Drop deformation dynamics and gel kinetics in a co-flowing water-in-oil system.

Drop deformation and superimposed gel kinetics were studied in a fast continuous-flow process for a water-in-oil system. Highly monodisperse drops were generated in a double capillary and then deformed passing through a narrowing rectangular channel geometry. Nongelling deformation experiments were used to establish the process and compare it with existing theories. Thereafter, temperature induced drop gelation was included to study its effect on deformation and gel kinetics on short timescales and at high temperature gradients. The disperse phase was a kappa-carrageenan solution with additional sodium and potassium ions for gelation experiments. Sunflower oil was used for the continuous phases. Nongelling experiments showed that shear forces are able to deform drops into ellipsoids. A comparison with the small deformation theory by Taylor was surprisingly good even when drop deformation and flow conditions were not in steady state. Superimposed gelation on the deformation process showed clearly the impact of the altered rheological properties of the dispersed and continuous phase. Deformation first increased on cooling the continuous phase until the onset of gel formation, where a pronounced decrease in deformation due to increasing droplet viscosity/viscoelasticity was observed. Drop deformation analyses were then used to detect differences in gelation kinetics at high cooling rate within process times as short as 1.8 s.

Formation of shaped drops in a fast continuous flow process.

Drop shaping, i.e., flow-induced deformation and fixation by gel formation, was studied under dynamic conditions in a fast continuous process for a water-in-oil system. The system consisted of sunflower oil with different surfactant concentrations (0.1-2% Admul Wol) and a 1.5% kappa-carrageenan solution with different Na(+) and K(+) concentrations. The continuous phase flowed in a 10-mm-wide straight channel into which the dispersed phase was injected via a thin needle. A subsequent shaping channel with a width of 1 or 2 mm deformed the drops. Gel formation was induced by a temperature gradient between the continuous and dispersed phase. Drop sizes in the range 220-roughly 1000 microm were produced at the needle tip by varying the ratio between the oil and carrageenan flow rate. A diffusion zone before the narrow channel allowed the surfactant to adsorb at the interface. In the elongation flow at the entrance of the shaping geometry, drops underwent initial elongation. In the narrow channel, the drops developed a parabolic shape within a residence time of 0.03-0.15 s. Choosing the correct parameter combinations made it possible to fix the deformation by gel formation within this time period. Shaped drops were shown to be functional. At a concentration of 25% in an emulsion, they increased the viscosity by about 15-20% compared to spherical drops even though 45% of the shaped drops had an aspect ratio of less than 1.2.

Shaping of gelling biopolymer drops in an elongation flow.

Shaping, defined as deformation in combination with gel formation of gelatine and kappa-carrageenan drops in an elongation flow, was studied. The focus was to investigate the possibility of shaping and fixating small drops in the diameter range 20 to 229 mum. In the shaping progress and the influence of experimental properties, the viscosity, temperature, and flow of the deforming fluid were examined on the final drop shape. In the experiments a hot emulsion of an aqueous biopolymer solution in silicone oil was injected into cold silicone oil where a deforming elongation flow field existed. After injection, a temperature decrease in the drops resulted in a gel formation of the biopolymer and a fixation of the deformed drop in the flow. The shape was measured and the effect on the drop aspect ratio was determined by image analysis. Over the total drop diameter range, kappa-carrageenan was more ellipsoid-shaped than gelatine, with a maximum aspect ratio of 6 compared to 4 for gelatine. For small drops, around 22 mum, it is possible to shape kappa-carrageenan, but for gelatine small drops tend to be unaffected. An increase in viscosity, temperature, and flow resulted in an increase in the final fixated shape of the drops. The differences in drop deformation between the biopolymers were explained by drop-viscosity/oil differences and differences in the kinetics of gel formation. The different gel formation kinetics resulted in a short, well-defined, shaping process for kappa-carrageenan, while for gelatine the process was more complex, with both deformation and relaxation present at different stages.