible transformation of crack/wrinkle microstructure in wettability,adhesion and optical properties.As the strain reaches 40%from 0%,the wrinkle amplitude drops dramatically,the surface roughness decreases,and the droplet contact with the PDMS surface changes from the Cassie state to the Wenzel state.When a 40%tensile strain is applied,the wrinkle surface adhesion is sufficient to lift the glass sphere.After the strain is released,the surface roughness of the sample increases to a critical value when the glass sphere falls off the sample due to loss of adhesion.When the stretch is large enough,the compression along the lateral direction causes wrinkling of the Si O_x stripes due to the Poisson effect,forming an orthogonal crack-wrinkle microstructure on the PDMS surface.After the tensile strain is released,the wrinkle structure flattens out,the crack channels close and the film surface returns to its initial smooth state.