Oscillatory technique

A more sensitive rheological techniques for following the stability of multiple emulsions is to use oscillatory techniques. In this case, a sinusoidal strain or stress is applied to the sample, which is placed in the gap of the concentric cylinder or cone-and-plate geometry the resulting stress or strain sine wave is followed at the same time. For a viscoelastic system, as is the case with multiple emulsions, the stress and strain sine waves oscillate with the same frequency, but out of phase. 

In oscillatory techniques, two types of experiment must be carried out... 

Syneresis may he predicted from measurement of the yield value (using steady-state measurements of shear stress as a function of shear rate) as a function of time, or hy using oscillatory techniques whereby the storage and loss modulus are measured as a function of strain amphtude and frequency of oscillation. 

As discussed in the introduction, the bulk rheology of emulsion systems can be investigated using steady-state (shear stress as a function of shear rate), constant stress, and oscillatory techniques. These methods are the same as those described for interfacial rheology. In this section, some results on various emulsion systems will be described to illustrate the use of rheological measurements in investigating the interaction between emulsion droplets. First, the viscosity-volume fraction relationship for o/w and w/o emulsions... 

Strain dependence. Furthermore, in steady shear a strong gel will rupture completely whereas a weak gel will flow, albeit without obeying the Cox-Merz superposition principal. In this case the complex viscosity (r] ) is higher than the flow viscosity ( //). This indicates the presence of weak interactions between the molecules that contribute to the viscosity measured by the non-destructive oscillatory technique but not to the viscosity measured by the destructive flow technique. 

Another problem encountered with many suspensions is that of syneresis , i.e. the appearance of a clear liquid film at the top of the suspension. Syneresis occurs with most flocculated and/or structured (i.e. those containing a thickener in the continuous phase) suspensions. Syneresis may be predicted from measurement of the yield value (using steady-state measurements of shear stress as a function of shear rate) as a function of time or using oscillatory techniques (whereby the storage and loss modulus are measured as a function of strain amplitude and frequency of oscillation). These techniques will be discussed in detail below. 

Dynamic (oscillatory) techniques is the response of the material to an oscillating stress or strain [49]. When a sample is constrained in, say, a cone and plate or concentric cylinder assembly, an oscillating strain at a given frequency o) (rads ) (o) = 2vn, where v is the frequency in cycles s or Hz) can be applied to the sample. After an initial start-up period, a stress develops in response to the applied strain, i.e. it oscillates with the same frequency. The change of the sine waves of the stress... 

It can be concluded from the above discussion that a combination of microscopy, sensory analysis and rheological properties (obtained under high and low deformation) using statistical evaluation methods can provide a correlation between sensory perception (as evaluated by expert panels) and the various characteristics of the gel. The relationship between microstructure and texture is important in optimizing the properties of food products as well as in the development of new products with the desirable properties. Modern techniques of microscopy (such as freeze fracture) can be applied to study the microstructure of gels. The viscoelastic properties of gels, which can be studied using oscillatory techniques (under various conditions of applied strain and frequency) can be correlated to the microstructure. 

In order to study viscoelastic materials, oscillatory techniques are often used. Oscillatory strains are generated and the response is recorded in terms of stress. The magnitude of the phase difference between strain and stress will be studied ... 

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