Dynamics of Water Entry

The hydrodynamics associated with water-entry of spheres can be highly variable with respect to the material and kinematic properties of the sphere. This series of five fluid dynamics videos illustrates several subtle but interesting variations. The first series of videos contrasts the nature of impact between a hydrophilic and hydrophobic sphere, and illustrates how surface coating can affect whether or not an air cavity is formed. The second video series illustrates how spin and surface treatments can alter the splash and cavity formation following water entry. The spinning sphere causes a wedge of fluid to be drawn into the cavity due to the no-slip condition and follows a curved trajectory. The non-spinning sphere has two distinct surface treatments on the left and right hemispheres: the left hemisphere is hydrophobic and the right hemisphere is hydrophilic . Interestingly, the cavity formation for the half-and-half sphere has many similarities to that of the spinning sphere especially when viewed from above. The third video series compares two millimetric nylon spheres impacting at slightly different impact speeds (Uo = 40 and 45 cm/s); the faster sphere fully penetrates the free surface, forming a cavity, whereas the slower sphere does not. The fourth series shows the instability of an elongated water-entry cavity formed by a millimetric steel sphere with a hydrophobic coating impacting at Uo = 600 cm/s. The elongated cavity forms multiple pinch-off points along its decent. Finally, a millimetric steel sphere with a hydrophobic coating breaks the free surface with an impact speed of Uo = 350 cm/s. The cavity pinches-off below the surface, generating a Worthington jet that pinches into droplets owing to the Rayleigh-Plateau instability.Comment: American Physical Society Division of Fluid Dynamics Gallery of Fluid Motion Video Entry Replaced previous version because abstract had LaTex markup and was too lon

Quantitative Flow Field Imaging about a Hydrophobic Sphere Impacting on a Free Surface

This fluid dynamics video shows the impact of a hydrophobic sphere impacting a water surface. The sphere has a mass ratio of m* = 1.15, a wetting angle of 110 degrees, a diameter of 9.5 mm, and impacts the surface with a Froude number of Fr = 9.2. The first sequence shows an impact of a sphere on the free surface illustrating the formation of the splash crown and air cavity. The cavity grows both in the axial and radial direction until it eventually collapses at a point roughly half of the distance from the free surface to the sphere, which is known as the pinch-off point. The second set of videos shows a sphere impacting the free surface under the same conditions using Particle Image Velocimetry (PIV) to quantify the flow field. A laser sheet illuminates the mid-plane of the sphere, and the fluid is seeded with particles whose motion is captured by a high-speed video camera. Velocity fields are then calculated from the images. The video sequences from left to right depict the radial velocity, the axial velocity, and the vorticity respectively in the flow field. The color bar on the far left indicates the magnitude of the velocity and vorticity. All videos were taken at 2610 fps and the PIV data was processed using a 16 x 16 window with a 50% overlap.Comment: American Physical Society Division of Fluid Dynamics 2008 Annual Meeting Replaced previous version because abstract had LaTex markup and was too long, missing periods on middle initial of first two name

Water entry of deformable spheres

When a rigid body collides with a liquid surface with sufficient velocity, it creates a splash curtain above the surface and entrains air behind the sphere, creating a cavity below the surface. While cavity dynamics have been studied for over a century, this work focuses on the water entry characteristics of deformable elastomeric spheres, which has not been studied. Upon free surface impact, elastomeric sphere deform significantly, resulting in large-scale material oscillations within the sphere, resulting in unique nested cavities. We study these phenomena experimentally with high speed imaging and image processing techniques. The water entry behavior of deformable spheres differs from rigid spheres because of the pronounced deformation caused at impact as well as the subsequent material vibration. Our results show that this deformation and vibration can be predicted from material properties and impact conditions. Additionally, by accounting for the sphere deformation in an effective diameter term, we recover previously reported characteristics for time to cavity pinch-off and hydrodynamic force coefficients for rigid spheres. Our results also show that velocity change over the first oscillation period scales with a dimensionless ratio of material shear modulus to impact hydrodynamic pressure. Therefore we are able to describe the water entry characteristics of deformable spheres in terms of material properties and impact conditions.Comment: 19 pages, 12 figure

Drop on a Bent Fibre

Inspired by the huge droplets attached on cypress tree leaf tips after rain, we find that a bent fibre can hold significantly more water in the corner than a horizontally placed fibre (typically up to three times or more). The maximum volume of the liquid that can be trapped is remarkably affected by the bending angle of the fibre and surface tension of the liquid. We experimentally find the optimal included angle ($\sim {36}{^\circ}$) that holds the most water. Analytical and semi-empirical models are developed to explain these counter-intuitive experimental observations and predict the optimal angle. The data and models could be useful for designing microfluidic and fog harvesting devices