Untersuchungen zum Formverhalten mikroskopisch kleiner Fluidtropfen in stationaren und instationaren Scherstromungen

Description

In emulsion systems there is a deformable interface between the dispersed droplets and the continuous phase. The droplets are deformed by the stresses acting in fluid flow. High stresses lead to droplet break up.The extent and the kinetics of droplet deformation and break up depend on the physical properties of the disperse and continuous phase fluids and the interface. The physical properties which influence deformation behaviour are the rheological behaviour of both phase fluids and the dynamic behaviour of the interface. Break up of fluid droplets is the aim of any emulsification process. In colloid mills or toothed-discs dispersing machines, emulsions are created by the transfer of "break up stresses" from the flowing continuous phase to the droplet interface. Process steps which follow the emulsification process often should not lead to droplet break up.In this work the relationship between time dependent deformation behaviour of a single fluid droplet under planar steady shear flow conditions, as well as relaxation behaviour, was studied. For the experimental study "optical flow devices" were constructed which allow direct observation of the deformation and break up behaviour of single droplets. The droplet shape can be measured using image analysing techniques. Deformation studies were carried out using Newtonian continuous phase fluids. The rheological behaviour of the disperse phase fluid was varied (Newtonian, structure viscous). The structure of the interface between the droplet and the continuous phase were influenced by addition of interface active agents (emulsifiers) to the disperse phase fluid. Different molecular structure of the interface leads to a different interfacial tension behaviour. Consequently the deformation and break up behaviour of the droplets change. In experimental studies it was found that besides the interfacial tension other interfacial properties influence the deformation and break up behaviour of fluid droplets.The shear flow conditions causing droplet deformation cannot be characterised only by the shear stress acting in the continuous phase regardless of local changes near the droplet. Besides shear stresses, normal stresses act at the droplet interface. The normal stresses in particular seem to be responsible for different deformation behaviour of droplets in continuous phase fluids with different Newtonian viscosities under equal "macroscopic" shear stresses (shear stress in unaffected continuous phase flow).The deformation behaviour of droplets under planar steady shear flow conditions, as well as relaxation behaviour were modelled theoretically with a "rheological model" using a combination of springs and dash pots. In the modelling, the rheological properties of the disperse phase fluid as well as the dynamical deformation behaviour of the interface were taken into account. The deformation behaviour of the interface was described with newly introduced interfacial rheological parameters. Coupling the solution of the equation of motion with the shear energy balance leads to an equation for the time dependent deformation and relaxation behaviour of the droplets. By comparing the model results with the available experimental results the interfacial model parameters were quantified for the fluid systems studied.Experimental studies on the break up behaviour of fluid droplets under shear flow conditions illustrated the different forms of droplet break up which are known from the literature. An influence of the deformation time on the break up behaviour was observed.Using the mechanism of droplet deformation in steady shear flow, a process to create "shape-fixed emulsions" was constructed. "Shape-fixed emulsions" are emulsions whose droplets were fixed (became rigid) in steady shear flow. The emulsion particles are flow-induced structured. As a consequence, the viscosity of these emulsions is reduced in comparison with "spherical emulsions".