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The Foodtexture Puff Device (FPD) is a non-contact rheological measurement device, which applies an air pulse on the sample and measures the subsequent deformation of the sample surface with a laser distance sensor. The deformation behavior is considered as a measure for the rheological properties of the sample. The applicability of this device was studied for use on viscous food products with a broad range of rheological characteristics. In this study, sugar and fat based systems with a viscosity range of respectively 0.001 Pa.s to 6.1 Pa.s and 0.01 Pa.s to 5.9 Pa.s were tested. Comparison of the FPD with classical rheological analyses showed that the maximum deformation created by the FPD is strongly correlated to the viscosity. Hence, the FPD is well suited for measurements on sugar based and fat based systems. It is capable of providing accurate, non-contact, fast, easy and non-destructive rheological measurements on food products.

Rheological properties of cell wall suspensions strongly depend on particle size and particle-particle interactions. In the present study, an experimental method was developed to study the effect of particle elasticity and electrostatic interactions on the rheological properties of cell suspensions. Enzymes were used to selectively depolymerize the pectin (backbone) and proteins in suspensions. The enzymatic treatments affected the physical properties, thus a hypothesis for the structure-function relationship of these biopolymers was formulated. The enzymatic treatment directly affected particle properties, resulting in looser cell walls as visualized by cryo-SEM. The effect of the enzymatic treatment on the storage modulus was measured as a function of total solid content (below critical packing fraction). Furthermore, experiments were performed in the presence of varying concentrations of sodium chloride in order to change the Debye screening length. Such method assisted in decoupling the electrostatic effects from particle elasticity. In addition, particle properties were measured directly by applying a compressive strain on the particles and measuring the normal force. By fitting the normal stress relaxation with a Maxwell model, particle properties such as time scale of relaxation and elasticity were obtained. It is suggested that for carrot suspensions, pectins on the cell walls could contribute to the particle hardness. The pectins on carrot cell walls are responsible for electrostatic interactions between particles.

The effects of geometrical confinement and viscosity ratio on droplet coalescence in shear flow are experimentally investigated by means of a counter rotating parallel plate device, equipped with a microscope. The ratio of droplet diameter to gap spacing is varied between 0.03 and 0.33 to study both bulk and confined conditions. Three grades of a Newtonian droplet material are combined with a Newtonian matrix, resulting in three different viscosity ratios, namely, 0.1, 1.1, and 2.6. The effects of confinement are qualitatively similar for all three viscosity ratios. For each system, confinement decreases the coalescence angle and renders coalescence possible up to higher capillary numbers and initial offsets. Moreover, for all three viscosity ratios, confinement induces a lower initial offset boundary below which the approaching droplets reverse flow direction without coalescence. However, there are quantitative differences between the systems. With increasing viscosity ratio, the critical capillary number and critical upper and lower offset boundaries decrease. Since the decrease of the upper offset boundary is more predominant, the coalescence efficiency decreases with viscosity ratio. The droplet trajectories of interacting droplets are affected by both the viscosity ratio and geometrical confinement, which clearly has implications on the coalescence behavior.

Droplet breakup in systems with either a viscoelastic matrix or a viscoelastic droplet is studied microscopically in bulk and confined shear flow, using a parallel plate counter rotating shear flow cell. The ratio of droplet diameter to gap spacing is systematically varied between 0.1 and 0.85. In bulk shear flow, the effects of matrix and droplet viscoelasticity on the critical capillary number for breakup are very moderate under the studied conditions. However, in confined conditions a profoundly different behaviour is observed: the critical capillary numbers of a viscoelastic droplet are similar to those of a Newtonian droplet, whereas matrix viscoelasticity causes breakup at a much lower capillary number. The critical capillary numbers are compared with the predictions of a phenomenological model by Minale et al. (Langmuir 26:126–132, 2010); the model results are in qualitative disagreement with the experimental data. It is also found that the critical dimensionless droplet length, the critical capillary number, and the dimensionless droplet length at breakup show a similar dependency on confinement ratio. As a result, confined droplets in a viscoelastic matrix have a smaller dimensionless length at breakup than droplets in a Newtonian matrix, which affects the breakup mode. Whereas confined droplets in a Newtonian matrix can break up into multiple parts, only two daughter droplets are obtained after breakup in a viscoelastic matrix, up to very large confinement ratios.

