Controlled structuring of dispersed food systems

Dispersed systems such as emulsions and suspensions are encountered in a large variety of applications areas such as food, cosmetics, pharmaceutics and polymers. To obtain the desired performance of an emulsion the dispersing process, i.e. droplet size distribution has to be monitored and controlled. By combining droplet generation, deformation and fixation in one experimental set-up we aim to produce equally shaped particles in order to manipulate the mircostructure of dispersions. In a first step we study a co-flowing liquid-liquid emulsification process aiming to generate monodispersed droplets. With such droplets at hand, deformation of each individual droplet can be achieved in a second step when the flow field of the downstream apparatus is known. Using gelling biopolymeric material as dispersed phase it is further possible to “freeze” and conserve the non-equilibrium shapes of such deformed droplet. As a consequence, suspensions of gelled emulsion droplets of various shape and shape functionality were obtained. These new shapes (sphere, fibrils, “hooky” bodies) provide an advanced tool to control and manipulate the flow properties and product performance of dispersed systems beside the well-known influence of the volume fraction of the dispersed particles . drop generation drop deformation drop fixation gelling encapsulation shear flow elongational flow Figure 1. Generation, deformation and fixation of a gelling biopolymer droplet toward a shaped suspension particle. INTRODUCTION The goal of this work is the continuous production of tailor-made droplets in size and shape in order to manipulate the microstructure of the dispersion. To achieve this the flow kinetics, fixation kinetics and kinetics at the interface (e.g. surfactant adsorption) have to be coupled (Fig. 1). Flow processes are used to deform liquid emulsion droplets on their way to suspended gelled particles and thus imprinting liquid-liquid deformation onto solid particles. In this investigation drop generation, drop deformation and gel kinetics are combined in a fast continuous flow process to study the impact of shear and elongation forces on drop deformation with and without simultaneous gel formation under high temperature gradients. Drops are generated in a double capillary and deformation and superimposed gel formation Controlled structuring of dispersed food systems Peter Fischer, Carsten Cramer, Erich J. Windhab, Bernhard Walther*, Lars Hamberg*, and Anne-Marie Hermansson* ETH Zurich, Laboratory of Food Process Engineering, Institute of Food Science and Nutrition, 8092 Zürich, Switzerland *SIK – The Swedish Institute for Food and Biotechnology, P.O. Box 5401, 402 29 Göteborg, Sweden occurs along a narrowing deformation channel. Droplet Generation To produce emulsions well-established dispersing devices are commonly used where droplets are subjected to shear and elongational stresses and fragmented into smaller droplets. Generally, the dispersed droplets display a certain size distribution and enormous effort has to be undertaken to achieve narrow size distributions. The demand for almost monodisperse emulsions has been rising due to the production of microcapsules or specially structured multiphase systems. Modern dispersing techniques such as membrane emulsification represent an example of a direct production technique of emulsions where the polydispersity lies in the range of 10% of the average droplet size. In microchannel emulsification, poly-dispersities of droplets below 5% was achieved with the droplet size depending primarily on the capillary size and channel geometry. Another dispersing technique is realized by injecting the disperse phase via a capillary into the continuous phase . It is distinguished into two different drop generation mechanisms: Either the drops break up at the capillary tip (dripping) or they are generated from an extended fluid jet (jetting) 2, . The fundamentals of dripping and jetting have been investigated extensively but these drop formation mechanisms have rarely been considered as a promising dispersing tool with relevance for technical applications. In the present work, the disperse phase is injected via a needle into a flowing ambient continuous phase as shown in Fig. 1 (drop generation). The droplet breakup from the capillary tip is promoted by the drag force of the continuous phase in comparison to the injection into a quiescent surrounding fluid. Thus, the material properties, such as interfacial tension, , viscosities, cont, disp, and density of the fluids are not the only governing parameters but the droplet size, ddrop, is rather controlled by the velocity of the continuous phase, vcont, and the needle diameter, dcap. An integral force balance accounts for the drag force of the flowing ambient fluid and the interfacial tension force (right side) [12]: 3 cont vcont ddrop dcap ( ) = dcap (1) The drag force (left side) was calculated according to a modified version of the Stokes formula for a solid sphere. Even though this model is considered as an approximation, Umbanhowar and co-worker found good agreement between theoretical predictions and experimental data by introducing a fitting parameter. Droplet Deformation In contrast to solid particles, droplets are deformable in flow when they are subject to stresses. The generated stresses depend on the geometry of the flow device, the drop's trajectory in the flow and the strength of the flow. In the present study the droplets are injected eccentrically into a narrowing flow channel. The flow stresses are controlled by the position of the injection point of the droplets into the channel and by the flow rates of the phases. In Fig. 1 (drop deformation) the droplet is deformed in shear flow on a well-defined streamline which is determined by the injection point. As a consequence, all droplets follow the same streamline and experience same stresses. Therefore, the deformation history of all droplets is identical under steady flow conditions along the flow channel. A welldefined, predictable deformation of the droplets is desired to investigate the influence of the particle shape on the product properties. The deformation of the droplet is described most conveniently by the Taylor deformation theory. When a droplet is subjected to a shear-rate, , it deforms from the initial spherical shape into an ellipsoidal one if the deformation is small. After initial transients have decayed, the droplet reaches a steady state deformation which is the result of an equilibrium between viscous forces, which tend to deform the droplet and even break it if they are large enough, and interfacial forces, which attempt to recover the initial state. A dimensionless deformation parameter, D, characterizes the deformation of the droplet as: