Session EJ: Bubbles III

Solid object impact creates supersonic air flow

When an object impacts on a water surface a sizeable cavity is formed below the surface which subsequently collapses due to hydrostatic pressure. The surrounding air is first sucked into the expanding cavity while at a later stage it is squeezed out of the - then shrinking - cavity. We measure this air flow using high-speed imaging of fine smoke particles. Numerical simulations combining a boundary-integral method with a fully compressible Euler solver reveal the intricate structure of the gas dynamics. Despite an impact velocity of merely 1 m/s and a cavity overpressure of only 0.02 atmospheres the air flow is shown to attain supersonic speeds. Consequently, there are significant effects of this air flow close to pinch-off which can be observed consistently in experiment and numerics: (i) the cavity wall is not smoothly curved but exhibits a kink and (ii) the cavity neck is pushed upwards.   

The water entry of decelerating spheres

We present the results of a combined experimental and theoretical investigation of the vertical impact of low-density spheres on a water surface. Particular attention is given to characterizing the sphere dynamics and the influence of its deceleration on the shape of the resulting air cavity. A theoretical model is developed that yields simple expressions for the pinch-off time and depth. Theoretical predictions compare favorably with our experimental observations, and allow us to rationalize the form of water-entry cavities resulting from the impact of buoyant and nearly buoyant spheres.   

Cavitation induced Particle Motion on Surfaces: A Model for Particulate Contamination removal

The motion of adherent polystyrene particles accelerated with a transient cavitation bubble is investigated experimentally. The bubble and particle dynamics is recorded with a high-speed camera. The particle trajectory is studied as a function of the initial separation and the particle diameter. For all particles investigated, i.e. 10, 4.5 and 2 $\mu m$ in diameter, we find the same intriguing dynamics: a short initial separation ($<$0.7 Rmax) leads to a final displacement away from the bubble while particles at larger separations are effectively attracted towards the bubble. All particles follow the same master curve when the initial separation is nondimensionalized with the bubble diameter and the final displacement with the particle diameter. Our comparison of the trajectory to a force balance model indicates that unsteady boundary layers have to be taken into account. We studied the rolling dynamics induced by the strong shear flow and find angular speeds in excess of 400,000 rps for very brief times. We find that both torque and drag are important mechanisms for the removal of particulate contamination in cavitation based cleaning methods.   

Numerical analysis on cavitation inception in the rotary valve of the power steering system

The power steering valve directs the power steering oil to either side of a power piston and relieves the driver of the effort to turn the wheel, when a driver begins to operate the vehicle. It is well known that hiss noise occurring at that moment is caused mainly by cavitation of the oil inside the rotary valve. However, it is very difficult to check the flow pattern and the cavitation inception experimentally because of the complex geometry and high pressure in the interior of the rotary valve. In spite of the increasing interest in this flow phenomenon inside the rotary valve, only few studies have been reported. In the present study, this complex oil flow inside the rotary valve has been analyzed numerically using three-dimensional cavitation model provided by the commercial code, FLUENT 6.2. It is confirmed that the location of cavitation inception is similar to that obtained by the existing two-dimensional numerical analysis. Then, the volume fraction of oil vapor has been compared with the hiss noise level measured experimentally in a semi-anechoic room.   

Probing nanotubes and red blood cells with laser-induced cavitation bubbles

A spatial light modulator (SLM) is used to simultaneously create arrays of laser-induced cavitation bubbles. The different bubble geometries result in the creation of a directed, transient and strong liquid flow. Due to the fast dynamics of the cavitation bubbles the flow is actuated on very short temporal ($\mu $s) and spatial ($\mu $m) scales. We show two examples of the use of laser-induced cavitation bubbles to probe and manipulate small objects: multiwalled carbon nanotubes (MWCNT) and red blood cells (RBCs). In the case of MWCNT, we use a pair of bubbles to displace and bend the nanotubes. By measuring the time it takes for the nanotube to recover its original shape we can estimate the flexural rigidity and the bending modulus. The shape recovery is recorded with a high-speed camera at up to 300,000 frames per second (fps). We found the flexural rigidity to be on the range of 0.98 -- 6.6 $\times $ 10$^{-19}$ Nm$^{2}$ and the Young's modulus on the order of 0.06-0.6 TPa for MWCNT with an average diameter of 117.8 $\pm $ 6.7 nm and a thickness of 4.6 $\pm $ 0.75 nm. A similar approach is used to study the mechanical properties of RBC's, where multiple cells are elongated due to the radial flow induced by a single bubble. We study the shape recovery of the RBCs and find a significant difference when they are treated with an enzyme.   

