Drop breakup interfacial tension

Atomization. A gas or Hquid may be dispersed into another Hquid by the action of shearing or turbulent impact forces that are present in the flow field. The steady-state drop si2e represents a balance between the fluid forces tending to dismpt the drop and the forces of interfacial tension tending to oppose distortion and breakup. When the flow field is laminar the abiHty to disperse is strongly affected by the ratio of viscosities of the two phases. Dispersion, in the sense of droplet formation, does not occur when the viscosity of the dispersed phase significantly exceeds that of the dispersing medium (13). 

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). 

Coalescence The coalescence of droplets can occur whenever two or more droplets collide and remain in contact long enough for the continuous-phase film to become so thin that a hole develops and allows the liquid to become one body. A clean system with a high interfacial tension will generally coalesce quite rapidly. Particulates and polymeric films tend to accumulate at droplet surfaces and reduce the rate of coalescence. This can lead to the ouildup of a rag layer at the liquid-hquid interface in an extractor. Rapid drop breakup and rapid coalescence can significantly enhance the rate of mass transfer between phases. 

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... 

Basic Breakup Modes. Starting from Lenard s investigation of large free-falling drops in still air,12671 drop/droplet breakup has been a subject of extensive theoretical and experimental studies[268] 12851 for a century. Various experimental methods have been developed and used to study droplet breakup, including free fall in towers and stairwells, suspension in vertical wind tunnels keeping droplets stationary, and in shock tubes with supersonic velocities, etc. These theoretical and experimental studies revealed that droplet breakup under the action of aerodynamic forces may occur in various modes, depending on the flow pattern around the droplet, and the physical properties of the gas and liquid involved, i.e., density, viscosity, and interfacial tension. 

Many authors have worked on drop deformation and breakup, beginning with Taylor. In 1934, he published an experimental work [138] in which a unique drop was submitted to a quasi-static deformation. Taylor provided the first experimental evidence that a drop submitted to a quasi-static flow deforms and bursts under well-defined conditions. The drop bursts if the capillary number Ca, defined as the ratio of the shear stress a over the half Laplace pressure (excess of pressure in a drop of radius R. Pl = where yint is the interfacial tension) ... 

Surface tension can be very important in deternhning drop and bubble sizes and shapes. This ultimately controls the size of drops and the breakup of films and drops. The presence of surface active agents that alter the interfacial tension between phases can have enormous influences in multiphase reactors, as does the surface tension of sohds and the wetting between solids and liquids. 

Although the dominant mixing mechanism of an immiscible liquid polymeric system appears to be stretching the dispersed phase into filament and then form droplets by filament breakup, individual small droplet may also break up at Ca 3> Ca. A detailed review of this mechanism is given by Janssen (34). The deformation of a spherical liquid droplet in a homogeneous flow held of another liquid was studied in the classic work of G. I. Taylor (35), who showed that for simple shear flow, a case in which interfacial tension dominates, the drop would deform into a spheroid with its major axis at an angle of 45° to the how, whereas for the viscosity-dominated case, it would deform into a spheroid with its major axis approaching the direction of how (36). Taylor expressed the deformation D as follows... 

Wang CY and Calabrese RV. Drop breakup in turbulent stirred-tank. Part II Relative influence of viscosity and interfacial tension. AIChE J 1986 32 667-676. 

Interception capture refers to the situation where drops are captured on the surface of the rock, in vugs, and in recirculation eddies. Re-entrainment of the captured droplets can occur when a repulsive hydrodynamic force on the droplet is much larger than the van der Waals electrostatic attraction between the droplet and the rock surface. Droplet breakup will occur when interfacial tension is low and hydrodynamic forces are high. 

Hydrocyclone drop diameter >20 pm feed concentration 6 to 60% v/v. Interfacial tension must be >10 mN/m to prevent drop breakup. 

They lower the interfacial tension of liquid interfaces, thereby facilitating bending of the interface, hence deformation and breakup of drops and bubbles. 

To make an emulsion (foam), one needs oil (a gas), water, energy, and surfactant. The energy is needed because the interfacial area between the two phases is enlarged, hence the interfacial free energy of the system increases. The surfactant provides mechanisms to prevent the coalescence of the newly formed drops or bubbles. Moreover it lowers interfacial tension, and hence Laplace pressure [Eq. (10.7)], thereby facilitating breakup of drops or bubbles into smaller ones. 

