Session L02: Mini-Symposium: Kitchen Flows

Live

For more than a century it was believed that thin-film hydraulic jumps which can be seen in kitchen sinks are created due to gravity. In 2018, we (Bhagat et al. 2018) demonstrated experimentally, supported by theory, that these jumps are caused by surface tension, and gravity does not play a significant role. Recently (Bhagat \& Linden (2020)), we have shown that our energy-based analysis is consistent with the conservation of momentum. We have also shown a fundamental flaw in the existing interfacial flow theory that in hydrodynamics, the influence of surface tension is not fully contained in Laplace pressure. Here we test the validity of our theory by comparing its predictions with the experimental results for jumps presented in the literature by other independent groups. We have compared ten sets of experimental data reported in the literature for jumps in the steady-state, for a range of liquids with different physical parameters, flow rates and experimental conditions, and the theory gives excellent prediction to the experimental data. We also show that beyond a critical flow rate, $Q_C^* \propto \gamma^2 /\nu \rho^2 g$, gravity plays a role, but at lower flow rates surface tension is the dominating force, confirming that kitchen sink jumps are caused by surface tension.   

Live

Mixing and rinsing of miscible liquids are common processes. At home, these include the washing plates by rinsing soap layers with jets of water and the dissolution of honey into water where viscous pendant or sessile drops of honey spread into low viscosity water. These processes, common to everyday experience, present fascinating fluid mechanical phenomena. These are also encountered in manufacturing operations, such formulating personal and food products, and in the cleaning of semiconductor substrates. This paper describes experiments and theoretical analyses of several classes of miscible liquid flows. Rinsing flows where jets of water impinging on precoated layers of polymer solutions provide very effective removal of colloidal particles adsorbed onto planar substrates (silicon wafers). When the lower substrate is set in rotation, as is the case in spin coating, similarity solutions are presented that explain the spreading dynamics of liquids flowing across coatings of miscible liquids. Pendant and sessile drops of viscous liquids residing within less viscous media undergo fascinating shape changes. It is demonstrated the dissolution dynamics of both drop geometries can be successfully scaled using convention/diffusion arguments.   

Live

Volatile binary liquid samples on wetting substrates are known to be subject to solutal Marangoni stresses fomenting either spreading or contraction tendencies, which depends on whether it is the more ``volatile" component that possesses a lower surface tension or not, respectively. We run experiments with sessile droplets (isopropanol-water and ethanol-water) for multiple combinations of the initial concentration in the liquid and controlled ambient humidity (water vapor only) essentially covering the entire admissible range of these parameters. Surprisingly, contraction regimes are thereby found for certain parameter ranges, in spite of the alcohols being more volatile than water. Furthermore, regime reversals occur for different liquid concentrations even at zero humidity. To rationalize these observations, a simple model is built highlighting the often overlooked role of the diffusion coefficient ratio of the two vapors and the non-ideality of the mixture. To emphasize the universality of our picture of the phenomenon, experiments are also conducted in other setups: drops with pinned contact lines and tears-of-wine menisci, drops with added microparticles and their deposition patterns.   

Live

Making pancake batter or bread dough requires mixing liquid, in this case, milk or water, with particles, here flour, ideally without forming lumps. The appearance of lumps, dry flour aggregates dispersed in the milk, is the main pitfall to be avoided by slowly and gradually adding the liquid. In all mixing processes, in the kitchen and beyond, the aim is to obtain a homogeneous mixture as quickly as possible and with the least effort. The blending of liquid in dispersed materials is also crucial in many industrial applications and environmental processes. This problem is reminiscent of the imbibition of a liquid in a fixed granular material, such as the liquid rise observed in a sugar cube. However, blending processes are more challenging to characterize as they involve an intricate coupling between the fluid flow and the granular dynamics. In this talk, I will present some of our recent work that aim to understand how a liquid imbibes moving grains, and how a jet of grains falling into a liquid bath is dispersed and can entrain air bubbles resulting in aggregates.   

Live

Thin liquid films (TLF) are ubiquitous in nature and in technological applications. Typically, TLFs form when two bubbles or droplets come into close proximity, and thus they can be present in various multiphase systems, such as foams, emulsions and bread. At equilibrium, the stability of such systems is directly related to the magnitude of the film's disjoining pressure, which was introduced by Derjaguin and Obuchov in 1936 as defined by the sum of all intermolecular forces that act between two opposing surfaces of the film, or alternatively as the derivative of the Gibbs energy per unit area with respect to separation distance. Other forces acting in thin films are those due to capillary pressures caused by curvature differences, and hydrodynamic stresses because of flow, both of which lead to a pressure jump across the film. When the disjoining pressure is larger than the pressure jump across the TLF, then film rupture and coalescence are arrested. The disjoining pressure and capillary forces are equilibrium properties that only depend on the state and geometry of the system. However, many applications entail non-equilibrium conditions, such as those encountered in draining and sheared multiphase systems. In such occasions, the dynamics of films are characterised by a complex interplay between the hydrodynamic, capillary, intermolecular forces and the interfacial stresses. We will discuss how these play a role in beer foam stability and in bread making.   

Live

One of the most important fluid-mechanical concepts in an introductory class is the fluid viscosity. While other properties are more intuitive, such as density or surface tension, viscosity is hard to assimilate: its units do not offer an immediate physical measure of its meaning and its formal definition is convoluted. In an effort to make our teaching laboratories adequate for the health emergency, we have completely redesigned them to be conducted individually at home. To learn about viscosity, instead of the classical sedimenting-sphere experiment, we ask the students to make pancakes: rapidly pour a known volume of any viscous kitchen fluid onto a horizontal surface. Using a phone to video-record the experiment, the time rate of change of the circular fluid blob radius can be determined with standard image processing techniques. By fitting the data to the viscous gravity current model by Huppert (JFM, 1982), $R(t) \sim (\rho g V t7\mu)^{1/8}$, it is possible to infer the value of viscosity. It works! Using oil, syrup, honey and, of course pancake batter, the students successfully measured the fluid viscosity (within the small-Re, large-Bo regime), with notable accuracy and repeatability. This simple flow, surprisingly, has rarely been used as teaching tool.