Amir Hirsa

Impact dynamics and rebound of water droplets on superhydrophobic carbon nanotube arrays

The authors report the impact response of water droplets impinging on superhydrophobic carbon nanotube arrays and observe that arrays with different wetting properties display significantly different responses. For an array with a static contact angle of 163°, the droplet bounces off the surface several times, while for an array with a reduced contact angle of 140°, the droplet does not rebound and remains pinned. The contact angle hysteresis and contact line pinning for the 140° array suggest that the momentum of the droplet during the initial impact enables it to penetrate and displace the air pockets that are responsible for the superhydrophobicity of the array under static conditions.

Electrochemically activated adaptive liquid lens

Manipulating the shape of liquids using surface tension is an important and useful strategy in designing small-scale fluidic devices. Here we present an example of such a device, a millimeter-scale variable focal length liquid lens consisting of two capillary surfaces. Surface tension is made to change on one capillary surface relative to the other by means of an electric voltage that works in conjunction with a redox surfactant. The change in curvature of the capillary surfaces induces a change in focal length in a process that is shown to be reversible. Focal length values are between 0.5mm and infinity depending on the volume, and electrochemical activation can change the focal length of the liquid lens by 50% or more.

On the interaction of vortex rings and pairs with a free surface for varying amounts of surface active agent

Observations are reported of the interaction with a free surface of vortex rings and vortex pairs moving normal to the surface when different amounts of surface active agents are present on the surface. At a vortex ring Reynolds number Γ/ν≈3800, the interaction with a contaminated free surface results in the generation of secondary and tertiary vortex rings that limited the outward motion of the vortex ring core. When the experiment was repeated with a cleaner surface the formation of the secondary vortex ring was delayed so that the outward motion and stretching of the vortex ring core was much more than for the contaminated surface. At a Reynolds number Γ/ν≈18 000, the vortex pair was observed to rebound from the free surface contrary to what one would expect for an inviscid flat boundary. When the surface was cleaned by draining away a portion of the contaminated surface water the amount of rebound was reduced. These changes in interaction are believed to be caused by the reduction in concentration of the surface active agent which, in turn, results in a reduced generation of secondary vorticity ahead of the vortex ring or pair before and during the interaction with the surface.

Directed rebounding of droplets by microscale surface roughness gradients

Impact dynamics of water droplets on superhydrophobic surfaces with different textures are known to vary dramatically, from total rebounding to complete sticking. Here we show that droplet rebounding on textured surfaces can be significantly influenced by the uniformity of the surface roughness. By engineering nonuniform textures (i.e., roughness gradients) on the surface, we are able to not only manipulate the axial rebound of the droplet, but also introduce a prescribed lateral component to the rebound trajectory. The measured directed rebounding is shown to fit a simple model balancing droplet inertia against the Young’s force imbalance from side to side.

Vortex pair generation and interaction with a free surface

Two vertical, rotating flaps are used to generate a vortex pair beneath a free surface. Vortex pair formation, propagation, and interaction with a free surface are described. Numerical simulations for inviscid flow about a constant upwash sheet of vorticity beneath a free surface agree with experiment up to the time that turbulent mixing occurs during interaction with the free surface. The spacing between the vortex pairs then becomes larger than the calculated spacing. The experiments and lines of marked particles included in the simulations show fluid ingestion and transport toward the free surface.

SYMMETRY BREAKING IN FREE SURFACE CYLINDER FLOWS

The flow in a stationary open cylinder driven by the constant rotation of the bottom endwall is unstable to three-dimensional perturbations for sufficiently large rotation rates. The bifurcated state takes the form of a rotating wave. Two distinct physical mechanisms responsible for the symmetry breaking are identified, which depend on whether the fluid depth is sufficiently greater or less than the cylinder radius. For deep systems, the rotating wave results from the instability of the near-wall jet that forms as the boundary layer on the rotating bottom endwall is turned into the interior. In this case the three-dimensional perturbations vanish at the air/water interface. On the other hand, for shallow systems, the fluid at radii less than about half the cylinder radius is in solid-body rotation whereas the fluid at larger radii has a strong meridional circulation. The interface between these two regions of flow is unstable to azimuthal disturbances and the resulting rotating wave persists all the way to the air/water interface. The flow dynamics are explored using three-dimensional Navier-Stokes computations and experimental results obtained via digital particle image velocimetry

Symmetry breaking to a rotating wave in a lid-driven cylinder with a free surface: Experimental observation

A systematic experimental investigation of the flow in an open cylinder, driven by the constant rotation of the bottom endwall, shows that axisymmetry is spontaneously broken via a supercritical Hopf bifurcation to a rotating wave with azimuthal wave number 4. The physical mechanism responsible for the symmetry breaking is shown to be due to the instability of the shear layer that is produced by the boundary layer on the bottom rotating endwall being turned into the interior by the stationary sidewall. Comparison with other experiments and numerical studies (restricted to axisymmetric subspaces) sheds new light on disparate observations in the literature and helps distinguish between spontaneous and forced (via imperfections) symmetry breaking.

Measurements of vortex pair interaction with a clean or contaminated free surface

Laminar vortex pairs with small Froude number were generated by a submerged delta wing at negative angle of attack or by a pair of vertically oriented, counter-rotating flaps. The vortex pairs thus generated rise and interact with the free surface. The surface and subsurface flow field was studied using flow visualization and particle image velocimetry. Initial surface deformations, striations, are shown to be caused by stretching and interaction of cross-stream vortices near the surface. With small amounts of surface contamination, contamination fronts (producing Reynolds ridges) form on the surface and secondary vorticity, generated beneath the surface beyond the fronts, rolls up to form vortices with opposite rotation outboard of the primary vortices

Low-dissipation capillary switches at small scales

A system of two coupled capillary surfaces is made to switch between its stable states via mechanical and electrochemical disturbances. The bistable switch is experimentally demonstrated using water droplets, where the mechanical activation or “toggle” is achieved by a momentary air-pressure change. Requirements for capillary switches to avoid viscous dissipation are described and strategies for utilizing capillary switches for transporting other liquids or solids are discussed. Addressability of individual switches is achieved using electrochemical activation via a redox surfactant, where surface tension of one element of the switch is changed relative to the other.

Determination of surface shear viscosity via deep-channel flow with inertia

Results of an experimental and computational study of the flow in an annular region bounded by stationary inner and outer cylinders and driven by the rotation of the floor are presented. The top is a flat air/water interface, covered by an insoluble monolayer. We develop a technique to determine the surface shear viscosity from azimuthal velocity measurements at the interface which extends the range of surface shear viscosity that can be measured using a deep-channel viscometer in the usual Stokes flow regime by exploiting flow inertia. A Navier{Stokes-based model of bulk flow coupled to a Newtonian interface that has surface shear viscosity as the only interfacial property is developed. This is achieved by restricting the flow to regimes where the surface radial velocity vanishes. The use of inertia results in an improved signal-to-noise ratio of the azimuthal velocity measurements by an order of magnitude beyond that available in the Stokes flow limit. Measurements on vitamin K1 and stearic acid monolayers were performed, and their surface shear viscosities over a range of concentrations are determined and found to be in agreement with data in the literature.