Theoretically, the cone plate geometry is used because it allows a uniform shear-rate (in the radial direction) provided the tip of the cone is on the plane. For practical and mechanical reasons, the cone is truncated, so the "tip" is replaced by the "gap". Thus the gap should be constant for a given cone angle, otherwise the shear rate is not uniform.

In the cone-plate geometry the recommended distance is set so that the velocity gradient is constant in the whole sample (and therefore the shear rate very well defined). In plate-plate geometry this is not the case: the velocity gradient at the outside of the sample is larger than at the in center (where it is zero). In the latter case an averaged shear rate is used to characterize the measurement. In your case, cone-plate with another than recommended plate-plate distance, the real shear rate in sample is ill defined. You cannot even determine the averaged shear rate. Therefore, it would be hard for other scientists to reproduce the experiments. When you want to have some flexibility in the inter-plate distance, I recommend the plate-plate geometry. The software of the rheometer automatically calculates the averaged shear rate.

Thank you Henny. I understand that, but most papers focused on starch gels rheology use 1 mm gap for a 40 mm cone and plate geometry. I would think that as long as everybody use the same configuration despite you mentioned before about constant gradient, experiments can be reproduced by other scientists. My point is that, on one side you have parallel plates which you can modify the gap but material properties i.e. modulus-compliances (strain, time) and zero shear viscosity can no be calculated accurately, and on the other side you have cone-plate geometry which you should not modify the gap but is no possible to test my samples because the will break apart. Is there any way of getting true material properties from measurements using parallel plate geometry?

Theoretically, the cone plate geometry is used because it allows a uniform shear-rate (in the radial direction) provided the tip of the cone is on the plane. For practical and mechanical reasons, the cone is truncated, so the "tip" is replaced by the "gap". Thus the gap should be constant for a given cone angle, otherwise the shear rate is not uniform.

As Didier said, the vertical position of the cone is fixed due to the truncation height. If you need larger gaps you should use other geometries like plate-plate (PP) systems and correct the influence of non-Newtonian effects on the shear stress distribution (Weissenberg correction) if you meassure in the non-linear regime of the material.

The use of larger cone angles in combination with larger truncation heights is not a good idea, because the independence of shear rate on the cone angle is valid only for small cone angles. Otherwise, if you increase the truncation height in CP systems with small cone angles, the area proportion of the PP regime increases and hence the inhomogeneity of the shear field.

To Henry: In plate-plate geometries the shear rate distribution in the gap is given geometrically. The shear stress must be corrected!

As long as I work in the linear viscoleastic region I can use parallel plates with the gap I want making sure I set the correct constants for the geometry configuration in order to calculate the right values for strain, lets say at the edge of the plate. Torque measurement is total, is not connected to any specific point in the geometry so ¿how can I get a local stress value as function of a local strain?

Hi, Alejandro Serrano

To avoid damage to the cone and plate the tip of the cone is removed (avoid contact). When testing a sample, the cone is positioned so that the "virtual" tip is in contact with the plate.

The stress and strain calculations of a cone are geometrically estimated and are based on the fact that the space between the cone and the plate is exactly the truncation distance of the cone. For this reason, it is essential to test samples with a cone-plate geometry only at the cone truncation distance. The truncation distances for different cones vary and range from about 50 to 100 microns and the angle and truncation distance of each cone is determined individually. These values ​​are included in the factory data for the geometry.

Unlike parallel plates, the space for a cone and a plate is fixed and is defined by the truncation distance of the cone. Due to the need to operate specifically at the truncation distance, a cone is normally used only for isothermal testing, as changes in temperature can cause changes in space due to thermal expansion.

The geometry of the cone and plate there is no velocity gradient across the diameter of the cone during testing. The cone and plate configuration is the desired configuration for normal stress measurements. For cone and plate geometry, the normal force, N, corresponds to the first normal stress difference, N1.

I work with a TA instruments ARES-G2 and this information is what we handle in relation to the cone-plate geometry.

Best regards

Hello,

Your comments are very helpfull so i have another questio. I am trying to measure the viscosity versus shear rate of a sample with very low particle size diameter (d50 about 1μm) and a value of viscosity 1.5. The test parameters are : Flow sweep test, Shear rate : 1 to 25000 s-1 Geometry: 40 mm Parallel plate Geometry gap: 200 μm. The experiment seems to be repeatable but according to the specifications of 40 mm Parallel plate is not suitable for low viscosity values. Do you thing that i am not getting the reeal value? Furthermore if i choose 60mm cone plate can i measure at maximum shear rate of 25000s-1?