The boundary layer equations for plane, incompressible, and steady flow are 

$$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

Boundary Layer Theory

The structure

Plasmonic colours are structural colours that emerge from resonant interactions between light and metallic nanostructures. The engineering of plasmonic colours is a promising, rapidly emerging research field that could have a large technological impact. We highlight basic properties of plasmonic colours and recent nanofabrication developments, comparing technology-performance indicators for traditional and nanophotonic colour technologies. The structures of interest include diffraction gratings, nanoaperture arrays, thin films, and multilayers and structures that support Mie resonances and whispering-gallery modes. We discuss plasmonic colour nanotechnology based on localized surface plasmon resonances, such as gap plasmons and hybridized disk–hole plasmons, which allow for colour printing with sub-diffraction resolution. We also address a range of fabrication approaches that enable large-area printing and nanoscale lithography compatible with complementary metal-oxide semiconductor technologies, including nanoimprint lithography and self-assembly. Finally, we review recent developments in dynamically reconfigurable plasmonic colours and in the laser-induced post-processing of plasmonic colour surfaces. Plasmonic colours can be used to colour large surfaces, can be mass-produced and dynamically reconfigured, and can provide sub-diffraction resolution. In this Review, basic properties of plasmonic colours, different platforms supporting them and recent developments in the field are discussed.

Plasmonic colour generation

Biophotovoltaics has emerged as a promising technology for generating renewable energy because it relies on living organisms as inexpensive, self-repairing, and readily available catalysts to produce electricity from an abundant resource: sunlight. The efficiency of biophotovoltaic cells, however, has remained significantly lower than that achievable through synthetic materials. Here, a platform is devised to harness the large power densities afforded by miniaturized geometries. To this effect, a soft-lithography approach is developed for the fabrication of microfluidic biophotovoltaic devices that do not require membranes or mediators. Synechocystis sp. PCC 6803 cells are injected and allowed to settle on the anode, permitting the physical proximity between cells and electrode required for mediator-free operation. Power densities of above 100 mW m-2 are demonstrated for a chlorophyll concentration of 100 μM under white light, which is a high value for biophotovoltaic devices without extrinsic supply of additional energy.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.201401299

A High Power-Density, Mediator-Free, Microfluidic Biophotovoltaic Device for Cyanobacterial Cells.

Book Review: Chlorophylla Fluorescence A Signature of Photosynthesis

Microalgae and cyanobacteria are some of nature's finest examples of solar energy conversion systems, effortlessly transforming inorganic carbon into complex molecules through photosynthesis. The efficiency of energy-dense hydrocarbon production by photosynthetic organisms is determined in part by the light collected by the microorganisms. Therefore, optical engineering has the potential to increase the productivity of algae cultivation systems used for industrial-scale biofuel synthesis. Herein, we explore and report emerging and promising material science and engineering innovations for augmenting microalgal photosynthesis.

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Photon management for augmented photosynthesis

Microalgae are a promising feedstock for sustainable biofuel production. At present, however, there are a number of challenges that limit the economic viability of the process. Two of the major challenges are the non-uniform distribution of light in photobioreactors and the inefficiencies associated with traditional biomass processing. To address the latter limitation, a number of studies have demonstrated organisms that directly secrete fuels without requiring organism harvesting. In this paper, we demonstrate a novel optofluidic photobioreactor that can help address the light distribution challenge while being compatible with these chemical secreting organisms. Our approach is based on light delivery to surface bound photosynthetic organisms through the evanescent field of an optically excited slab waveguide. In addition to characterizing organism growth-rates in the system, we also show here, for the first time, that the photon usage efficiency of evanescent field illumination is comparable to the direct illumination used in traditional photobioreactors. We also show that the stackable nature of the slab waveguide approach could yield a 12-fold improvement in the volumetric productivity.

/pdf/slab-waveguide-photobioreactors-for-microalgae-based-biofuel-4qtpryof7o.pdf

Slab waveguide photobioreactors for microalgae based biofuel production

The spectral distribution of light influences microalgae productivity; however, development of photobioreactors has proceeded largely without regard to spectral optimization. Here, we use monochromatic light to quantify the joint influence of path length, culture density, light intensity, and wavelength on productivity and efficiency in Synechococcus elongatus. The productivity of green light was ∼4× that of red at the highest levels of culture density, depth, and light intensity. This performance is attributed to the combination of increased dilution and penetration of this weakly absorbed wavelength over a larger volume fraction of the reactor. In contrast, red light outperformed other wavelengths in low-density cultures with low light intensities. Low-density cultures also adapted more rapidly to reduce absorption of longer wavelengths, allowing for prolonged cultivation. Taken together, these results indicate that, particularly for artificially lit photobioreactors, wavelength needs to be included as a critical operational parameter to maintain optimal performance. Biotechnol. Bioeng. 2017;114: 1160-1169. © 2017 Wiley Periodicals, Inc.

/pdf/light-dilution-via-wavelength-management-for-efficient-high-3518uecqyn.pdf

Light dilution via wavelength management for efficient high-density photobioreactors.

Hydrothermal liquefaction uses high temperatures and pressures to break organic compounds into smaller fractions, and is considered the most promising method to convert wet microalgae feedstock to biofuel. Although, hydrothermal liquefaction of microalgae has received much attention, the specific roles of temperature, pressure, heating rate and reaction time remain unclear. We present a microfluidic screening platform to precisely control and observe reaction conditions at high temperature and pressure. In situ observation using fluorescence enables direct, real-time monitoring of this process. A strong shift in the fluorescence signature from the algal slurry at 675 nm (chlorophyll peak) to a post-HTL stream at 510 nm is observed for reaction temperatures at 260 °C, 280 °C, 300 °C and 320 °C (P = 12 MPa), and occurs over a timescale on the order of 10 min. Biocrude formation and separation from the aqueous phase into immiscible droplets is directly observed and occurs over the same timescale. The higher heating values for the sample are observed to increase over shorter timescales on the order of minutes. After only 1 minute at 300 °C, the higher heating value increases from an initial value of 21.97 MJ kg−1 to 33.63 MJ kg−1. The microfluidic platform provides unprecedented control and insight into this otherwise opaque process, with resolution that will guide the design of large scale reactors and processes.

/pdf/biomass-to-biocrude-on-a-chip-via-hydrothermal-liquefaction-1frlwfyak6.pdf

Biomass-to-biocrude on a chip via hydrothermal liquefaction of algae

In this work, cultivation of photosynthetic microbes in surface plasmon enhanced evanescent fields is demonstrated. Proliferation of Synechococcus elongatus was obtained on gold surfaces excited with surface plasmons. Excitation over three days resulted in 10 μm thick biofilms with maximum cell volume density of 20% vol/vol (2% more total accumulation than control experiments with direct light). Collectively, these results indicate the ability to (1) excite surface-bound cells using plasmonic light fields, and (2) subsequently grow thick biofilms by coupling light from the surface. Plasmonic light delivery presents opportunities for high-density optofluidic photobioreactors for microalgal analysis and solar fuel production.

Matthew D. Ooms

Papers

Photon management for augmented photosynthesis

Slab waveguide photobioreactors for microalgae based biofuel production

Light dilution via wavelength management for efficient high-density photobioreactors.

Biomass-to-biocrude on a chip via hydrothermal liquefaction of algae

Culturing photosynthetic bacteria through surface plasmon resonance