“Kitchen Earth Science,” which is scientific, educational, and public outreach activities, aims at understanding a natural phenomenon in Earth and planetary sciences by analogue experiments using goods and tools in our daily life. Analogue experiments have a function to reveal the fundamental physics governing the phenomenon. On the other hand, they essentially include uncertainties so that unexpected results are frequently obtained, which have a potential for surprising discoveries. These findings also provide a good opportunity for deeply thinking and raise new questions to explore. Such experiences are precious not only for young researchers, but also non-expert people who need a scientific thinking to live wisely. Here, we introduce our recent activities of “Kitchen Earth Science” and the experimental studies rerated to the multiphase flow in Earth and planetary sciences.

Dense suspension of non-Brownian particles such as potato starch often shows counter-intuitive behaviors due to its discontinuous rheological property. Using this particular rheological property, people can run on a dense suspension. However, once the motion of leg becomes too slow, the leg is arrested, namely, it is difficult to walk on the surface. To explain this peculiar behavior, propagation of jamming front caused by an impact onto a dense suspension has been considered recently. The effectively solidified zone is developed by the jamming-front propagation. This solidified zone effectively increases viscosity of the suspension. In addition, when the jamming front reaches the bottom of the container, a solidified pillar which causes effective elasticity is developed. Because the lifetime of the solidified zone is short, the suspension recovers usual viscosity after a while. To characterize the viscoelastic properties of the impacted dense suspension, a set of simple impact experiments was performed. A solid sphere was dropped from a certain height onto a suspension. The impact and rebound dynamics were recorded by a high-speed camera. Using the kinematic data, viscoelasticity of the impacted suspension was measured. The obtained data are consistent with the idea of jamming-front propagation. In addition to the impact onto a static target, impact experiments onto a dense suspension under the influence of mechanical vibration were also performed. Although the mechanical vibration can significantly deform the free surface of suspension, the impact response was hardly affected. In this article, a brief review of the impact-induced jamming-front propagation and the resultant effective viscoelasticity is presented.

Explosive volcanic eruptions produce pumice including bubbles (mainly H₂O gas) which form by the vesiculation process under decompression during the magma ascent in the volcanic conduit. Shape of bubbles varies from spherical to tubular or irregular depending on the eruption style, intensity and magnitude. Interestingly, huge eruptions such as caldera-forming eruptions characteristically include elongated bubbles. We conduct the bread baking experiments to examine what controls the shape of bubbles by using the vesiculation process due to the fermentation of yeast. We use two experimental setups, baking within and without a glass conduit, to evaluate the effect of 1 dimensional expansion flow on the bubble shape. Results show that breads baked within the glass conduit typically include the elongated bubbles. We make the textural analysis for the cross section of bread after fermentation and baking with a time interval of 5 min to understand the evolution of bubble texture and with variable concentrations of yeast to evaluate the effect of amount of yeast on fermentation, consequent expansion of dough and bubble texture. By experiments with the variable concentration of yeast, it is found that the maximum expansion occurs at the certain concentration of yeast approximately 0.5 g per 50 g dough, and that the number of bubbles monotonically decreases with increasing the concentration of yeast. Time series experiments show that the number of bubbles decreases and average size of bubble increases with time for both within and without conduit. Aspect ratios of bubbles within a glass conduit are larger in smaller size of bubbles and smaller (i.e., elongated) in larger size. Numerical model simulating the evolution of bubble shape under 1 D expansion demonstrates that smaller bubbles relax to the spherical shape by surface tension effect and larger bubbles maintain the elongated shape formed by 1 D expansion.

