Thin Liquid Shell Research Articles

Large-scale production of compound bubbles using parallelized microfluidics for efficient extraction of metal ions.

Recent advances in microfluidic technologies have enabled production of micro-scale compound bubbles that consist of gaseous cores surrounded by thin liquid shells, achieving control and uniformity not possible using conventional techniques. These compound bubbles have demonstrated enormous utility as functional materials for drug delivery, as ultra-lightweight structural materials, as engineered acoustic materials, and also as separating agents for extraction of metal ions from waste fluid streams. Despite these successful demonstrations, compound bubbles have largely remained at the laboratory-scale due to the slow production rates endemic to microfluidics (<10 mL h-1). Although parallelization approaches have enabled large-scale production of simple emulsions and bubbles, its application to the production of higher order dispersions such as compound bubbles has been limited because the optimal processing window for the production of uniform compound bubbles is relatively narrow and the required channel geometry is quite complex. In this report, we demonstrate the parallelization of multi-stage flow focusing droplet generators that produce compound ternary bubbles. We parallelize 400 multi-stage FFG devices, generating up to 3 L (∼1011 bubbles) of monodispersed (CV < 5%) compound bubbles in less than 1 hour. We show that it is critical to use multi-height channels and operate each individual generator in a flow regime that is minimally sensitive to variations in the flow rate to reliably produce uniform compound bubbles. To demonstrate the utility of our parallelized device, we take advantage of the buoyancy and the high mass transfer rate that comes from the thin shells of gas-in-oil-in-water compound bubbles to rapidly extract Nd ions from a model waste stream.

The morphologies of Lennard-Jones liquid encapsulated by carbon nanotubes

Abstract Using molecular dynamics simulations, we studied the morphologies of Lennard-Jones liquid encapsulated in carbon nanotubes (CNTs) for a wide range of liquid–CNT interaction, system size and temperature. The morphology of liquid is found to be sensitive to the filling ratio of liquid (a ratio of liquid volume to the available volume of CNT pore) and the liquid–CNT interaction. The ‘phase diagram’, namely by the morphologies versus the liquid–CNT interaction and the filling ratio, is obtained. In most cases, the liquid inside CNTs forms a thin liquid shell attached to a carbon wall when the filling ratio is small. With the increasing of the filling ratio, liquid tends to form droplet. As the filling ratio increases further, liquids form a cylinder with finite length. Finally, the whole inner space of CNT was filled with liquid when the filling ratio is large enough. Current studies could shed light on the adsorption and flow of liquid inside CNTs.

Mechanistic investigation of the sonochemical synthesis of zinc ferrite

In this investigation, an attempt has been made to establish the physical mechanism of sonochemical synthesis of zinc ferrite with concurrent analysis of experimental results and simulations of cavitation bubble dynamics. Experiments have been conducted with mechanical stirring as well as under ultrasound irradiation with variation of pH and the static pressure of the reaction medium. Results of this study reveal that physical effects produced by transient cavitation bubbles play a crucial role in the chemical synthesis. Generation of high amplitude shock waves by transient cavitation bubbles manifest their effect through in situ micro-calcination of metal oxide particles (which are generated through thermal hydrolysis of metal acetates) due to energetic collisions between them. Micro-calcination of oxide particles can also occur in the thin liquid shell surrounding bubble interface, which gets heated up during transient collapse of bubbles. The sonochemical effect of production of OH radicals and H2O2, in itself, is not able to yield ferrite. Moreover, as the in situ micro-calcination involves very small number of particles or even individual particles (as in intra-particle collisions), the agglomeration between resulting ferrite particles is negligible (as compared to external calcination in convention route), leading to ferrite particles of smaller size (6nm).

Visualization of low Reynolds boundary-driven cavity flows in thin liquid shells

Classic examples of low-Reynolds recirculating cavity flows are typically generated from lid-driven boundary motion at a solid–fluid interface, or alternatively may result from shear flow over cavity openings. Here, we are interested in an original family of boundary-driven cavity flows occurring, in contrast to classic setups, at fluid–fluid interfaces. Particle image velocimetry (PIV) is used to investigate the structure of internal convective flows observed in thin liquid shells. Under the specific configuration investigated, the soap bubble’s liquid shell is in fact in motion and exhibits sporadic local “bursts”. These bursts induce transient flow motion within the cavity of order Re ∼ O(1). The combination of PIV and proper orthogonal decomposition (POD) is used to extract dominant flow structures present within bubble cavities. Next, we show that thermally induced Marangoni flows in the liquid shell can lead to forced, (quasi) steady-state, internal recirculating flows. The present findings illustrate a novel example of low-Reynolds boundary-driven cavity flows.

