The Yen−Mullins model, also known as the modified Yen model, specifies the predominant molecular and colloidal structure of asphaltenes in crude oils and laboratory solvents and consists of the following: The most probable asphaltene molecular weight is ∼750 g/mol, with the island molecular architecture dominant. At sufficient concentration, asphaltene molecules form nanoaggregates with an aggregation number less than 10. At higher concentrations, nanoaggregates form clusters again with small aggregation numbers. The Yen−Mullins model is consistent with numerous molecular and colloidal studies employing a broad array of methodologies. Moreover, the Yen−Mullins model provides a foundation for the development of the first asphaltene equation of state for predicting asphaltene gradients in oil reservoirs, the Flory−Huggins− Zuo equation of state (FHZ EoS). In turn, the FHZ EoS has proven applicability in oil reservoirs containing condensates, black oils, and heavy oils. While the development of the Yen−Mullins model was founded on a very large number of studies, it nevertheless remains essential to validate consistency of this model with important new data streams in asphaltene science. In this paper, we review recent advances in asphaltene science that address all critical aspects of the Yen−Mullins model, especially molecular architecture and characteristics of asphaltene nanoaggregates and clusters. Important new studies are shown to be consistent with the Yen−Mullins model. Wide ranging studies with direct interrogation of the Yen−Mullins model include detailed molecular decomposition analyses, optical measurements coupled with molecular orbital calculations, nuclear magnetic resonance (NMR) spectroscopy, centrifugation, direct-current (DC) conductivity, interfacial studies, small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS), as well as oilfield studies. In all cases, the Yen−Mullins model is proven to be at least consistent if not valid. In addition, several studies previously viewed as potentially inconsistent with the Yen−Mullins model are now largely resolved. Moreover, oilfield studies using the Yen−Mullins model in the FHZ EoS are greatly improving the understanding of many reservoir concerns, such as reservoir connectivity, heavy oil gradients, tar mat formation, and disequilibrium. The simple yet powerful advances codified in the Yen−Mullins model especially with the FHZ EoS provide a framework for future studies in asphaltene science, petroleum science, and reservoir studies.
The resolution of asphaltene nanoscience is becoming increasingly important for a variety of purposes. One key molecular attribute of the asphaltenes is the size distribution of their polycyclic aromatic hydrocarbons (PAHs). Comparison of measured spin singletÀsinglet absorption and emission transitions with exhaustive molecular orbital (MO) calculations on 523 PAHs indicates that asphaltene PAHs have a population centroid of ∼7 fused rings. To further test this understanding of asphaltene PAHs, it is desirable to consider the dynamics of triplet states. Nevertheless, triplet-state spectroscopy is complex, especially on polydisperse materials such as asphaltenes. For validation, we compare simple expectations for asphaltenes against both experimental and theoretical results. Measurements were conducted on crude oil and asphaltene samples of dramatically different heavy end content to identify specific transitions being investigated. Experimental results include spectra at several wavelengths, lifetimes in the presence and absence of molecular oxygen, and temperature effects. Specifically, we use classic techniques [Horrocks and Wilkinson, Proc. R. Soc. London A 1968, 306, 257À273] to measure tripletÀtriplet spectra for crude oils and asphaltenes. These are compared with corresponding MO calculations. Again, using classic methods [Guzeman et al., J. Chem. Soc. Faraday Trans. 1973, 69, 708À720], quenching effects of asphaltene triplet states by molecular oxygen are measured and compared with simple diffusion expectations. The temperature dependence provides further stringent testing. Spectral comparisons versus crude oil composition rule out significant spectral contributions from free radicals. Simple expectations regarding triplet-state spectroscopy of asphaltenes and crude oils apply and corroborate previous conclusions from singlet-state spectroscopy of crude oils and asphaltenes. The data herein are consistent with asphaltene PAHs being relatively large (e.g., 7 fused rings); this, in turn, is consistent with the predominance of a single PAH per asphaltene molecule (the "island" molecular architecture). Smaller PAHs dominate the triplet transitions for the crude oil samples and optical wavelengths used herein.
A new type of solid-state variable focal length lens is described. It is based on shape changes in an elastomeric membrane driven by compression of a reservoir of a polymer gel. A novel fabrication process based on individual lens components allows for customization of lens power based on the desired application. The lens shape as a function of applied compressive strain is measured using direct surface profile measurements. The focal length of a solid state lens was reversibly changed by a factor of 1.9. Calculated back focal lengths of the lens were consistent with experimental measurements.
A synthetic polymeric lens was designed and fabricated based on a bio-inspired, "Age=5" human eye lens design by utilizing a nanolayered polymer film-based technique. The internal refractive index distribution of an anterior and posterior GRIN lens were characterized and confirmed against design by µATR-FTIR. 3D surface topography of the fabricated aspheric anterior and posterior lenses was measured by placido-cone topography and exhibited confirmation of the desired aspheric surface shape. Furthermore, the wavefronts of aspheric posterior GRIN and PMMA lenses were measured and simulated by interferometry and Zemax software, respectively. Their results show that the gradient index distribution reduces the overall wavefront error as compared a homogenous PMMA lens of an identical geometry. Finally, the anterior and posterior GRIN lenses were assembled into a bio-inspired GRIN human eye lens through which a clear imaging was possible.
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