Helical structures exhibit novel optical and mechanical properties and are commonly used in different fields such as metamaterials and microfluidics. A few methods exist for fabricating helical microstructures, but none of them has the throughput or flexibility required for patterning a large surface area with tunable pitch. In this paper, we report a method for fabricating helical structures with adjustable forms over large areas based on multiphoton polymerization (MPP) using single-exposure, three dimensionally structured, self-accelerating, axially tunable light fields. The light fields are generated as a superposition of high-order Bessel modes and have a closed-form expression relating the design of the phase mask to the rotation rate of the beam. The method is used to fabricate helices with different pitches and handedness in the material SU-8. Compared to point-by-point scanning, the method reported here can be used to reduce fabrication time by two orders of magnitude, paving the way for adopting MPP in many industrial applications.
Microfabrication based on two-photon polymerization (TPP) is typically achieved by scanning a focal spot point-by-point. This is a type of serial processing that significantly limits fabrication speed. Bessel beams known for their nondiffracting property are suitable for the fabrication of high-aspect-ratio microstructures without scanning the beams. The zero-order Bessel beam generated by an axicon or a spatial light modulator (SLM) has been used to fabricate such structures as polymer fibers with an aspect ratio exceeding 500:1. However, the fabrication speed is still limited by the serial exposure of a single Bessel beam. In this paper, the authors explore a method for parallel fabrication of high-aspect-ratio microstructures using an array of high-order Bessel beams. An optics system is built in which high-order and superposed high-order Bessel beams generated by an SLM are demagnified and relayed to the photopolymer. These beams retain the same nondiffracting property as the zero-order beam while expanding the exposure light field to arrays of beams. Beam profiles are characterized and compared with theoretical predictions. The power efficiency of the system is measured and analyzed. The influence of off-axis illumination on the SLM is studied. Combined with suitable photopolymer and exposure parameters, this method could be useful for high-speed, volumetric fabrication in TPP.
Biofuel production needs to be more efficient than its current status to increase its competitiveness. The multistep biofuel production is consisted of processes on biomass preprocessing and bioconversion stages. As a crucial parameter, biomass particle size has significant effects on both stages. It is essential to have an insightful understanding of the effects of particle size on sugar yield. Although numerous studies have been performed to meet this objective, there is no commonly accepted guideline on how to select particle size. One possible reason for this gap is the effects of particle size vary when different biomass pretreatment methods are employed. In this study, an assessment on the relationship between particle size and sugar yield was performed for four pretreatment methods. Three particle sizes (1, 4, and 8 mm) of corn stover and switchgrass biomass were used in supercritical CO2, dilute acid (H2SO4), dilute alkaline (Na2CO3), and metal oxide (MgO) pretreatments. Biomass compositional analyses were conducted before and after each pretreatment. Pretreatment solid recovery and sugar recovery rates were calculated. Enzymatic hydrolysis sugar yield and efficiency were used to evaluate the performance of hydrolysis and total sugar yield was used to interpret how much sugar a unit dry weight of biomass (before pretreatment) can yield through pretreatment and enzymatic hydrolysis combined. It was found that particle size was a weak indicator of enzymatic hydrolysis efficiency. There was little value in reducing particle size below 8 mm in order to overcome the resistance imposed by biomass structure on cellulose and xylan hydrolysis.
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