Molecular Dynamics Simulation to Investigate the Interaction of Asphaltene and Oxide in Aggregate

The asphalt-aggregate interface interaction (AAI) plays a significant role in the overall performances of asphalt mixture, which is caused due to the complicated physicochemical processes and is influenced by various factors, including the acid-base property of aggregates. In order to analyze the effects of the chemical constitution of aggregate on the AAI, the average structure C65H74N2S2 is selected to represent the asphaltene in asphalt and magnesium oxide (MgO), calcium oxide (CaO), aluminium sesquioxide (Al2O3), and silicon dioxide (SiO2) are selected to represent the major oxides in aggregate. The molecular models are established for asphaltene and the four oxides, respectively, and the molecular dynamics (MD) simulation was conducted for the four kinds of asphaltene-oxide system at different temperatures. The interfacial energy in MD simulation is calculated to evaluate the AAI, and higher value means better interaction. The results show that interfacial energy between asphaltene and oxide reaches the maximum value at 25°C and 80°C and the minimum value at 40°C. In addition, the interfacial energy between asphaltene and MgO was found to be the greatest, followed by CaO, Al2O3, and SiO2, which demonstrates that the AAI between asphalt and alkaline aggregates is better than acidic aggregates

Exploring how sensory dominance modulated by modality-specific expectation: an event-related potential study

The Colavita visual dominance effect refers to the phenomenon in which tend to respond only or preferentially to visual stimuli of bimodal audiovisual stimulus. Previous evidence has indicated that sensory dominance can be modulated by top-down expectation. However, it remains unclear how expectations directed toward a single sensory modality influence Colavita visual dominance at the electrophysiology level. Using event-related potential (ERP) measurements, we investigated how modality expectation modulates sensory dominance by manipulating the different unimodal target probabilities used in previous related Colavita studies. For the behavioral results, a significantly larger visual dominance effect was found when the modality expectation was directed to the visual sensory condition (40% V:10% A). Further ERPs results revealed that the mean amplitude of P2 (200–250 ms) in the central-parietal region was larger in the visual precedence auditory response (V_A) type than in the auditory precedence visual response (A_V) type when modality expectation was directed to visual sensory stimuli (40% V:10% A). In contrast, the mean amplitude of N2 (290–330 ms) in the frontal region was larger for the V_A type than in the A_V type when modality expectation was directed to the auditory sensory stimuli (10% V:40% A). Additionally, for the A_V type N1 (150–170 ms) in the frontal region was larger in visual versus auditory expectation condition. Overall, the study tentatively suggested that increasing unimodal target probability may lead to greater top-down expectation direct to target modality stimulus, and then sensory dominance emerges in the late phase when participant response to visual stimuli of bimodal audiovisual stimulus

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Multiscale study of the bitumen-aggregate interfacial behavior based on molecular dynamics simulation and micromechanics

