Wall friction is determined by measuring the force required to secure and sustain slip of a bulk solids that is subjected to a range of normal loads, that is, the load pressing the material against the contact surface. It can be measured with a Jenike type tester, but these have a very restricted translation and measurement should be taken at different contact pressures without resetting the material so a long strain tester should be used, as the Ajax wall friction device, see. www.ajax.co.uk/testers.

By 'Kinetic friction', I presume that you mean 'dynamic friction', or 'sliding friction', as opposed to 'static friction', which is the dfference between initiating slip and sustaining motion once it has commenced. 'Dynamic friction' is the value normally employed for wall friction in hopper design. These friction values must all be defined against a specific contact surface against which the material is slipping.

A different value is 'Internal Friction', that is assessed from a yield locus, and represents how the material will slide on itself in a shear condition. This internal friction applies as the boudary layer of the flow channel in a 'funnel flow' situation and reflects internal deformation of the mass during flow with a velocity gradient across the flow channel.

In all cases of friction it is necessary to understand that any attractive forces at the interface will distort the apparent friction angle measurement, particularly at low contact pressures. If you care to forward your email address to lyn@ajax.co.uk, I will send notes on powder characterisation and testing. ■

Dear Mr. Bates, what normal load in pre and shear phases do you recommend to determinate flow properties of granular and powders materials using Jenike shear cell?

Thank you ■