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1. Morphology and kinetics of asphalt binder microstructure at gas, liquid and solid interfaces

   https://doi.org/10.1111/jmi.12842  

   RAMM, A. ; DOWNER, M. C. ; SAKIB, N. ; BHASIN, A. ( November 2019 , Journal of Microscopy)

   We combined optical and atomic force microscopy to observe morphology and kinetics of microstructures (typically referred to as bees) that formed at free surfaces of unmodified Performance Graded (PG) 64‐22 asphalt binders upon cooling from 150°C to room temperature (RT) at 5°C min^–1, and changes in these microstructures when the surface was terminated with a transparent solid (glass) or liquid (glycerol) overlayer. The main findings are: (1) at free binder surfaces, wrinkled microstructures started to form near the crystallization temperature (∼45°C) of saturates such as wax observed by differential scanning calorimetry, then grew to ∼5 µm diameter, ∼25 nm wrinkle amplitude and 10–30% surface area coverage upon cooling to RT, where they persisted indefinitely without observable change in shape or density. (2) Glycerol coverage of the binder surface during cooling reduced wrinkled area and wrinkle amplitude three‐fold compared to free binder surfaces upon initial cooling to RT; continued glycerol coverage at RT eliminated most surface microstructures within ∼4 h. (3) No surface microstructures were observed to form at binder surfaces covered with glass. (4) Submicron bulk microstructures were observed by near‐infrared microscopy beneath the surfaces of all binder samples, with size, shape and density independent of surface coverage. No tendency of such structures to float to the top or sink to the bottom of mm‐thick samples was observed. (5) We attribute the dependence of surface wrinkling on surface coverage to variation in interface tension, based on a thin‐film continuum mechanics model.

   Asphalt binder, or bitumen, is the glue that holds aggregate particles together to form a road surface. It is derived from the heavy residue that remains after distilling gasoline, diesel and other lighter products out of crude oil. Nevertheless, bitumen varies widely in composition and mechanical properties. To avoid expensive road failures, bitumen must be processed after distillation so that its mechanical properties satisfy diverse climate and load requirements. International standards now guide these mechanical properties, but yield varying long‐term performance as local source composition and preparation methods vary.In situdiagnostic methods that can predict bitumen performance independently of processing history are therefore needed. The present work focuses on one promising diagnostic candidate: microscopic observation of internal bitumen structure. Past bitumen microscopy has revealed microstructures of widely varying composition, size, shape and density. A challenge is distinguishing bulk microstructures, which directly influence a binder's mechanical properties, from surface microstructures, which often dominate optical microscopy because of bitumen's opacity and scanning‐probe microscopy because of its inherent surface specificity. In previously published work, we used infrared microscopy to enhance visibility of bulk microstructure. Here, as a foil to this work, we use visible‐wavelength microscopy together with atomic‐force microscopy (AFM) specifically to isolatesurfacemicrostructure, to understand its distinct origin and morphology, and to demonstrate its unique sensitivity to surface alterations. To this end, optical microscopy complements AFM by enabling us to observe surface microstructures form at temperatures (50°C–70°C) at which bitumen's fluidity prevents AFM, and to observe surface microstructure beneath transparent, but chemically inert, liquid (glycerol) and solid (glass) overlayers, which alter surface tension compared to free surfaces. From this study, we learned, first, that, as bitumen cools, distinctly wrinkled surface microstructures form at the same temperature at which independent calorimetric studies showed crystallization in bitumen, causing it to release latent heat of crystallization. This shows that surface microstructures are likely precipitates of the crystallizable component(s). Second, a glycerol overlayer on the cooling bitumen results in smaller, less wrinkled, sparser microstructures, whereas a glass overlayer suppresses them altogether. In contrast, underlying smaller bulk microstructures are unaffected. This shows that surface tension is the driving force behind formation and wrinkling of surface precipitates. Taken together, the work advances our ability to diagnose bitumen samples noninvasively by clearly distinguishing surface from bulk microstructure.

    

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2. Height-modulation of diffraction gratings by light-controlled capillary force lithography for structural coloring

   https://doi.org/10.1117/12.2650950  

   Ji, Myung Gi ; Kim, Jaeyoun ( March 2023 , PROCEEDINGS SPIE OPTO, MOEMS and Miniaturized Systems XXII)

   Piyawattanametha, Wibool ; Park, Yong-Hwa ; Zappe, Hans (Ed.)

