MOTION OF A HIGHLY WATER-REPELLENT SPHERE IN NEWTONIAN FLUIDS

摘要

It has, recently noticed the flow characteristics on a highly water-repellent wall on which a gas-liquid interface occurs near the solid surface. We have reported the drag reduction of flow around a circular cylinder with a highly water-repellent wall where fluid slip occurred at the wall and the separation point moved downstream compared with the case of a circular cylinder with a smooth solid surface. On the other hand, we have discovered that a gas-phase film of about 60-90 μm covered the sphere with a highly water-repellent surface, when the sphere was soaked in water. A new practical use of this phenomenon is to give buoyancy to the sphere while the sphere retains the gas-phase film in the flow field, for surfaces with the hydrophobic property. We clarified the effect of the gas-phase film on the fall velocity of a highly water-repellent sphere in Newtonian fluids. Experiments were carried out to measure the thickness of the film by the contact needle method and the fall velocity by the falling sphere method in the Reynolds number range of 171 < Re < 10032. Consequently, it was clarified that the thicknesses of gas-phase films are almost constant. Films can be made to form at the thicknesses of Duracon and Nylon, 91μm and 61μm, on the highly water-repellent surface. The terminal velocity of the highly water-repellent sphere decreases more than that of the normal smooth sphere. The maximum velocity deceleration ratios were 61% for Nylon and 42% for Duracon spheres. It was clarified that a sphere with density greater than that of water floats on the water surface when it is covered with the gas-phase film. The results have obtained that the phenomenon can be applied to the prevention of the precipitation of solid spheres and particles.