Research Projects

Very little is known about how floating, buoyant bubbles interact with Faraday waves that are created by the vertical vibration of the liquid surface. The surface waves generated are subharmonic, with instability in the form of parallel lines, which then change to moving cusps and troughs, which break at larger amplitudes to create drop ejections. The thin film of liquid on the top surface of floating bubbles drain due to the upward buoyant force, resulting in their collapse. When the liquid layer is vibrated, the film on the top of the bubble could be replenished, resulting in the stabilisation of the bubbles at the surface. In contrast, in some regimes, the bubbles could be punctured by the downward plunging wave crests or by the upward moving wave troughs, causing violent collapse. Further, vertical acceleration of the displaced mass of fluid due to the bubbles cause additional kinematic buoyancy, that can vertically migrate the bubbles. Further, oscillating curvatures of the liquid surface create oscillating capillary pressure fields. Moreover, bubbles colliding with the vibrating liquid surface could create pilot waves that can affect the motion of the bubbles themselves. These complex interactions are yet unexplored; exploration of these are expected to give novel insights as well discover novel physical phenomena. The main hypothesis is that when bubbles float on a liquid layer, with the liquid layer subjected to a vertical vibration creating Faraday waves, when the drainage time of the liquid in the film above the bubble is smaller than the wave time scale, then the bubbles will not collapse. Such a situation can be then tuned to pump more and more bubbles into a liquid layer to create foams of novel properties, hitherto unattainable. In addition, from a fundamental perspective, dynamics of bubbles on faraday waves involve interaction of buoyancy force of the bubble, kinematic buoyancy of the displaced fluid subjected to an acceleration due to vibration, forces due to streaming motions, oscillating capillary pressures due to change in curvatures of the liquid surface and the force exerted by likely pilot waves initiated by the collision of the bubble with the liquid surface. We hypothesise that novel phenomena will occur in such situation, which needs to be investigated. The main experiments to be carried out involve visualisation of bubble dynamics at the surface of a vertically vibrating liquid layer. The experiments will be conducted in a deep layer where the wavelength of Faraday waves is smaller than the liquid layer height, sufficiently wide so that the end effect of the container are not felt substantially at the centre of the layer. The bubble sizes, the frequencies and amplitudes of vibration and the liquid properties will be changed to conduct a range of experiments so as to find regimes of stabilisation or collapse of bubbles. Similar experiments with swarm of bubbles will also be attempted so as to explore creation of polymer or metal foams of novel properties. In such experiments the hold up volume and average bubble sizes will also be measured by combination of flow meter and visualisation. The significance of the present investigation of dynamics of bubbles at a parametrically excited surface is that the research is expected to result in discovery of new physical phenomena due to the complex interaction of the multitudes of phenomena discussed above. Application wise, the present research could result in novel methods to create polymer and metal foams, with properties not achievable by the present methods of manufacture. The present research would also be helpful in suppression of formation, stabilisation and migration of bubbles in rocket liquid fuel tanks where they are invariably subjected to vibration in space flight. Further, the present research could suggest methods to create thin high velocity jets; this will be of use ink jet printing, painless injections, micro explosives etc.