Syneresis strain rate

Thus the syneresis strain rate can be regarded as the result of an effective stress on the network equal to Pj. 

Two more features are needed to complete the constitutive equation for a gel allowance for the syneresis strain rate (which occurs in the absence of applied stress) and for the presence of the liquid phase. When a force F is applied to the face of the sample cube (see Fig. 11), a portion of that force is borne by the solid phase and a portion F is borne by the liquid (and = Fxs + F i). The pressure in the liquid phase, is equal to the force on the liquid divided by the area of the liquid, The negative... 

From Eq. 43 we see that a = 3pP = per, so the total force on the solid phase is p(/ i + Fy + Fj) that is, the load is borne by the solid phase in proportion to the volume fraction that it occupies. Finally, we identify the linear syneresis strain rate as and modify the constitutive equation by adding that quantity to the right side of each line in Eq. 46. That is, we assume that the stress-induced strain rate and the inherent syneresis strain rate are linearly additive. Thus the constitutive equations for an isotropic viscous gel are ... 

Syneresis is contraction of the network driven by condensation reactions. There is a limit, P, to the stress that this mechanism can generate if the liquid were prevented from leaving, the network would squeeze the liquid until the pressure rose to that level (P = PJ, then contraction of the network would stop. (P is the stress in the liquid, so it is negative when—as in this case—the liquid is compressed.) This is a small stress (typically <1 atm) compared to the capillary pressure that develops during drying. For a plate of thickness 2L drying by evaporation from both faces (each with area A), the rate of loss of liquid is 2/1P b, so the total volumetric strain rate of the drying body is... 

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. 

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. 

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