The surfactant also lowers the interfacial tension, thereby facilitating droplet breakup. The effective y value during breakup depends on surfactant type and concentration and on the rate of transport to the drop surface. Approximate equations are available for this rate, and also for the stresses acting on a drop, the drop size resulting from breakup, and the frequency at which the drops encounter each other. 

Tokita [1977] calculated the total number of collisions per unit volume and time. The author assumed that coalescence is proportional to it and to the number of particles. The latter was assumed to increase with mixing time, being proportional to the shearing energy, and inversely proportional to the interfacial tension coefficient, Vj2. At equilibrium, the rates of coalescence and breakup are equal. Thus, the equilibrium drop size can be expressed as ... 

The microrheology makes it possible to expect that (i) The drop size is influenced by the following variables viscosity and elasticity ratios, dynamic interfacial tension coefficient, critical capillarity number, composition, flow field type, and flow field intensity (ii) In Newtonian liquid systems subjected to a simple shear field, the drop breaks the easiest when the viscosity ratio falls within the range 0.3 < A- < 1.5, while drops having A- > 3.8 can not be broken in shear (iii) The droplet breakup is easier in elongational flow fields than in shear flow fields the relative efficiency of the elongational field dramatically increases for large values of A, > 1 (iv) Drop deformation and breakup in viscoelastic systems seems to be more difficult than that observed for Newtonian systems (v) When the concentration of the minor phase exceeds a critical value, ( ) >( ) = 0.005, the effect of coalescence must be taken into account (vi) Even when the theoretical predictions of droplet deformation and breakup... 

Tokita [1977] suggested that the drop diameter in polymer blends originates from the two competitive processes continuous breakup and coalescence of the dispersed particles. The equUibrium drop diameter should increase with concentration, number of drops, and the interfacial tension coefficient, but decrease with shear stress, <5. The dependence qualitatively agrees with experiments [Liang et al., 1983 White and Min, 1985 Willis et al., 1991]. 

The reactor vessel is usually a stirred tank. The monomer phase is subjected either to turbulent pressure fluctuations or to viscous shear forces, which break it into small droplets that assume a spherical shape under the influence of interfacial tension. These droplets undergo constant collisions (collision rate >1 s ), with some of the collisions resulting in coalescence. Eventually, a dynamic equilibrium is established, leading to a stationary mean particle size. Individual drops do not retain their unique identity, but undergo continuous breakup and coalescence instead. In some cases, an appropriate dispersant can be used to induce the formation of a protective Aim on the droplet surface. As a result, pairs of clusters of drops that tend to coalesce are broken up by the action of the stirrer before the critical coalescence period elapses. A stable state is ultimately reached in which individual drops maintain their identities over prolonged periods of time [247]. 

Some authors report the next guide principles that may be applied for blend morphology after processing, (i) Drops with viscosity ratios higher than 3.5 cannot be dispersed in shear but can be in extension flow instead, (ii) The larger the interfacial tension coefficient, the less the droplets will deform, (iii) The time necessary to break up a droplet (Tj,) and the critical capillary number (Ca ) are two important parameters describing the breakup process, (iv) The effect of coalescence must be considered even for relatively low concentrations of the dispersed phase. 

The results of Esselink et al. [16] also shed some light on the solubilization of oils in micelles and indicate three mechanisms of transfer of solute molecules from an oil phase into micelles. In one, solubilization results from the adsorption of surfactants on oil drops, which lowers the interfacial tension and causes the breakup of the oil drop into smaller droplets. In the second, the solute molecules that leave the drops are trapped by micelles in the immediate neighborhood of the drops. The last mechanism represents the exchange of surfactant and solute during a micelle-oil drop collision. The results of Esselink et al. also demonstrate quantitatively that the shorter solute molecule ci is solubilized to a greater extent (due to its higher solubility in the solvent) than the longer C3. 

The occurrence of these two minima may be interpreted by analyzing the variation of the interfacial tension (Fig. 14, left) and emulsion stability (Fig. 14, center) along the same formulation scan (81,82). The difference between the two variations is essentially due to the fact that the tension variation takes place over a much wider temperature interval than the -stabiiity change. The combination of these two factors that have opposite effects on the drop size generates the shown variation. When optimum formulation is approached (from any side), the first effect to be fell is the tension reduction, which makes breakup easier with a resulting drop size decrease. Then, when the rapid reduction in emulsion stability takes place, the coalescence rate increases very quickly and the trend is reversed to produce larger drops. Consequently, the minimum size drop is not attained at optimum formulation, where the tension exhibits its lowest value, but at some distance" from it, where the best compromise between low tension and not too... 

---