Erich J. Windhab

Stress- and strain-controlled measurements of interfacial shear viscosity and viscoelasticity at liquid/liquid and gas/liquid interfaces

An interfacial rheometer for both stress- and strain-controlled measurements of shear rheological properties at liquid/liquid and gas/liquid interfaces is presented. The device is based on a rotating or oscillating biconical bob design in combination with a low friction electronically commutated motor system. The interfacial shear stress, viscosity, and dynamic moduli are obtained by solving the Stokes equations (low Reynolds number) along with the Boussinesq–Scriven interfacial stress tensor, which is used for the boundary conditions at the interface. An improved and simple numerical method for the calculation of the velocity distribution in the measuring cell is presented. The scope and limitations of the rheometer are discussed. Results from steady shear and oscillatory experiments as well as creep recovery and stress relaxation tests at both oil/water and air/water interfaces with adsorbed films of a globular protein (ovalbumin) and spread films of a surfactant (sorbitan tristearate) are presented.

Flow characteristics and modelling of foam generation in a continuous rotor/stator mixer

Within this study, the Newton/Reynolds characteristics of a continuous rotor/ stator mixer for foam production were determined with special geometrically adapted Newton and Reynolds numbers. A critical Reynolds number of 105 was derived. Equations for the laminar and turbulent flow field for pre-calculating the maximum bubble size in rotor/stator mixers were derived and experimentally confirmed. These equations combine process, recipe and geometry parameters and a direct scale-up is possible. An increase of the rotor speed from 600 to 2000 rpm resulted in a decrease in bubble size of about a factor of 4.5 for a low viscous liquid, whereas for a highly viscous liquid, the decrease in bubble size in the same rotor speed range was only 20%. Drainage stability improvement of about 100% and an increase in foam viscosity of about 100% were achieved in the same rpm range.

Influence of emulsifiers on ice cream produced by conventional freezing and low-temperature extrusion processing

Ice cream at six different levels of emulsification was produced by freezing in a conventional scraped surface freezer and with a serial configuration of a conventional freezer followed by a low-temperature extruder. The aim was to examine the influence of emulsifiers on the process, since both emulsifier addition and low-temperature extrusion may have similar effects on promotion of colloidal structure in ice cream. Ice cream samples from both processes were analyzed for stiffness at draw, fat destabilization indices, melting performance, and microstructure. Low-temperature extrusion generally promoted enhanced fat destabilization, however, fat particle size and solvent extractable fat showed different dependencies on emulsification level from the two processing systems. Although solvent extractable fat reached high levels with increasing emulsification, fat particle size data suggested that fat agglomerate size was controlled by mechanical shearing. Significant difference between the two systems was seen also in the meltdown test, where melting rates for unemulsified and slightly emulsified mixes led to a very low melting rate and high shape retention in extruded ice cream. Scanning electron microscopy detected generally smaller air bubbles for extruded ice creams. Enhanced fat structuring around the air bubble and into the serum phase was also shown for unemulsified extruded samples.

Deformation of single emulsion drops covered with a viscoelastic adsorbed protein layer in simple shear flow

The small-deformation behavior of single Newtonian oil drops covered by an adsorbed viscoelastic protein layer is investigated in simple shear flow. Adsorption and network formation of the protein (lysozyme) at the oil/water interface are tracked by interfacial rheology and tension. While uncovered drops deform to the expected steady ellipsoidal shape, protein-covered drops are able to resist the bulk shear stress to a much higher degree, leading to a smaller average deformation and oscillating drop shapes. The results show direct evidence for a commanding role of in-plane interfacial stresses of a viscoelastic protein network on the macroscopic drop deformation as opposed to the equilibrium interfacial tension.

Comparison of precrystallization of chocolate

Abstract Two continuous processes for the precrystallization of chocolate were investigated. An Aasted temper AMT 250 is compared to a newly developed high shear crystallizer. The main features of the high shear crystallizer are the compact design, the improved heat and mass transfer, and the highly efficient homogeneous energy dissipation. Starting from the same chocolate recipes, the differences in precrystallization between the conventional and the high shear crystallization technique are compared. Several analytical methods were used for comparison of the chocolate at process outlet and storage temperatures. In the liquid state, viscosity function and temper curves were established. The final chocolate bars, after moulding and cooling, were compared by calorimetric and color measurements. The comparison of the two processes shows that with a high shear crystallizer the same chocolate crystallization quality can be achieved at a significantly reduced residence time and lower outlet temperature. Furthermore, the new process can be easily adapted to different chocolate recipes by changing the rotational speed. Due to the improved secondary nucleation in shear flow, even lower temper values produce bloom stable chocolates.

Simultaneous control of pH and ionic strength during interfacial rheology of β-lactoglobulin fibrils adsorbed at liquid/liquid Interfaces.

Proteins can aggregate as amyloid fibrils under denaturing and destabilizing conditions such as low pH (2) and high temperature (90 °C). Fibrils of β-lactoglobulin are surface active and form adsorption layers at fluid-fluid interfaces. In this study, β-lactoglobulin fibrils were adsorbed at the oil-water interface at pH 2. A shear rheometer with a bicone geometry set up was modified to allow subphase exchange without disrupting the interface, enabling the investigation of rheological properties after adsorption of the fibrils, as a function of time, different pH, and ionic strength conditions. It is shown that an increase in pH (2 to 6) leads to an increase of both the interfacial storage and loss moduli. At the isoelectric point (pH 5-6) of β-lactoglobulin fibrils, the maximum storage and loss moduli are reached. Beyond the isoelectric point, by further increasing the pH, a decrease in viscoelastic properties can be observed. Amplitude sweeps at different pH reveal a weak strain overshoot around the isoelectric point. With increasing ionic strength, the moduli increase without a strain overshoot. The method developed in this study allows in situ subphase exchange during interfacial rheological measurements and the investigation of interfacial ordering.