Low‐Reynolds boundary driven cavity flows in a thin liquid shell

AbstractLow‐Reynolds recirculating cavity flows are traditionally generated from lid‐driven boundary motion at a solid‐fluid interface or result from shear flow over an opening. Such flows are typically described by the equations of creeping motion, where viscous forces are dominant. We illustrate using Particle Image Velocimetry (PIV) an original family of boundary‐driven cavity flows occurring, in contrast to classic configurations, at a liquid‐gas interface: thermally‐induced Marangoni flows in a thin liquid shell generate forced, steady‐state recirculating flows inside the cavity. Forcing relies on viscous mechanisms at the boundary but resulting flow patterns are, however, inviscid. Here, the inviscid equations of fluid motion are not used as an approximation, but rather come as a result from the solution of the creeping motion equations in the region inside the sphere. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Keeping One's Head

CHEMISTRY Foam consists of a mass of bubbles—air trapped within thin liquid shells—that forms on agitation, such as when beer is poured into a glass. The stability of a foam depends on the nature of the liquid: evanescent in some cases, hardier in others because of additives that extend their

Pulsed laser evaporation: equation-of-state effects

Theoretical study of laser ablation is usually based on the assumption that the vapor is an ideal gas. Its flow is described by gas dynamics equations [1, 2]. The boundary conditions at vaporization front are derived from the solution of the Boltzmann equation that describes the vapor flow in the immediate vicinity of the vaporizing surface (so-called Knudsen layer) [1]. This model is applicable within the range of temperatures much lower than the critical temperature of target material. In the present work, a general case is considered when the temperature of the condensed phase is comparable to or higher than the critical temperature. The dynamics of both condensed and gaseous phases can be described in this case by the equations of hydrodynamics. The dynamics of vaporization of a metal heated by an ultrashort laser pulse is studied both analytically and numerically. The analysis reveals that the flow consists of two domains: thin liquid shell moving with constant velocity, and thick low-density layer of material in two-phase state.

Expansion of matter heated by an ultrashort laser pulse

Recent experiments have utilizied high-power subpicosecond laser pulses to effect the ultrafast heating of a condensed material to temperatures far above the critical temperature. Using optical diagnostics it was established that a complicated density profile with sharp gradients, differing substantially from an ordinary rarefaction wave, forms in the expanding heated matter. The present letter is devoted to the analysis of the expansion of matter under the conditions of the experiments reported by D. von der Linde, K. Sokolowski-Tinten, and J. Bialkowski, Appl. Surf. Science 109/110, 1 (1996); K. Sokolowski-Tinten, J. Bialkowski, A. Cavalleri et al., Proc. Soc. Photo-Opt. Instum. Eng. 3343, 46 (1998); and, K. Sokolowski-Tinten, J. Bialkowski, A. Cavalleri et al., Phys. Rev. Lett. 81, 224 (1998). It is shown that if the unloading adiabat passes through the two-phase region, a thin liquid shell filled with low-density two-phase matter forms in the expanding material. The shell moves with a constant velocity. The velocity in the two-phase material is a linear function of the coordinate (flow with uniform deformation), and the density is independent of the coordinate and decreases with time as t−1.

Solitons in disperse systems with electric double layers

An investigation is made of the behavior of disperse systems with electric double layers in external time-dependent electric fields, taking into account the interaction between the moving concentrated charges in the shells of the neighboring grains. It is shown that an external electromagnetic excitation at ultralow frequencies can be transmitted to the bulk of a disperse system in the form of solitary waves. It is proposed that the real observation of these waves is possible, for example, in disperse systems with nonconducting spherical inclusions having thin liquid shells and suspended in a solid insulator.

Balance equations for fluid lines, sheets, filaments and membranes

Fluid structures can be classified into two categories: fluid filaments and fluid membranes. If the cross section of a fluid filament vanishes, the fluid filament is called a fluid line, whereas if the thickness of a fluid membrane vanishes, the fluid membrane is called a fluid sheet. In this paper, the local instantaneous balance equations are derived for a line and a sheet of fluid moving in a three-dimensional geometrical space. For a filament, the balance equations are obtained by using quantities averaged over the cross section of the filament. For a membrane, the balance equations are obtained by using quantities averaged over the height of the membrane. It is shown that the local and averaged formulations are consistent. All these balance equations have been rigorously derived by applying original mathematical theorems given in the appendices to the paper. Such balance equations can be used to model single-phase flow in bends or coils, the dynamic centering of thin liquid shells in capillary oscillations and the instability of an annular jet.

Shape oscillations of bubbles driven by modulated ultrasonic radiation pressure: The complex frequency response.

The shape of a bubble in water should change in response to the radiation pressure of an ultrasonic wave. Modulation of the radiation pressure at the resonance frequencies for shape oscillations should facilitate the stable excitation of such modes [P. L. Marston, J. Acoust. Soc. Am. 67, 15–26 (1980)]. Levitation and detection of shape oscillations were previously discussed for bubbles in the size range 1–5 mm diameter [P. L. Marston etal., J. Acoust. Soc. Am. 89, 1885(A) (1991)]. Levitator design improvements have allowed stable trapping and shape oscillation detection for bubbles in the size range 1–12 mm diameter. The quadrupole mode was observed with resonance frequencies in the range 400–15 Hz. Oscillations over a more limited range were studied with a laser light scattering method that allowed the response phase and amplitude to be measured. The phase is referenced to the amplitude modulation of the acoustic field. Further experiments have involved the effects of surfactants, levitation of thin liquid shells, splitting of bubbles, and surface instabilities. [Research supported by ONR and NASA.]

The centring dynamics of a thin liquid shell in capillary oscillations

The physical mechanism governing the centring of a hollow liquid shell in capillary oscillations, which has been observed in experiments, is investigated theoretically. First, the shell is assumed to be inviscid and to have a thickness that is much less than its spherical radius. A system of one-dimensional nonlinear equations of motion is derived using a thin-sheet model. From a numerical study the nonlinear effects of the wave are found to cause the core to oscillate slowly relative to the shell while the centre of mass of the whole system remains stationary. The effects of small viscosity are then considered in an approximation. Finally the strength of the centring mechanism is compared with that of the decentring effect due to buoyancy. The findings are consistent with the limited experimental information available.