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[PPS06-17] The formation environment of compound chondrules constrained by collision experiments between viscous droplets and solid spheres
Keywords:compound chondrule, protosolar disk, shock wave heating, planetesimal collision
"Compound chondrules", in which two chondrules are combined, are found at a rate of 2-4% of all chondrules. In many cases, one chondrule maintains its spherical shape, while the other deforms from its spherical shape and joins together. One chondrule that maintains its spherical shape is thought to have already solidified at the time of bonding, while the other was still molten. In order for composite chondrules to form, chondrules must collide, and a certain number density is required depending on the collision velocity. Arakawa & Nakamoto (2016: Icarus 276, 102) estimated that the number density required for compound chondrule formation is 8 m-3 chondrules, assuming that the collision speed is 1 m s-1 and the probability of merging at the time of collision is 1. As dust aggregate falls to the midplane of the primordial solar system disk, the gas:dust ratio increases to 1:1 at the midplane, and if all the dust becomes chondrule precursors, the number will be only 2 m-3, which is insufficient for the formation of compound chondrules. However, if the collision velocity becomes larger than 1 m s-1, the collision frequency will increase and the required number density will decrease. On the other hand, as the collision speed increases, the possibility that the chondrules will not coalesce and break up during the collision. In addition, there is a large range in the viscosity coefficient during the formation of compound chondrules, and it is thought that the smaller the viscosity, the easier it is to split. We clarified the coalescence probability as a function of collision velocity and viscosity coefficient through collision experiments between viscous droplets and solid spheres.
There are many researches of droplet collision experiments, and water is used in most cases. This is because water can efficiently accelerate many droplets when sprayed. However, the viscosity coefficient of rock melt is at least 1 Pas, which is 1000 times the viscosity coefficient of water. For this reason, it is difficult to release highly viscous droplets by spraying. When the viscosity coefficient was less than 10 Pa s, the droplet was rolled on a copper mesh coated with water-repellent spray (Pan et al. 2019: Adv. Sci. 2019, 1). In cases of high viscosity, a droplet was dropped from a syringe. The droplet materials used were water (10-3 Pa s), honey (10 Pa s), and starch syrup mixed with water. By changing the water mixing ratio, we conducted experiments in three ways: 5 Pa s, 100 Pa s, and 200 Pa s. On the other hand, a glass sphere with a diameter of 7 mm was used as the solid sphere. When colliding at high speed, a mid-air collision was achieved by ejecting from a side-knock ballpoint pen. When colliding at low speed, the glass sphere was fixed to the suction device and released to cause the collision. The collision results were categorized by the We number, which is the ratio of inertial force to surface tension, and dimensionless collision parameters.
There were three outcomes of the collision: sticking, seperation, and bouncing. If the viscosity is relatively small and the collision parameter is small, they will stick. Seperation occurs when the collision parameter becomes larger than a certain value. If bc is the collision parameter that causes seperation, then bc2 is the sticking probability. Bouncing occurs when the viscosity coefficient exceeds 100 Pa s. By organizing the experimental results using the We number on the horizontal axis and the collision parameter on the vertical axis, we found that the boundary line between sticking and seperation shifts toward the larger collision parameter as the viscosity increases. From this, we found an empirical formula that gives bc. By using this empirical formula, we were able to obtain the coalescence probability as a function of the viscosity coefficient and collision velocity. It was found that the sticking probability is 1 at low speeds and approximately inversely proportional to the fourth power of the collision speed at high speeds. The sticking probability becomes the lowest at around 1 m s-1 of collision velocity, and is 8 m-3. The required number density increases from 8 m-3 even if the collision velocity increases or decreases from 1 m s-1. This is because on the low velocity side, the sticking probability is 1, but the collision frequency decreases, and on the high velocity side, the sticking probability decreases in inverse proportional to the fourth power of the collision velocity. This result suggests that it is unlikely that the compound chondrules were formed from precursors floating in the proto-Solar System disk, but rather in an environment where higher number densities could be achieved, such as planetesimal collisions.
There are many researches of droplet collision experiments, and water is used in most cases. This is because water can efficiently accelerate many droplets when sprayed. However, the viscosity coefficient of rock melt is at least 1 Pas, which is 1000 times the viscosity coefficient of water. For this reason, it is difficult to release highly viscous droplets by spraying. When the viscosity coefficient was less than 10 Pa s, the droplet was rolled on a copper mesh coated with water-repellent spray (Pan et al. 2019: Adv. Sci. 2019, 1). In cases of high viscosity, a droplet was dropped from a syringe. The droplet materials used were water (10-3 Pa s), honey (10 Pa s), and starch syrup mixed with water. By changing the water mixing ratio, we conducted experiments in three ways: 5 Pa s, 100 Pa s, and 200 Pa s. On the other hand, a glass sphere with a diameter of 7 mm was used as the solid sphere. When colliding at high speed, a mid-air collision was achieved by ejecting from a side-knock ballpoint pen. When colliding at low speed, the glass sphere was fixed to the suction device and released to cause the collision. The collision results were categorized by the We number, which is the ratio of inertial force to surface tension, and dimensionless collision parameters.
There were three outcomes of the collision: sticking, seperation, and bouncing. If the viscosity is relatively small and the collision parameter is small, they will stick. Seperation occurs when the collision parameter becomes larger than a certain value. If bc is the collision parameter that causes seperation, then bc2 is the sticking probability. Bouncing occurs when the viscosity coefficient exceeds 100 Pa s. By organizing the experimental results using the We number on the horizontal axis and the collision parameter on the vertical axis, we found that the boundary line between sticking and seperation shifts toward the larger collision parameter as the viscosity increases. From this, we found an empirical formula that gives bc. By using this empirical formula, we were able to obtain the coalescence probability as a function of the viscosity coefficient and collision velocity. It was found that the sticking probability is 1 at low speeds and approximately inversely proportional to the fourth power of the collision speed at high speeds. The sticking probability becomes the lowest at around 1 m s-1 of collision velocity, and is 8 m-3. The required number density increases from 8 m-3 even if the collision velocity increases or decreases from 1 m s-1. This is because on the low velocity side, the sticking probability is 1, but the collision frequency decreases, and on the high velocity side, the sticking probability decreases in inverse proportional to the fourth power of the collision velocity. This result suggests that it is unlikely that the compound chondrules were formed from precursors floating in the proto-Solar System disk, but rather in an environment where higher number densities could be achieved, such as planetesimal collisions.