The bitumen-aggregate interfacial phenomena are an area where chemistry, physics, and engineering intersect. While continuous research efforts have been devoted to this issue in the past decades, much less is known about the fundamental mechanisms controlling the origins and the evolution of interfacial failure. The complexity lies in the multifactorial and multiscale nature of bitumen-aggregate interfacial behavior. The interaction between bitumen and aggregate relies directly on an intricate interplay of bitumen chemistry, aggregate mineralogy, and surface topography; and the interfacial performances in service are also closely related to the random heterogeneous microstructure of asphalt mixture, the periodical climate conditions, and the repeated vehicle loads. Moreover, the heterogeneity of the interacting components ranges across nine orders of magnitude in length scales. The current thesis is dedicated to developing a “bottom-up” approach which handles the enormous number of factors across the micro-to-macro length and time scales. A mechanistic study using molecular dynamics simulation was carried out to uncover the adsorption configuration of bitumen-aggregate interface at the molecular scale and how the aggregate mineralogy affects it. The microstructural of the adsorbed bitumen layer was found to be a superposition of two configurations: the layered configuration in the near-surface region arising from aggregation and parallel orientation of the bitumen molecules, and the gradient descent configuration in the region further away from the surface. The degree of concentration and radius of influence are significantly impacted by the mineral surface. The hypothesis of selective adsorption was tested by probing the distribution characteristics of different fractions in bitumen, and the results suggest a rejection of the hypothesis.For purpose of investigating the water-induced damage between bitumen and aggregate, the rolling bottle tests were conducted for six kinds of aggregates, and the ternary bitumen-water-aggregate interface models were established to perform molecular dynamics simulations. The results indicate the existence of competitive adsorption between bitumen and water molecules at the mineral surface, and the penetration capacity of bitumen molecule is greatly affected by the mineral property. Aggregates with higher content of nepheline, chlorite, pyroxene and olivine minerals are more likely to exhibit better moisture damage resistance while aggregates with higher content of quartz, plagioclase, and calcite minerals do the opposite. The influence of surface topography on the adhesion and water-induced debonding behaviors of bitumen on aggregate surface was studied through wetting theory. The contact angle tests were performed to measure the surface energies of bitumen and aggregate surfaces varying in both mineralogy and roughness, based on which the interaction energies between bitumen and aggregate in both air and water environments were quantified, respectively. The negative interfacial adhesive energy for the air/bitumen/aggregate interface and interfacial debonding energy for the water/bitumen/aggregate interface imply that both bitumen wetting and water-induced bitumen dewetting on flat surface are thermodynamically favorable. The Wenzel model was found to describe the rough interface systems well. The interfacial adhesive energy and interfacial debonding energy are enhanced geometrically by the roughness factor r, which indicates that the textured aggregate surface is in favor of force-induced interfacial cracking resistance but leading to an adverse effect on moisture damage resistance. The interfacial cracking behavior of asphalt mixture was exploited at the mesoscale through micromechanics method. A micromechanical damage model was established by incorporating the bilinear Cohesive Zone Model (CZM) into the Mori-Tanaka model. It is found that the interfacial debonding between bitumen and asphalt mortar exhibits a significant dependency on the aggregate size. A critical aggregate size has been identified, above which the damage behavior of asphalt mixtures changes from hardening to softening. The critical aggregate size increases as the mortar modulus increases but decreases with the increase of the interfacial stiffness, Poisson’s ratio of mortar, and aggregate volume fraction. The interface strength and fracture energy also show significant influences on the fracture behavior of asphalt mixtures. The strength of asphalt mixtures increases as the interface strength increases, but it is independent of the fracture energy. Increased fracture energy can improve the fracture resistance of the asphalt mixture, while increased interface strength has the opposite effect.In general, this thesis has exploited a mechanistic investigation on the interfacial interaction between bitumen and aggregate. The fundamental knowledge regarding the influence factors as well as the way how they works were created at multiple length/time scales. The findings from this thesis open up an avenue for predicting the bitumen-aggregate interfacial behavior based on the material genomes

Development of a Full-Depth Wheel Tracking Test for Asphalt Pavement Structure: Methods and Performance Evaluation

The rutting performance of asphalt pavement structure relies on the high temperature properties of asphalt mixture as well as the pavement structure and thickness. In order to investigate the influence of the structure and thickness, a full-depth wheel tracking test is developed in this research by improving the conventional wheel tracking test apparatus. The newly proposed test method is capable of varying its load speed and load size, controlling its specimen temperature gradient, and simulating the support conditions of actual asphalt pavement. The full-depth wheel tracking test based rutting performance evaluation of different asphalt pavement structures indicates that it is not reasonable to explain the rutting performance of asphalt pavement structure from the point of view of single-layer asphalt mixture rutting performance. The developed full-depth wheel tracking test can be used to distinguish rutting performance of different asphalt pavement structures, and two of five typical asphalt pavement structures commonly used in Shanxi Province were suggested for use in practical engineering