The dynamics of single droplets in a bounded shear flow is experimentally and numerically investigated for blends that contain one viscoelastic component. Results are presented for systems with a viscosity ratio of 1.5 and a Deborah number for the viscoelastic phase of 1. The numerical algorithm is a volume-of-fluid method for tracking the placement of the two liquids. First, we demonstrate the validation of the code with an existing boundary integral method and with experimental data for confined systems containing Newtonian components. This is followed by numerical simulations and experimental data for the combined effect of geometrical confinement and component viscoelasticity on the droplet dynamics after startup of shear flow at a moderate capillary number. The viscoelastic liquids are Boger fluids, which are modeled with the Oldroyd-B constitutive model and the Giesekus model. Confinement substantially increases the viscoelastic stresses and the elongation rates in and around the droplet. We show that the latter can be dramatic for the use of the Oldroyd-B model in confined systems with viscoelastic components. A sensitivity analysis for the choice of the model parameters in the Giesekus constitutive equation is presented.

Using a counter rotating parallel plate shear flow cell, the shape relaxation of deformed droplets in a quiescent matrix is studied microscopically. Both the effects of geometrical confinement and component viscoelasticity are systematically explored at viscosity ratios of 0.45 and 1.5. The flow conditions are varied from a rather low to a nearly critical Ca number. Under all conditions investigated, viscoelasticity of the droplet phase has no influence on shape relaxation, whereas matrix viscoelasticity and geometrical confinement result in a slower droplet retraction. Up to high confinement ratios, the relaxation curves for ellipsoidal droplets can be superposed onto a master curve. Confined droplets with a sigmoidal shape relax in two stages: the first consists of a shape change to an ellipsoid with a limited amount of retraction, and the second is the retraction of this ellipsoid. The latter stage can be described by means of one single relaxation time that can be obtained from the relaxation of initially ellipsoidal droplets. The experimental results are compared to the predictions of a recently published phenomenological model for droplet dynamics in confined systems with viscoelastic components (Minale et al., Langmuir 26:126–132, 2010). However, whereas the model predicts additive effects of geometrical confinement and component viscoelasticity, the experimental data reveal more complex interactions.

The breakup of confined drops in shear flow between parallel plates is investigated as a function of viscosity ratio and confinement ratio. Using a boundary-integral method for numerical simulations and a counter-rotating experimental device, critical capillary numbers in shear flow are obtained. It is observed that different viscosity ratios yield different trends with increasing confinement ratio: a low viscosity ratio drop shows an increase in critical capillary number, at a viscosity ratio of unity no major trend is seen, and the critical capillary number for a high viscosity ratio drop decreases significantly. A generalized explanation for all viscosity ratios is that confinement affects the orientation of the drop with respect to the direction of the local strain field. At moderate confinement ratios, the drop orients more towards the strain direction, where it experiences a stronger flow and hence, the critical capillary number is decreased. As the drop gets more confined, it aligns more in the flow direction. Hence, the drop experiences a weaker flow and thus, additionally stabilized by wall effects, it breaks at a higher critical capillary number. In principle, this behavior is the same for all viscosity ratios, but transitions occur at different confinement ratios. Most of the breakup is of a binary nature, but ternary breakup can occur if the drop length is larger than 6 undeformed drop radii, consistent with arguments based on the Rayleigh-Plateau instability.

The effect of microstructure on the rheology of clay/polymer nanocomposites is investigated using dispersions of organically treated clay in nearly Newtonian poly(dimethylsiloxane). Degree of dispersion and floc size are altered by using two different dispersion procedures and by changing the shear history. The scaling for dynamic moduli of attractive colloids applies, except for a possible relaxation mechanism at very low frequencies. The time to reach the crossover at a given frequency is found to be extremely sensitive to the dispersion procedure used. Hydrodynamic and elastic components of the steady state stress, on the other hand, evolve in a very similar fashion for the different systems. Although the relaxation times of the elastic stress components change drastically with flow-induced changes in structure, the dispersion process hardly has an effect at all. Intermittent start-up flows in the forward and reverse directions show that anisotropy persists long after the flow has been arrested, even at shear rates where no large reversible flocs are present. The degree of dispersion only had a limited effect on the anisotropy. Finally, the effect of shear on structure recovery has been studied. Very low shear rates are found to increase the rate of recovery, even for small strains.