   Diffraction gratings are ubiquitous in many optical applications such as sensors, filters, and optical security devices. Capillary force lithography, which utilizes the capillary rise of photopolymer into nanoscale cavities, is a simple and rapid method to construct diffraction gratings without necessitating expensive instruments or complex steps. With the help of spatial light modulators, such as the digital micromirror device, the height of the grating can also be spatially modulated, printing spatially height-modulated gratings. When white light normally impinges on the grating, the light propagates into the grating interferes with light that propagates into air. By varying the height of the grating, the optical path lengths of two lights can be varied, leading to different interference effects and structural coloring. Judicious design of the grating’s parameters and patterning process will even allow encoding of multiple images. In this work, by tuning the height of the grating through the light-controlled capillary force lithography, we demonstrate grating-based structural color printing. This technique is promising for producing the custom patterns for anti-counterfeiting, authentication, and cryptography. 

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3. Covert infrared image encoding through imprinted plasmonic cavities

   https://doi.org/10.1038/s41377-018-0095-9  

   Franklin, Daniel ; Modak, Sushrut ; Vázquez-Guardado, Abraham ; Safaei, Alireza ; Chanda, Debashis ( November 2018 , Light: Science & Applications)

   Functional surfaces that can control light across the electromagnetic spectrum are highly desirable. Plasmonic nanostructures can assume this role by exhibiting dimension-tunable resonances that span multiple electromagnetic regimes. However, changing these structural parameters often impacts the higher-order resonances and spectral features in lower-wavelength domains. In this study, we discuss a cavity-coupled plasmonic system with resonances that are tunable across the 3–5 or 8–14 μm infrared bands while retaining near-invariant spectral properties in the visible domain. This result is accomplished by regime-dependent resonance mechanisms and their dependence on independent structural parameters. Through the identification and constraint of key parameters, we demonstrate multispectral data encoding, where images, viewable in the infrared spectral domain, appear as uniform areas of color in the visible domain—effectively hiding the information. Fabricated by large area nanoimprint lithography and compatible with flexible surfaces, the proposed system can produce multifunctional coatings for thermal management, camouflage, and anti-counterfeiting.

    

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4. Optical characterization of marine phytoplankton assemblages within surface waters of the western Arctic Ocean

   https://doi.org/10.1002/lno.11199  

   Reynolds, Rick_A ; Stramski, Dariusz ( May 2019 , Limnology and Oceanography)

   An extensive data set of measurements within the Chukchi and Beaufort Seas is used to characterize the optical properties of seawater associated with different phytoplankton communities. Hierarchical cluster analysis of diagnostic pigment concentrations partitioned stations into four distinct surface phytoplankton communities based on taxonomic composition and average cell size. Concurrent optical measurements of spectral absorption and backscattering coefficients and remote‐sensing reflectance were used to characterize the magnitudes and spectral shapes of seawater optical properties associated with each phytoplankton assemblage. The results demonstrate measurable differences among communities in the average spectral shapes of the phytoplankton absorption coefficient. Similar or smaller differences were also observed in the spectral shapes of nonphytoplankton absorption coefficients and the particulate backscattering coefficient. Phytoplankton on average, however, contributed only 25% or less to the total absorption coefficient of seawater. Our analyses indicate that the interplay between the magnitudes and relative contributions of all optically significant constituents generally dampens any influence of varying phytoplankton absorption spectral shapes on the total absorption coefficient, yet there is still a marked discrimination observed in the spectral shape of the ratio of the total backscattering to total absorption coefficient and remote‐sensing reflectance among the phytoplankton assemblages. These spectral variations arise mainly from differences in the bio‐optical environment in which specific communities were found, as opposed to differences in the spectral shapes of phytoplankton optical properties per se. These results suggest potential approaches for the development of algorithms to assess phytoplankton community composition from measurements of seawater optical properties in western Arctic waters.

    

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5. Temporal reflection and refraction of optical pulses inside a dispersive medium: an analytic approach

   https://doi.org/10.1364/JOSAB.416058  

   Zhang, Junchi ; Donaldson, W_R ; Agrawal, G_P ( February 2021 , Journal of the Optical Society of America B)

   We develop an analytic approach for reflection of light at a temporal boundary inside a dispersive medium and derive frequency-dependent expressions for the reflection and transmission coefficients. Using the analytic results, we study the temporal reflection of an optical pulse and show that our results agree fully with a numerical approach used earlier. Our approach provides approximate analytic expressions for the electric fields of the reflected and transmitted pulses. Whereas the width of the transmitted pulse is modified, the reflected pulse is a mirrored version of the incident pulse. When a part of the incident spectrum lies in the region of total internal reflection, both the reflected and transmitted pulses are distorted considerably.

    

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