Two-photon laser fabrication of large-format microporous structures and the study of irreversible microfluidic processes

Microporous structures are central to many fields of science and engineering, ranging from biological tissue to fluidic filters to porous aquifers. Most of these systems are complex with little or no symmetry. Although multilayer microstructures have been fabricated using broad-illumination photolithography, arbitrary three-dimensional (3D) microfluidic structures are too complex to be made with linear exposure. Multiple-exposure of two-beam interference can be used for periodic structure fabrication such as for photonic crystals, instead of arbitrary microstructures. Two-photon polymerization (2PP), on the other hand, would be a means to generate arbitrarily complex microfluidic systems. Furthermore, 2PP has been shown to push microstructure dimensions beyond the diffraction limit as an advantage of fabricating submicron structures. I apply 2PP and femtosecond laser direct-writing techniques to fabricate 3D microporous structures in photopolymers. Large-scale 3D microporous structures (450 μm × 450 μm × 40 μm) are built up from 2D patterned-microchannels with random geometry. The height and the width of the features are about 40 μm and 5 μm, respectively. 3D microchannels were fabricated with random apertures and obstacles in two layers that allow liquid to flow through multiple random flow paths. I also fabricated three-layered lattice-like microstructures. In each layer, the height and the width of the walls are about 18 μm and 2 μm, respectively. A wedge-shaped micro-channel that eliminates geometrical contributions to hysteresis was fabricated by 2PP and laser direct-writing technique to simulate one pore throat in a porous medium. Capillary pressure – wetting phase saturation (Pc – Sw) hysteresis was examined in this smooth-walled micro-model. The 3D structures of interfaces in a simple channel were directly imaged and quantified using laser scanning confocal microscopy. The data are used to study the relationship between the area under the Pc – Sw scanning curves and the irreversible surface free energy changes that occur as the common line between the solid phase and the two mobile phases of liquid (water and air) moves along the interface. Confocal microscopy provides an effective method to directly image 3D thin films and to directly measure film thickness, volume, and other parameters. The same micro-channel and flow setup were used to investigate the film effects on contact angle hysteresis, interfacial area per volume, and capillary pressure – saturation hysteresis. Film relaxation is discussed and compared to that without film