Air-water interface deformation by impacting bioinspired cylinder arrays of varying geometry

Many animals have bristled appendages whose function varies depending on inter-bristle spacing, orientation, Weber number, Bond number, and Reynolds number. In a single-phase environment (e.g. water or air), such appendages may function as paddles for swimming or rakes for sensing. Some semi-aquatic insects use these appendages to facilitate takeoff from the water surface; however, the fluid mechanical interaction of the bristled appendage with the water surface is poorly understood compared to the single-phase case. We idealize the bristled appendage as a parallel array of long cylinders at equidistant spacing (ranging from 2 to 8 cylinder diameters), and experimentally investigate how such an array impacts and interacts with an air-water interface when the forces of surface tension and buoyancy are equally important (Bo~1). We also vary the geometry of the array: while each cylinder remains parallel to the undisturbed air-water interface (so that the problem is two-dimensional), we raise the outer cylinders, so that the overall array takes on a "flattened vee" shape from the side view. Using high-speed videography, we measure the deformation of the air-water interface as the array impacts the surface and infer the resulting vertical reaction force on the cylinder array. We find that by varying its geometry, the array can achieve a greater plunge depth without breaking the surface. Using computational modeling, we also explore the effects of decreasing cylinder diameter, thereby investigating the relative roles of surface tension vs. buoyancy. Our findings offer insight into the biomechanics of interfacial locomotion and open new possibilities in designing bio-inspired devices.