Session BF: Interfacial and Thin Film Instabilities II

Sunday, November 20, 2005

Experiments on the rupture of a free plain viscous film are reported.The relatively thick film, with the typical thickness of order 0.1-0.6 mm, rests between two long parallel needles. When the film is ruptured, a hole is formed with the rim on the front. The hole grows, reaches the needles, and propagates along them with a constant velocity of order 5-50 cm/s. Expression for propagation velocity of the rim is derived and compared well with the experimental data. The derived rupture profile, visually similar to brachistochone curve is consist ent with the experimental observations.   

Sunday, November 20, 2005

Owing to its high surface area, a fluid sheet is not stable. The nucleation of a hole in such a film leads to its bursting. We present experimental results about the bursting of fluid sheets in a viscous atmosphere (as it occurs as drops of water coalesce in a water/oil emulsion). Contrasting with the explosion of a soap film in air, the environment plays a dominant role in the dynamics of the opening of a hole in the sheet. A simple model is provided to explain the observed bursting speeds. We also describe different hydrodynamic instabilities that occur in such a process.   

Sunday, November 20, 2005

By using thin-film asymptotics, the dynamics of a 2D film with Plateau borders can be described. This non-planar flowing state is tested for instability to van der Waals pinching and precise instability conditions are obtained for the film.   

Sunday, November 20, 2005

Pinch-off in conventional 3D droplets and bubbles have been of considerable recent interest. In 3D, pinch-off is driven by surface tension, and various flow regimes have been observed which depend on the viscosity and density ratios of the interior and exterior fluids. In contrast, 2D line tension alone can not drive a film to break apart. Liquid lenses provide an interesting intermediate case in which surface tensions are small and the geometry approaches 2 dimensions. We have developed an experimental system to reproducibly observe pinch-off in hydrocarbon liquid lenses. The shape of the pinch region is qualitatively different than that in 3D, and involves a hierarchy of multiple spontaneous singularities and satellite lenses. High-speed video will be shown which illustrates these phenomena.   

Sunday, November 20, 2005

In our experiment, an interface between a viscous liquid and air is deformed by a sink flow of constant flow rate to form a sharp tip. The interface shape is recorded using a microscope with 1 $\rm{\mu m}$ resolution. The curvature at the tip is controlled by the distance $h$ between the tip and the sink. As a critical distance $h^{\star}$ is approached, the curvature diverges like $1/(h-h^{\star})^3$ and the tip becomes cone-shaped. As the distance to the sink is decreased further, the opening angle of the cone vanishes like $h^2$. No evidence for air entrainment was found, except when the tip was inside the orifice.   

Sunday, November 20, 2005

Recent experiments reveal a novel jetting instability in a micellar fluid system near a second-order phase transition [Casner and Delville, Phys.Rev.\ Lett.\textbf{90} 144503]. When a horizontal interface between the immiscible phases is illuminated by a laser from above at large power, a shape transition occurs. A narrow jet of liquid from the upper layer is propelled into the lower layer, eventually breaking up into droplets. We show the net transport of liquid across the interface results from a flow driven by light scattering off the density fluctuations. When the interface is illuminated from below, the light-driven-flow creates a large-scale hump. We calculate the hump shape and show it agrees well with experimental measurements.   

Sunday, November 20, 2005

The flow of a fluid down the exterior of a vertical, rigid fibre is considered. An evolution equation for the interface in the long-wavelength approximation is derived; this model is similar to those previously used to investigate the dynamics of slender viscous threads in the absence of a fibre. Numerical solutions of this evolution equation reveal the rich interfacial dynamics which manifests itself via the formation of beads that propagate down the fibre undergoing coalescence. Our results demonstrate the existence of different flow regimes depending on system parameters such as the fibre radius, liquid flow rate and physical properties. These are found to be in good agreement with available experimental data as well as those obtained as part of the present work. Connections with models already available in the literature are also established.   

Sunday, November 20, 2005

New modes of striking drop breakup instabilities of a thin film of micro magnetic drops are experimentally observed under a constant perpendicular field. These modes are shown to depend strongly on the initial sizes of drops. Three modes of well-ordered breakup instability patterns are recorded for drop sizes ranging from diameter \textit{d=400$\mu $m} to\textit{ 1,400$\mu $m}. Mode I instability (\textit{d$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $400$\mu $m}) shows a fully evolved central droplet associated with numerous incompletely developed droplets in a formation of evenly distributed waves at a separated outer fluid annulus. These waves at the outer annulus are further destabilized and fully evolve into Mode II instability (\textit{500$\mu $m$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $d$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $1,000$\mu $m)} which forms an additional outer circular array of droplets$.$ A more vigorous Mode III instability with an additional array of droplets in the middle region and smaller secondary droplets in the outer array is observed for a even bigger initial drop size (\textit{1,100$\mu $m$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $d$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $1,400$\mu $m}). For drop sizes \textit{d$\ge $1,500$\mu $m, }a more complex Mode IV instability is recorded, which shows disorderly arrangements of numerous droplets in the middle region and derivative droplets at the outer array.