**3. Detailed rheometry of mixtures without coarse**

#### **3.1 Material**

Detailed rheology measurements are possible only when the coarse fraction is removed from the mixture. An alternative is to revert to a system where natural segregation has already removed the sand fraction. Natural fluid muds constitute such a system and as mentioned earlier it also appears to display wall slip signature. Results of rheometry [33] conducted on natural fluid mud from Beerkanaal, the port of Rotterdam, The Netherlands, with an initial density of 1263 kg/m3 will be presented. Earlier measurements of the fluid mud from the port of Hamburg and Emden are also analysed [11, 34–37]. The yield stress of natural fluid muds is considered decisive for navigable depth in ports and waterways.

*Rheology, Rheometry and Wall Slip DOI: http://dx.doi.org/10.5772/intechopen.108048*


#### **Table 1.**

*Applied measuring elements and protocols (B = bob CC25DIN, gBC = grooved bob CC25Din, C = cup CC25 DIN, V = vane FL22, PP = parallel plate P35).*

#### **3.2 Configuration and protocol**

Configurations with well-defined shear rates such as concentric bob-cup (BC) cylinders are preferred in rheometry. However, as already mentioned, a BC system is prone to slippage: following Boger [29] on usage of vane, slippage appears also to be radically prevented in a vane-cup (VC) configuration, interestingly. The shear rate in a VC, on the other hand, has to be defined.

Two standard rheometry protocols, namely controlled shear rate (CSR) and controlled shear stress (CSS), are applied and compared to investigate the occurrence of wall slip in BC, PP and VC configurations. The vane is applied with 1 mm bottom clearance. **Table 1** provides information on the applied measuring elements and testing protocols. Tests are conducted by Haake Mars 1. The basis protocol consists of a linear ramp-up of either shear rate or shear stress during a certain time, followed by a constant phase and a ramp-down phase.

#### **3.3 Results and discussion**

#### *3.3.1 Wall slip signature in bob-cup (BC) configuration*

A definite difference is found between tests conducted in BC and VC. **Figure 4a** depicts the result of CSR and CSS tests in a BC configuration. At low shear rates (< 10 [1/s]), the ramp-down part of the curve crosses the ramp-up part in both CSR and CSS tests. This is because the measured shear stresses during the initial part of the ramp-up are underestimated, a typical wall slip signature in a BC configuration, compared with **Figure 3**. **Figure 4b** shows the result of CSR and CSS tests in a VC configuration where no wall slip is observed. The viscometer's outputted rotational velocity has been transformed to an equivalent shear rate at the rim of a virtual cylinder encompassing the vane, similar to the BC configuration, to allow comparisons. The shear stress-shear rate relationships over the ramp-down (remoulded) phase are in excellent agreement.

#### **Figure 4.**

*Example of rheometric results on fluid mud from Beerkanaal, port of Rotterdam, The Netherlands, flow curve (a, c) and viscosity plot (b, d) with different elements and protocols.*