Beautiful patterns such as snowflakes, wind ripples, and cloud patterns are ubiquitous in nature. Some of those are formed by interfacial instability. We focus on two pattern formations caused by the Rayleigh-Taylor instability. Rayleigh-Taylor instability is an interfacial instability between two liquids that occurs when a heavier liquid is on a lighter liquid. The first pattern we discuss is a fractal/cell pattern in a coffee cup. If a droplet of coffee solution is placed on milk, the coffee solution spreads on the surface of the milk. Then the Rayleigh-Taylor instability occurs at the interface between the coffee solution and milk, and the coffee solution starts to sink into the milk. We found that the fractal or the cell pattern is formed at the surface of the milk in this process. We showed that an aspect ratio between the radius of the container and the depth of the milk affects vertical flow and it leads to a transition between the fractal pattern and the cell pattern. We also showed that the fractal pattern is formed by a similar mechanism of a viscous fingering. The second pattern is a breakup of a droplet. A sinking droplet in a viscous solution spontaneously deforms to a vortex ring and then breaks up spontaneously. We experimentally investigated relations among breakup number, radii of droplets, viscosities and density differences between two solutions. We also proposed a phenomenological model considering the Rayleigh-Taylor instability. The phenomenological model provides a non-dimensional parameter derived from a radius of a droplet, viscosity and a density difference between two liquids. And, the model states that the breakup number is classified by the parameter. Our experimental results obey the parameter. It means that the competition between a driving force of gravity and the viscosity dissipation at the interface of two solutions determines the breakup number.

Expantion of foods during cooking sometimes causes crack and burst. Burst is undesirable, while crack is desirable in some cases. Karinto with no sugar occasionally causes burst involving danger and cracks in doughnuts are generally favorable. Crack during deep frying is caused when the internal pressure of sample exceeds the cracking pressure of outer layer. Factors of internal pressure of samples are temperature increase and expanding ratio, and those of cracking pressure are thickness, tensile strength, and size and shape of the sample. The value of both internal pressure and cracking pressure can be calculated by using these data. The predicted timing of cracks in doughnut were confirmed in the experiment under various conditions. Mechanism of burst and crack during deep frying was proposed and it was expected to be applicable to the prediction and control of crack in food processing.

Micro bubbles measuring tens of micrometers or less in diameter have recently been paid much attention. Swirl-type, pressurizing dissolution, and perforated plate methods have been proposed for micro bubble generation. The swirl-type method can generate micro bubbles even under low-pressure power in a simple structure. The enlarged inlet diameter of the swirl chamber can reduce the average bubble diameter. This study examined the effect of the swirl chamber inlet diameter on the flow around chamber outlet in a swirl-type micro bubble generator by numerical analysis. The Inlet diameter of the swirl chamber was changed to 30 mm, 60 mm, and 90 mm, the flow velocity distribution and the wall shear stress around the chamber outlet were calculated. As a result, when the inlet diameter of the swirl chamber was enlarged, it was clarified that the circumferential flow velocity and the wall shear stress around the chamber outlet increased, which reduced the average bubble diameter.

A novel ultrafine bubble generation method has been developed in this study. Water with adjusted dissolved oxygen concentration in a pressure vessel was pressurized with oxygen. After sufficient shaking, the water in the pressure vessel was sprayed through the nozzle into the receiver. The formation of ultrafine bubbles was observed in the discharged water, and its number concentration changed depending on the initial dissolved oxygen concentration in the water. The discharged water was returned to the pressure vessel again, pressurized and shaken with oxygen. A series of the operations for spray discharge were repeated many times. The number concentration of ultrafine bubbles generated with the number of repetitions increased. We succeeded in obtaining a number density higher than the number concentration of ultrafine bubbles obtained by repeatedly applying and reducing pressure without spraying. In addition, when the pressurizing pressure was 0.5 MPa the maximum number concentration of ultrafine bubbles was obtained. Therefore, generation of ultrafine bubbles was promoted not only by repeated pressurization and depressurization, but also by spraying from a pressure vessel to a receiver.

Air entrained phenomenon can often occur in hydraulic structures, for example, head-tank spillway of hydro power plant, stormwater systems, and so on. When the air-water mixture flow is discharged to the submerged outlet tank, there is both shock and noise due to the instantaneous release of large bubbles; that is, the air hammer phenomenon occurs. This study was a numerical investigation using OpenFOAM for pressure fluctuations created by the horizontal air-water two-phase slug flow on submerged outlet condition. A comparison between the numerical results of three different solvers and experimental data indicated that the numerical result using compressibleInterFoam solver could reproduce the pressure fluctuation with sufficient accuracy.