Bubble Density Functionals and Stochastic Fluid Flow Models

Arbitrary spatial domains containing a stochastic density of gas pressurized bubbles in a fluid material are statistically decomposed such that the probable relative position between two arbitrary spatial points is a time dependent, but path independent, integral functional of fluid material and the bubble density function. Since the relative position vector is an invariant physical quantity, the line integral is stochastically path independent. Using this spatial decomposition vector, arbitrary spatial domains for probable fluid and probable bubble areas and volumes are derived. To address instability in bubble density dependent fluid flows, bubble density evolution in fluids is then formulated with: (1). A Boltzmann bubble density field equation to describe discrete bubble species size(radius) and gas content(atoms) evolution; (2). Relative deformation and velocity functionals of bubble density; (3). Fluid material and bubble gas mass transport functionals of bubble density; and (4). Momentum transport and stress/bubble-pressure functionals of bubble density. These stochastic equations embed finite physical length scales of bubble density fluids.   

Numerical and experimental study of the coefficient of restitution for colliding bubbles

We have studied the motion of bubbles colliding with solid walls both numerically and experimentally. The simulations were performed considering a VOF method of the JADIM code (Bonometti \& Mahnaudet IJMF 2007) that permits to reproduce accurately motion bubbles for a large range of Eotvos and Morton numbers. To be able to compare with the numerical results, the experiments were carried out using silicon oils, for which the interface remains clean under ordinary laboratory conditions. By measuring the approach and rebound velocities, we calculated the coefficient of restitution of the collision, $\epsilon$, which was found to scale as $-\log\epsilon \sim (Ca/St^*)^{1/2}$, as suggested by Zenit and Legendre (PoF, 2009) (where $Ca$ is the capillary number and $St^*$ is a modified Stokes number). Since the numerical results were validated (through direct comparisons with experiments), we conducted a vast parametric study of the coefficient of restitution, varying all the fluid properties in an independent manner. We will discuss these results and their implications in the study of solid/fluid particle collisions in general.   

Enhanced slip on micro-patterned substrates due to contact line depinning

We present numerical simulations of a shear flow over a periodically patterned substrate with entrapped gas bubbles. A diffuse-interface model is employed to handle the liquid--gas interface deformation and the three-phase contact line. Depending on the shear rate and the pattern geometry, four flow regimes are observed. The contact lines can be pinned, depinned or eliminated depending on the competition between the shear force and the surface tension. The effective slip length is found to be dependent on the morphology of the menisci and hence on the shear rate. In particular, the bubbles are transformed into a continuous gas film when the shear rate is larger than a critical value, resulting in a significantly enhanced slip length proportional to the liquid--gas viscosity ratio. The present results have interesting implications for effective slip on superhydrophobic surfaces.   

Generation of mysterious bubbles by irradiation of femtosecond pulses in ultrapure water

The femtosecond pulse laser (fs pulse), which is a high-intensity and ultrashort light pulses, produces the optical nonlinear phenomenon such as multiphoton absorption, and induces the completely new phenomenon which is not obtained by using conventional laser pulses. In this research, fs pulses were irradiated into pure water, and micro bubbles were generated. We conducted time-resolved measurement in order to investigate the process of the bubble generation in detail by using the pump probe method. As a result, it was observed that strong light was emitted in the middle of the region of refractive index changed in the water at 900 ps, and subsequently, a bubble was generated with about 1 ns after the fs pulse irradiation into the water.   

Enhanced Boiling Heat Transfer using Acoustic Interfacial Actuation

Low-power acoustic actuation is used to enhance boiling heat transfer on a submerged surface and inhibit the instabilities that lead to film boiling at the critical heat flux by controlling the formation and evolution of the vapor bubbles. The effects of the acoustic field are investigated using a flat boiling heat transfer surface having a central, isolated hot spot that is designed to control the location, growth, and detachment of a single vapor bubble using a thin hydrophobic coating centered around the hot spot. Specific emphasis is placed on the coupling between the frequency and sound pressure and bubble diameter. It is shown that the acoustic field induces interfacial instabilities that affect the bubbles' contact line with the surface leading to their detachment. In addition to contact line dynamics, the primary and secondary Bjerknes forces play an important role in the detachment and advection of vapor bubbles.