A numerical procedure for calculating droplet deformation in dispersing flows and experimental verification

Abstract A three-step numerical procedure for studying droplet deformation in mixed, dispersing-type, flow fields is described. Finite element and numerical particle tracking techniques are used to obtain the history of shear and elongation rates along a particle trajectory in the flow field, and from this history, boundary integral techniques are used to determine the deformation a drop would experience along this path. This approach is then used to investigate the effect of a small change in geometry on the breakup behavior of drops in the annular gap flow between two eccentric cylinders. This flow geometry serves as an idealization of a rotor–stator dispersing device used for highly viscous fluid systems. It is found that an increase in eccentricity produces an increase in dispersing capability. Experiments in an eccentric cylinder geometry were performed to verify the simulation procedure. Under the experimental conditions considered, it is found that the simulations perform well, correctly predicting whether or not drop breakup occurs and the qualitative drop evolution behavior. The simulation procedure outlined in this paper can serve as an effective tool to determine drop breakup in dispersing geometries and hence to optimize dispersing procedures.

Effects of flow behaviour on the aggregation of whey protein suspensions, pure or mixed with xanthan

Abstract The effects of the flow behaviour at different temperatures on whey protein isolate (WPI) suspensions, pure or mixed with xanthan, have been investigated at pH 5.4. The experiments were performed in a continuous rotor/stator device, on the pilot-scale, using different rotational speeds and rotor geometries. The effects of a defined shear flow in a concentric cylinder system were compared with the shear and elongational flow resulting in a geometry with two scraping blades. The aggregation was studied by means of a phase inverse light microscope and quantified by particle laser diffraction measurements. The suspensions were rheologically analysed by a steady stress sweep. A decrease in aggregate size was found with an increased rotational speed, independent of temperature and rotor geometry. For the pure suspensions, the increase in rotational speed from 100 to 500 rpm, in the absence of scrapers, was accompanied by Taylor vortices. These changes in flow behaviour resulted in a dramatic decrease in aggregate size for the suspensions close to denaturation at 70°C. The rotor geometry also influenced the aggregate size. At 70°C, smaller aggregates were formed in the presence of scrapers compared to in the absence of scrapers, while below denaturation at 50°C, the opposite behaviour was found. The viscosity of the processed suspensions was lower compared to that for an unprocessed sample. The processed sample showed shear thickening behaviour. In the presence of xanthan, the effects of temperature and flow behaviour on the WPI aggregation were similar but less pronounced than those in the pure suspensions. This indicates that xanthan inhibits the effects on the WPI aggregation. When scrapers were attached to the rotor, WPI aggregates with a tendency to a long and rod-like shape were formed. These shaped aggregates were explained in terms of the flow behaviour and the pseudoplastic nature of xanthan. All mixtures showed this pseudoplastic behaviour, independent of process history.

Extensional Properties of Hydroxypropyl Ether Guar Gum Solutions

The extensional properties of 2-hydroxypropyl ether guar gum solutions were investigated using a capillary breakup extensional rheometer (CaBER). Optimization of the geometric parameters of this device allowed for the measurement of the characteristic relaxation times and the apparent extensional viscosities of a series of dilute to semidilute guar gum solutions. The measured relaxation times were compared with predicted Zimm relaxation times, assuming that the hydrophobically modified guar was in a good solvent. Good agreement was found at low concentrations (0.01 wt % approximately 0.17 c*, where c* is the polymer overlap concentration), and this technique allowed for relaxation times on the order of 1 ms to be measured for solutions with shear viscosities of 2 mPa.s. Both the shear and (apparent) steady-state extensional viscosities of this set of industrially relevant fluids exhibited two regions of dependency on polymer concentration: linear up to concentrations of 0.2 wt % ( c/ c* approximately 3) and power law thereafter, where interchain interactions became significant. The extracted relaxation times followed the same trend (i.e., having a near linear dependency on concentration up to 0.2 wt % and a power-law dependency on concentration up to 9 c*). The results indicate that the transition from dilute to semidilute behavior occurs at a nominal concentration of approximately 3 c* instead of c*. The results presented suggest that interchain interactions for this modified guar are weak overall, and the solutions investigated are absent of entanglements over the whole range of frequencies and concentrations explored ((0.17-9) c*).

Direct measurement of thermal fat crystal properties for milk-fat fractionation

The temperature dependency of the solid-fat content of a fat is important for predicting the consistency of the final product and for process control. Two direct analytical methods for determining the solid-fat content and the melting characteristics of milk-fat fractions have been developed. The fractions are analyzed during crystallization, and the results give reproducible information about the thermal behavior of the fractions and the actual crystal amount in the suspensions. The results are obtained within minutes, and no separation of the fractions before the measurement is necessary. New ways of reducing process time are proposed. It is possible to adapt crystallization time to seasonal variations of the initial milk fat and to the required properties of the final product. Partially crystallized milk fat was directly measured in a calorimeter and a pulsed nuclear magnetic resonance spectrometer. The data showed good correlations and even more accurate results than the conventional methods. The calorimetric method makes it possible to detect polymorphic changes and co-crystallization in the crystals so that the influence of processing parameters, such as energy input, on crystallization behavior can be investigated.