Mechanism drop breakup

Janssen, J.M.H. and Meijen H.E.H. (1993) Drop breakup mechanisms stepwise equilibrium versus transient dispersion. J. Rhetd., 87, 597. 

Inteifacial tension. A high interfacial tension promotes rapid coalescence and generally requires high mechanical agitation to produce small droplets. A low interracial tension allows drop breakup with low agitation intensity but also leads to slow coalescence rates. Interfacial tension usually decreases as solubility and solute concentration increase and falls to zero at the plait point (Fig. 15-10). 

Mechanical compatibilization is accomplished by reducing the size of the dispersed phase. The latter is determined by the balance between drop breakup and coalescence process, which in turn is governed by the type and severity of the stress, interfacial tension between the two phases, and the rheological characteristics of the components [9]. The need to reduce potential energy initiates the agglomeration process, which is less severe if the interfacial tension is small. Addition... 

Bubble and drop breakup is mainly due to shearing in turbulent eddies or in velocity gradients close to the walls. Figure 15.11 shows the breakup of a bubble, and Figure 15.12 shows the breakup of a drop in turbulent flow. The mechanism for breakup in these small surface-tension-dominated fluid particles is initially very similar. They are deformed until the aspect ratio is about 3. The turbulent fluctuations in the flow affect the particles, and at some point one end becomes... 

Illustration Satellite formation in capillary breakup. The distribution of drops produced upon disintegration of a thread at rest is a unique function of the viscosity ratio. Tjahjadi et al. (1992) showed through inspection of experiments and numerical simulations that up to 19 satellite drops between the two larger mother drops could be formed. The number of satellite drops decreased as the viscosity ratio was increased. In low-viscosity systems p < 0(0.1)] the breakup mechanism is self-repeating Every pinch-off results in the formation of a rounded surface and a conical one the conical surface then becomes bulbous and a neck forms near the end, which again pinches off and the process repeats (Fig. 21). There is excellent agreement between numerical simulations and the experimental results (Fig. 21). 

The breakup or bursting of liquid droplets suspended in liquids undergoing shear flow has been studied and observed by many researchers beginning with the classic work of G. I. Taylor in the 1930s. For low viscosity drops, two mechanisms of breakup were identified at critical capillary number values. In the first one, the pointed droplet ends release a stream of smaller droplets termed tip streaming whereas, in the second mechanism the drop breaks into two main fragments and one or more satellite droplets. Strictly inviscid droplets such as gas bubbles were found to be stable at all conditions. It must be recalled, however, that gas bubbles are compressible and soluble, and this may play a role in the relief of hydrodynamic instabilities. The relative stability of gas bubbles in shear flow was confirmed experimentally by Canedo et al. (36). They could stretch a bubble all around the cylinder in a Couette flow apparatus without any signs of breakup. Of course, in a real devolatilizer, the flow is not a steady simple shear flow and bubble breakup is more likely to take place. 

The most efficient mechanism of drop breakup involves its deformation into a fiber followed by the thread disintegration under the influence of capillary forces. Fibrillation occurs in both steady state shear and uniaxial extension. In shear (= rotation + extension) the process is less efficient and limited to low-X region, e.g. X < 2. In irrotatlonal uniaxial extension (in absence of the interphase slip) the phases codeform into threadlike structures. 

Another important application of the front-tracking method is to simulate the drop/bubble formation in flow-focusing devices. Production of mono disperse drops ubbles in microchannels is of fundamental importance for the success of the concept of lab-on-a-chip. It has been shown that flow-focusing can be effectively used for this purpose. Filiz and Muradoglu performed front-tracking simulations in order to understand the physics of the breakup mechanism and effects of the flow parameters on the droplet/ bubble size in the flow-focusing devices [11]. 

When values of the capillarity number and the reduced time are within the region of drop breakup, the mechanism of breakup depends on the viscosity ratio, X. In shear, four regions have been identified [Goldsmith and Mason, 1967] ... 

Following the early work by Thorsen et al., focused on the formation of monodisperse aqueous droplets in an organic carrier fluid performed on a microfluidic chip, and then followed by others works, the breakup mechanism responsible of droplet formation was later analyzed by Garstecki et al. ° showing that when is order of 1 the dominant contribution to the dynamics of breakup at low capillary numbers is not dominated by shear stresses, but it is driven by the pressure drop across the emerging droplet. 

Lin, S. P. Regimes of jet breakup and breakup mechanisms (mathematical aspects). In Recent Advances in Spray Combustion Spray Atomization and Drop Burning Phenomena, Vol. 1, ed. K.K. Kuo. Restrai AIAA Inc. (1996), pp. 137—160. 

S. S. Hwang, Z. Liu, R. D. Reitz Breakup Mechanisms and Drag Coefficients of High-Speed Vaporizing Liquid Drops, Atom. Sprays 6(3), 353-376 (1996). 

Fig. 9.1 Schematic illustration of a drop breakup caused by Kelvin-Helmholtz (KH) or Rayleigh-Taylor (R-T) instabilities. The breakup mechanisms are ciassified with respect to the (increasing) Weber number as bag, stripping (shear) and catastrophic breakup...

A fragmented liquid core is simulated by injecting large drops which break up into smaller and smaller product droplets, until the latter reach a stable condition. The primary breakup, that is, the first drop breakup after injection, is modeled by delaying the initial drop breakup in accordance with experimental correlations. The drop distortion and the breakup criterion are obtained from Taylor s drop oscillator. The properties of the product droplets are derived from principles of population dynamics and are modeled after experimentally observed droplet breakup mechanisms. 

The DPF method [5-7] assumes spray formation is a combination of random and nonrandom processes. An instability analysis is used to describe primary breakup, which is uniquely determined for a given set of initial conditions (fluid physical properties and atomizer parameters) and a model of the breakup mechanism. The drop size distribution arises from fluctuations in the initial conditions due to such factors as gas and liquid turbulence, atomizer passage surface roughness, vortex shedding, liquid mixture composition, etc. 

For K > 2 the drops deform into stable filaments, which only upon reduction of k disintegrate by the capillarity forces into mini-droplets. The deformation and breakup processes require time - in shear flows the reduced time to break is tb > 100- When values of the capillarity number and the reduced time are within the region of drop breakup, the mechanism of breakup depends on the viscosity ratio, A, - in shear flow, when X > 3.8, the drops may deform, but they cannot break. Dispersing in extensional flow field is not subjected to this limitation. Furthermore, for this deformation mode Kcr (being proportional to drop diameter) is significantly smaller than that in shear (Grace 1982). 

Twin-screw extruder In the melting section of CORI extruders, virtually all the degradation mechanisms that can essentially be distinguished, such as quasi-steady drop breakup, folding, end pinching, and decomposition through capillary instabilities, take place in parallel Polente et al. 2001... 

F. Mighri, M.A. Huneault, Drop deformation and breakup mechanisms in viscoelastic model fluid systems and polymer blends. Canad. J. Chem. Eng. 80, 1028-1035 (2002)... 

The critical time for drop breakup also varies with = / cr Furthermore, when k and % are within the drop breakup regions, according to Goldsmith and Mason [70] four mechanisms of drop breakup in shear lead to different blend morphologies ... 

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