An observational study of engineering online education during the COVID-19 pandemic.

The COVID-19 pandemic compelled the global and abrupt conversion of conventional face-to-face instruction to the online format in many educational institutions. Urgent and careful planning is needed to mitigate negative effects of pandemic on engineering education that has been traditionally content-centered, hands-on and design-oriented. To enhance engineering online education during the pandemic, we conducted an observational study at California State University, Long Beach (one of the largest and most diverse four-year university in the U.S.). A total of 110 faculty members and 627 students from six engineering departments participated in surveys and answered quantitative and qualitative questions to highlight the challenges they experienced during the online instruction in Spring 2020. Our results identified various issues that negatively influenced the online engineering education including logistical/technical problems, learning/teaching challenges, privacy and security concerns and lack of sufficient hands-on training. For example, more than half of the students indicated lack of engagement in class, difficulty in maintaining their focus and Zoom fatigue after attending multiple online sessions. A correlation analysis showed that while semi-online asynchronous exams were associated with an increase in the perceived cheating by the instructors, a fully online or open-book/open-note exams had an association with a decrease in instructor's perception of cheating. To address various identified challenges, we recommended strategies for educational stakeholders (students, faculty and administration) to fill the tools and technology gap and improve online engineering education. These recommendations are practical approaches for many similar institutions around the world and would help improve the learning outcomes of online educations in various engineering subfields. As the pandemic continues, sharing the results of this study with other educators can help with more effective planning and choice of best practices to enhance the efficacy of online engineering education during COVID-19 and post-pandemic.

Analytic solutions for colloid transport with time- and depth-dependent retention in porous media.

Elucidating and quantifying the transport of industrial nanoparticles (e.g. silver, carbon nanotubes, and graphene oxide) and other colloid-size particles such as viruses and bacteria is important to safeguard and manage the quality of the subsurface environment. Analytic solutions were derived for aqueous and solid phase colloid concentrations in a porous medium where colloids were subject to advective transport and reversible time and/or depth-dependent retention. Time-dependent blocking and ripening retention were described using a Langmuir-type equation with a rate coefficient that respectively decreased and increased linearly with the retained concentration. Depth-dependent retention was described using a rate coefficient that is a power-law function of distance. The stream tube modeling concept was employed to extend these analytic solutions to transport scenarios with two different partitioning processes (i.e., two types of retention sites). The sensitivity of concentrations was illustrated for the various time- and/or depth-dependent retention model parameters. The developed analytical models were subsequently used to describe breakthrough curves and, in some cases, retention profiles from several published column studies that employed nanoparticle or pathogenic microorganisms. Simulations results provided valuable insights on causes for many observed complexities associated with colloid transport and retention, including: increasing or decreasing effluent concentrations with continued colloid application, delayed breakthrough, low concentration tailing, and retention profiles that are hyper-exponential, exponential, linear, or non-monotonic with distance.

Langmuirian Blocking of Irreversible Colloid Retention: Analytical Solution, Moments, and Setback Distance.

Soil and aquifer materials have a finite capacity for colloid retention. Blocking of the limited number of available retention sites further decreases the rate of retention with time and enhances risks (e.g., pathogens or colloid-associated contaminants) or benefits (e.g., remediation by microorganisms or nanoparticles) of colloid migration. Our objective was to use a straightforward procedure, based on variable transformation and Laplace transform, to solve the problem of advective colloid transport with irreversible retention and Langmuirian blocking for a pulse-type condition. Formulas for the mean breakthrough time and retardation factor were obtained using zero- and first-order time moments of the breakthrough curves. Equations for the time and position (setback distance) for a particular colloid concentration were obtained from this information. D21 g breakthrough curves and retention profiles in fine sand at four ionic strengths were well described by the model when parameters were optimized. Illustrative simulations demonstrated that blocking becomes more important for smaller retention capacity () and for larger retention rate coefficient (), input concentration (), and pulse duration. Blocking tended to delay colloid arrival time at a particular location relative to a conservative tracer, and produced larger setback distances for smaller and /.

Numerical investigation of the impact of ethanol on flow in the vadose zone.

There is a need to elucidate the impact of ethanol on the subsurface environment because of the application of ethanol as automotive fuel. This study quantifies the effects of changes in surface tension, viscosity, and density induced by ethanol on the transmission and retention of water in the vadose zone. The HYDRUS-1D model was modified to simulate two different scenarios of flow in a sandy loam involving ponding (constant head) or spillage with a subsequent rainfall event (constant flux). Solutions containing different amounts of the highly miscible ethanol (10, 50, and 100% by weight) as well as pristine water were considered. During ponding, ethanol reduced the amount of fluid entering the soil and slowed down the advancement of the wetting front. Viscosity effects were predominant for this scenario, reducing the average depth of the infiltrating liquid up to 44%. The total amount of pure ethanol that entered the soil was 11.38 vs. 17.64 cm for pure water. For the spillage scenario, the results suggest that density has little impact on the liquid movement. Surface tension effects are predominant in the upper portion of the soil. The changes in hydraulic conductivity due to ethanol-induced modifications of solution viscosity are responsible for the slower advancement of the moisture front. The 10% ethanol solution moved 43.1% faster than pure ethanol during the first 2 d because of viscosity and surface tension effects.