Speaker: Andy Woods, University of Cambridge
Title: On sediment dispersion during deep sea mining
Video: cambridge.org/flowwebinar/woods
Abstract: In this talk, I will present a series of new experiments in which we explore the dynamics of sediment plumes moving through the ocean. The sediment plumes are produced from a source of sediment and water discharged into a flume tank with a moving source to mimic the effect of a background flow. We will describe how the dynamics evolves from a convecting sediment plume to a sedimenting stream of particles, and we will present some simple models to provide bounds on the trajectories of the particles in the water column.
The modelling and dynamics are relevant for assessing some of the potential environmental impacts of deep sea mining and we will discuss this in the presentation.
Speaker: Martin Bazant, MIT
Title: Monitoring carbon dioxide to quantify the risk of indoor airborne transmission of COVID-19
Video: cambridge.org/flowwebinar/bazant
Abstract: A new guideline for mitigating indoor airborne transmission of COVID-19 prescribes a limit on the time spent in a shared space with an infected individual (Bazant & Bush, Proceedings of the National Academy of Sciences of the United States of America, vol. 118, issue 17, 2021, e2018995118). Here, we rephrase this safety guideline in terms of occupancy time and mean exhaled carbon dioxide (CO2) concentration in an indoor space, thereby enabling the use of CO2 monitors in the risk assessment of airborne transmission of respiratory diseases. While CO2 concentration is related to airborne pathogen concentration (Rudnick & Milton, Indoor Air, vol. 13, issue 3, 2003, pp. 237–245), the guideline developed here accounts for the different physical processes affecting their evolution, such as enhanced pathogen production from vocal activity and pathogen removal via face-mask use, filtration, sedimentation and deactivation. Critically, transmission risk depends on the total infectious dose, so necessarily depends on both the pathogen concentration and exposure time. The transmission risk is also modulated by the fractions of susceptible, infected and immune people within a population, which evolve as the pandemic runs its course. A mathematical model is developed that enables a prediction of airborne transmission risk from real-time CO2 measurements. Illustrative examples of implementing our guideline are presented using data from CO2 monitoring in university classrooms and office spaces.
Speaker: Aimee Morgans, Imperial College London
Title: Simulation and control of wake bi-modal switching
Video: cambridge.org/flowwebinar/morgans
Abstract: The turbulent wake behind blunt or “squareback” bluff bodies is relevant to many engineering applications, including road vehicles. In the last decade, a new feature of these wake flows has been identified: wake bi-modality or multi-modality, which is associated with enhanced drag. Although it may be symmetric in the long time average, instantaneously the wake is asymmetric, undergoing switching events between different asymmetric positions. Switchings occur randomly, and over very large timescales. The preferred wake asymmetric positions have been shown to be the result of laminar flow instabilities/bifurcations, with switching events triggered by flow disturbances. The large timescales and dependence on boundary layer disturbances makes computational simulation challenging. We will present insights into how to simulate these wake switching events, as well as feedback control strategies for their suppression.
Speaker: Phillippe Renaud, EPFL
Title: A tutorial on transport phenomena in nanofluidics
Video: cambridge.org/flowwebinar/renaud
Abstract: Nanofluidics is the study of fluid dynamics within channels or cavities typically below 100nm. In the 10nm to 100nm range, the Stokes equation still apply for liquids. However, electrokinetic phenomena related to surface charges of the solid walls start to have strong effect on fluid flow and molecular diffusion.
In this short tutorial, we will restrict the discussion to the ion transport and molecular diffusion in conditions of partial or total overlap of the electrical double layers from the surfaces. Under such conditions, the surface conductance due to the counter ions of the diffused layer increases the overall conductance of the channel. The effective surface charge which depends on ion concentration and temperature lead to corrections to usual nanochannel conductance models [1]. Because of proximity to the surfaces, the molecular interactions affect the apparent diffusion characteristics. A careful analysis of single molecule diffusion reveals specials regimes related to electrostatic screening of the surface [2]. When channel dimensions reach nanometric size or below, new effects have to be taken in consideration. This will briefly be addressed in the conclusion [3].
- [1] An improved model for predicting electrical conductance in nanochannels, M. Taghipoor, A. Bertsch and Ph. Renaud, Phys. Chem. Chem. Phys., 2015, 17, 4160-4167
- [2] Direct Observation of Transitions between Surface-Dominated and Bulk Diffusion Regimes in Nanochannels, N. Durand, C. Dellagiacoma, R. Goetschmann, A. Bertsch, I. Märki, T. Lasser, and Ph. Renaud, Analytical Chemistry, 81, 13, 2009
- [3] N. Kavokine, R. Netz, and L. Bocquet, Annu. Rev. Fluid Mech. 2021. 53:377–410
Speaker: Moran Bercovici, Technion – Israel Institute of Technology
Title: Fluidic Shaping of Optical Components
Video: cambridge.org/flowwebinar/bercovici
Abstract: Fabrication of optical components has not changed significantly in the past century, and is based on mechanical grinding, machining and polishing that rely on complex and expensive infrastructure. Modern manufacturing methods such as 3-D printing, while capable of producing nearly arbitrary structures, cannot provide the required surface quality for optical applications.
I will present our theoretical and experimental work on leveraging the basic physics of liquid-fluid interfaces for fabrication of a wide range of high-quality optical components, without the need for any mechanical processing.
I will first discuss the use of light projection to create desired deformations in thin polymer films by the thermocapillary effect. Polymerization of the film results in diffractive optical elements with sub-nanometric surface quality. To create larger components such as eyeglasses or telescope lenses, the elimination of gravity is crucial. Relying on neutral buoyancy, we developed a passive method wherein we engineer boundary conditions on fluidic interfaces to drive liquid volumes into minimum energy states that correspond to desired optical topographies.
Frumkin, V., & Bercovici, M. (2021). Fluidic shaping of optical components, Flow.
Speaker: Maurizio Porfiri, New York University
Title: Modeling zebrafish collective behavior: emergence of in-line swimming patterns
Video: cambridge.org/flowwebinar/porfiri
Abstract: Mathematical models promise new insights into the mechanisms underlying the emergence of collective behaviour in fish. Here, we establish a mathematical model to examine collective behaviour of zebrafish, a popular animal species in preclinical research. By integrating the paradigms of the vortex dipole and the jump persistent turning walker, we formulate a mathematical model that accounts for social and hydrodynamic interactions between individuals, along with the burst-and-coast swimming style of zebrafish. Each fish is described as a system of coupled stochastic differential equations, which govern the time evolution of their speed and turn rate. Model parameters are calibrated using our previous experimental observations of zebrafish pairs swimming in a shallow water tank. The model successfully captures the main features of the collective response of live animals, by predicting their preference to swim in-line, with one fish leading and the other trailing. Mathematically backed results presented in this talk support the application of dynamical systems theory to unveil the inner workings of fish collective behaviour.