By using a counter rotating plate-plate device, single droplets in shear flow have been microscopically studied at confinement ratios ranging from 0.1 to 0.75. The droplet-to-matrix viscosity ratio was fixed at 0.45 and 1.5. Results are presented for systems with a viscoelastic Boger fluid matrix or a viscoelastic Boger fluid droplet, at a Deborah number of 1. Although the separate effects of confinement and component viscoelasticity on dropletdynamics in shear flow are widely studied, we present the first systematic experimental results on confined dropletdeformation and orientation in systems with viscoelastic components. Above a confinement ratio of 0.3, wall effects cause an increase in dropletdeformation and orientation, similar to fully Newtonian systems. To describe the experimental data, the Shapira–Haber theory [Shapira, M., and S. Haber, Int. J. Multiph. Flow16, 305–321 (1990)] for confined slightly deformeddroplets in Newtonian-Newtonian systems is combined with phenomenological bulk models for systems containing viscoelastic components [Maffettone, P. L., and F. Greco, J. Rheol48, 83–100 (2004); M. Minale, J. Non-Newtonian Fluid Mech.123, 151–160 (2004)]. The experimental results are also compared to a recent model for confined dropletdynamics in fully Newtonian systems [M. Minale, Rheol. Acta47, 667–675 (2008)]. For different values of the viscosity ratio, component viscoelasticity and Ca-number, good agreement was generally obtained between experimental results and predictions of one or more models. However, none of the models can accurately describe all experimental data for the whole range of parameter values.

An experimental study of drop dynamics under shear is conducted for five fluid pairs: a reference Newtonian system, two systems with a viscoelastic drop in a Newtonian matrix, one with a Newtonian drop in a viscoelastic matrix, all at drop to matrix viscosity ratio ¿=1.5¿=1.5, and a separate case at ¿=0.75¿=0.75. The viscoelastic liquids are either a Boger fluid or a shear-thinning viscoelastic fluid satisfying an Ellis model. Deborah numbers in the range 1–2 and a range of capillary numbers from low to above breakup conditions are addressed. The results focus on three aspects: relaxation after cessation of shear, a new viscoelastic drop breakup scenario, and the effect of shear flow history on drop breakup. Numerical simulations with the 3D volume-of-fluid PROST method complement the experimental results.

The deformation and orientation of droplets during transient shear flow is studied in a counterrotating device using microscopy. The effect of the degree of confinement and viscosity ratio is systematically investigated. The system consists of polydimethylsiloxane droplets of varying sizes and viscosities dispersed in a polyisobutylene matrix. The observations are compared with the predictions of an adapted version of the Maffettone and Minale model [Maffettone, and Minale, J. Non-Newtonian Fluid Mech.78, 227–241 (1998)] which includes confinement effects [Minale, Rheol. Acta47, 667–675 (2008)]. For flow start-up at low capillary numbers, the deformation of confined droplets and their orientation towards the flow direction are increased with respect to the unconfined situation for all viscosity ratios under investigation. The confined model results for start-up and the experimental data at low capillary numbers are in good agreement both showing similar monotonous transients. At high degrees of confinement and high shear rates, one or more overshoots in the dropletdeformation are experimentally observed, depending on the viscosity ratio. In addition, droplets become sigmoidal when highly confined. Under these conditions, the confined Maffettone and Minale model, which assumes an ellipsoidal droplet shape, cannot be used to predict the droplet behavior. The relaxation of confined droplets upon cessation of steady-state shear flow is also studied. It is experimentally observed that confinement only affects the relaxation at degrees of confinement above 60% of the gap spacing. Highly confined droplets experience a slightly slower relaxation with respect to bulk conditions. The relaxation predictions of the confined model are in rather good agreement with the experimental data

The dynamics of confined droplets in shear flow is investigated using computational and experimental techniques for a viscosity ratio of unity. Numerical calculations, using a boundary integral method (BIM) in which the Green's functions are modified to include wall effects, are quantitatively compared with the results of confined droplet experiments performed in a counter-rotating parallel plate device. For a viscosity ratio of unity, it is experimentally seen that confinement induces a sigmoidal droplet shape during shear flow. Contrary to other models, this modified BIM model is capable of predicting the correct droplet shape during startup and steady state. The model also predicts an increase in droplet deformation and more orientation toward the flow direction with increasing degree of confinement, which is all experimentally confirmed. For highly confined droplets, oscillatory behavior is seen upon startup of flow, characterized by an overshoot in droplet length followed by droplet retraction. Finally, in the case of a viscosity ratio of unity, a minor effect of confinement on the critical capillary number is observed both numerically and experimentally. ©2008 American Institute of Physics