Concentric droplet separation-layer

P 5] Layers of 4,4 -bipyridyl (0.3 mol 1 in dichloromethane), ethyl bromoacetate (0.3 mol in dichloromethane) and a separation layer of dichloromethane were fitted into each other by means of a concentric separation mixer (three-fluid nozzle with three tubes having diameters of 1.5, 3 and 4 mm, slotted into each other) [78]. Thereby, two circular liquid layers of a thickness of 200 pm and a center stream of 1.5 mm diameter were generated. The reaction temperature was 22 °C. The reaction solution was inserted as droplets or a continuous stream either directly or via... 

To fractionate a polymer by precipitation, a precipitant is slowly added to the polymer solution (concentration of polymer 0.1-1 wt%) at constant temperature until a persistent cloudiness appears. After some time the droplets separate as a second liquid (or swollen gel) phase. This fraction contains the highest molecular weight components and is separated by decantation or centrifugation. Further precipitant is then added to the majority layer until further phase separation is observed. Then the above procedure is repeated several times until all polymer is separated off. A disadvantage of fractional precipitation is that the residual solutions become more and more dilute so that separation of the late fractions might be difficult. Furthermore, the method is rather time consuming since the formation of the gel phase occurs very slowly. 

Another process which leads to HIPE instability is gravitational syneresis, or creaming, where the continuous phase drains from the thin films as a result of density differences between the phases. This produces a separated layer of bulk continuous phase and a more concentrated emulsion phase. The separated liquid can be located either above or below the emulsion, depending on whether the continuous phase is more or less dense, respectively, than the dispersed phase. This process has been studied by Princen [111] who suggests that it can be reduced by a number of parameters, including a high internal phase volume, small droplet sizes, a high interfacial tension and a small density difference between phases. 

P 49] CFD simulations were made for monitoring the flow patterns within a droplet which is generated at a concentric separation-layer micro mixer [39], Diffusion-convection equations of two user scalars have to be solved in addition to the corresponding equation for the volume fraction of the fluids within a multiphase CFD simulation. 

Layers of calcium nitrate (40 mmol T1 in water CaN03 4 H20), potassium carbonate (40 mmol 1 1 in water) and a separation layer of water are fitted into each other by means of the concentric separation mixer [53], The reaction temperature is 22 °C. The reaction solution forms droplets in a dodecane reservoir and inserted as such a segmented flow in the tubular reactor [137, 138],... 

M 54] [P 48] Separation-layer micro mixers with concentric multi-layered outlets can be operated in a droplet-forming mode [53] If fast precipitating solutions are contacted in this way with a solvent layer for initial separation, the part of the droplet close to the tube outlets remains transparent, which demonstrates that a tri-layered system still exists with the two reacting solutions not being intermixed, as evidenced by calcium carbonate formation in aqueous solutions as described in [39,136], At the droplet end cap the layers collide and circulation flow sets in. As a result, mixing is achieved and precipitation occurs. The circulation patterns are visualized by the particle trajectories. 

Figure 1.122 Photographs documenting the extent and location of calcium carbonate precipitation in droplets generated by the concentric separation layer mixer. The photos were rotated by 90° to allow a better comparison ([53] source IMM).

The creaming process, in which emulsion droplets rise or settle without significant coalescence, is not, by itself, emulsion breaking. Creaming produces two separate layers of emulsion that have different droplet concentrations and are usually distinguishable from each other by colour or opacity. Creaming does promote coa-... 

Schematically, the structure of the a-gel state is given in Figure 9.2, which can be deduced from the photographs (Figures 9.7 and 9.8) in the reference of Heertje et al. (1998). The structure consists of a continuous phase of bilayers, which are stacked parallel to one another in regions with a speeifie domain size, and of droplets being embedded in this continuous phase. The droplets consist of bilayers that are stacked concentrically, like the layers in an onion, and are separated by water layers. For obvious reasons these droplets are sometimes denoted by the term onions.

Emulsions were prepared with the Shell crude and distilled water with sodium hydroxide concentrations varying from 1.0 x 10 to 8.0 x 10 moles NaOH/gram oil. The emulsions were very stable and no separation into two phases occurred when NaOH concentration was above 4.0 x 10 At 4.0 x 10 J and lower NaOH concentrations, some separation occurred after one or two days but even at 3.0 x 10 moderate stability was observed. Microscopic examination showed that the top layer contained a high fraction of oil with large particle sizes predominating and the bottom layer contained a high fraction of water with the small particle size oil droplets predominating. 

The most widely studied deformable systems are emulsions. These can come in many forms, with oil in water (O/W) and water in oil (W/O) the most commonly encountered. However, there are multiple emulsions where oil or water droplets become trapped inside another drop such that they are W/O/W or O/W/O. Silicone oils can become incompatible at certain molecular weights and with different chemical substitutions and this can lead to oil in oil emulsions O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics and foodstuffs the droplets are in contact and deform. Volume fractions in excess of 0.90 can be achieved. The drops are separated by thin surfactant films. Selfbodied systems are multicomponent systems in which the dispersion is a mixture of droplets and precipitated organic species such as a long chain alcohol. The solids can form part of the stabilising layer - these are called Pickering emulsions. 

When a biopolymer mixture is either close to phase separation or lies in the composition space of liquid-liquid coexistence (see Figure 7.6a), the effect of thermodynamically unfavourable interactions is to induce biopolymer multilayer formation at the oil-water interface, as observed for the case of legumin + dextran (Dickinson and Semenova, 1992 Tsapkina et al, 1992). Figure 7.6b shows that there are three concentration regions describing the protein adsorption onto the emulsion droplets. The first one (Cprotein< 0.6 wt%) corresponds to incomplete saturation of the protein adsorption layer. The second concentration region (0.6 wt% < 6 proiem < 6 wt%) represents protein monolayer adsorption (T 2 mg m 2). And the third region (Cprotein > 6 wt%) relates to formation of adsorbed protein multilayers on the emulsion droplets. 

Let us consider a spherical entity (particle, bubble, droplet) surrounded by a fluid medium (gas, liquid). The fluid and the sphere are separated by a fluid-side concentric boundary layer of thickness 8 (Fig. 19.16). The concentration at the surface of the sphere is Cs the fluid concentration outside the boundary layer is CF. The equilibrium relation between S and F is defined by a partition coefficient KSfF = Cs/ CFq (Eq. 19-16). 

The lamellae in a foam contain two gas/liquid interfaces separated by a layer of fluid, thin film, each interface having a surface tension. For this reason the term film tension is sometimes used, the film tension being equal to twice the surface tension of the surfaces. It should be noted that, while the film tension is twice the surface tension of the surfaces, this is not necessarily the same as twice the surface tension of the bulk solution. In fact, the surface tension of a fluid film surface is similar to that of the bulk solution when the fluid film is thick, but departs from the bulk solution value as the fluid film thins. The situation is similar for the thin films between droplets in a concentrated emulsion. 

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