Micro/Nanofluidic Control Based On Patterned Microstructures And Surfaces

In recent years,microfluidics has shown broad applications in chemistry,biochemistry,and medical detection because of their feature advantages such as reduced sample volume,low cost,portability,and shortened analysis time.In a microchannel fluidic chip,guidance and operation of the fluid play important roles in rapid and precise analytical functions.To date,researchers have fabricated a lot of microvalves for the fluid control in microfluidics.Active microvalves utilize magnetic,mechanical,and pneumatic mechanisms to control the fluid behavior.Usually,such microvalves are miniaturized models of their conventional macro counterparts and consist of actuating,moving,and sealing components in the channel,which increases the cost and the size of the integrated microfluidic device.Passive microvalves include check valves and capillary valves,which do not require external force for the switching of on/off state but only depend on the property of the channel surface.Abrupt geometry change and altered wetting properties are the most common approaches for the fabrication of capillary microvalves.In addition to these approaches,directional wetting of asymmetric structures has become a promising alternative to control the liquid flow in recent years.For example,slanted nanohairs and Janus Si elliptical pillar arrays with anisotropic wettability were utilized to control the flow speed and flow direction in microfluidics.But the fabrication processes of the aforementioned anisotropic structures were relatively complex,which may limit their practical applications in microfluidics.Nanofluidic technologies,a natural extension of microfluidics and nanotechnology,have been developed rapidly in recent years.The development of nanofluidics heavily depends on the micro and nanofabrication technologies.Conventional nanochannel fabrication methods include photolithography,electron beam lithography,focused ion beam milling,interference lithography,and nanoimprint lithography.The advantages of these nanofabrication methods are high resolution and reproducibility,however,the high complexity and fabrication costs limit their rapid prototyping and utilization in resource-limited settings.Recently,unconventional nanofabrication technologies have been developed to compensate for this or substitute conventional technologies.For example,techniques based on sacrificial nanowires and the mechanical deformation behaviors of elastomeric materials,including wrinkling,cracking,and structure collapsing,have been employed to fabricate nanofluidic channels.These techniques facilitate the use of disposable nanofluidic devices in many applications with reduced cost and time,but they also possess some weaknesses in repeatability and reproducibility with high resolution.In this dissertation,we prepare anisotropic wetting surfaces by patterned microstructures for the fluid control in microfluidics.In addition,we show a new strategy for the fabrication of nanofluidic channels with an ultra-high surface-to-volume ratio based on nanoscale gaps in nanopillar arrays.In chapter 2,we prepared hydrophilic-hydrophobic chemically patterned stripes structures and realized the anisotropic flow of fluid by introducing the patterned structures into microfluidic channels.The anisotropic flow of the fluid is attributed to the distinct surface energy of the modified materials.We investigated the flow behavior of water with different driving pressure and found that the anisotropic flow characteristic was weakened by increasing the driving pressure.Thus,there exist a threshold pressure for the microfluidic system based on the chemically patterned surface,and the threshold pressure is a judging criteria for the controlling capability of the surfaces.We studied the effect of period of patterns,hydrophilic-hydrophobic ratio,surface energy difference between the modified materials,and the height of the microchannel on the controlling capability of the chemically patterned surfaces for microfluidic control.Based on the anisotropic flow behavior in the microchannel,the chemically patterned surfaces could act as microvalves of microfluidics and the on/off switching of the prepared microvalves could be realized by tailoring the driving pressure for fluid.Finally,by reasonably combining the patterned surface and microchannel together,we realized the transportation of water in a microchannel along a ―virtual‖ wall,which provides a gas-liquid interface for microfluidics.In chapter 3,we introduced morphology patterned anisotropic wetting surfaces into microchannels and the flow direction of the fluid was controlled in a simple and effect manner.Water shows unidirectional flow in the microchannel due to the energy barrier difference in the directions parallel and perpendicular to the morphology stripes.In the prepared microfluidic system,the energy barrier in the perpendicular direction is larger than that in the parallel direction.Two factors induce the energy barrier in the perpendicular direction: Laplace pressure resulted from the change of height of the surface and Gibbs inequality condition.We investigated the flow behavior of water with different driving pressure and found that water did not show unidirectional flow behavior when the driving pressure was increased to some extent.We defined the critical pressure as failure pressure,and the failure pressure indicates the microfluid control capability of the morphology patterned surface.Next,we studied the effect of period of patterns,depth of the structures,surface energy of the modified materials,and the size of the microchannel on the value of failure pressure.Due to the stable unidirectional flow behavior in the microchannel,the morphology patterned surfaces could act as microvalves of microfluidics and the switching between the on and off states of the microvalves could be realized by changing the driving pressure for fluid.Finally,gas-liquid separation was realized in the microchannel using the morphology patterned surfaces,which provided an alternative method for the separation of immisible phases in microfluidics.In chapter 4,we show a new strategy for the fabrication of nanofluidics based on nanoscale gaps in nanopillar arrays.Silicon nanopillar arrays are prepared in a designed position by combining conventional photolithography with colloidal lithography.The nanogaps between the pillars are used as nanochannels for the connection of two polydimethylsiloxane-based microchannels in microfluidics.Compared to conventional single nanochannels,the prepared nanochannels are massive,interconnected nanogap networks.The gap between neighbouring nanopillars can be accurately controlled by changing the etching condition,which further determines the dimensions of the nanochannels.In addition,we also prepared nanochannels with a large height to width ratio and high-density,and realized size-tunable and flexable fabrication of nanochannels.Besides the PDMS-Si micro-nanofluidic chips,our fabrication method is also suitable for the fabrication of PDMS-glass and Si-glass micro-nanofluidic chips.At a low ionic strength,the surface charge-governed ion transportation shows that the nanochannels possess the same electrokinetic properties as typical nanofluidics.Due to the perm-selectivity of the nanochannels,the nanofluidic chips can be used to preconcentrate low concentration samples.The nanochannels based on nanogaps are compatible with conventional photolithography and microfluidic channels,which facilitate the integration of LOC devices with versatile functions.In addition,the prepared nanochannels preserve the properties of high-throughput and ultra-high surface-to-volume ratio,which indicates that these nanochannels have great potential for the nanofluidic applications such as energy conversion and molecular separation.