Monday, June 27, 2011

Modeling of wall friction (a.k.a. What is Wall Friction? Part 6)

The mentioned previous studies produced clear trends in the measured wall friction, as it varied with respect to a parameter (such as particle size, plate roughness, etc.). But the conclusions in many of these projects were qualitatively in nature. In only a couple of studies I found, a model was proposed. So the question comes: why has nobody come up with a model, given all the data that has been produced? The answer came to me until recently: there is no clear way of deriving a model from first principles (say a continuum equation). Other than just fitting a curve to the data, given the know parameters that affect wall friction, there is no evident method that would lead us to a function that could be used for predicting and modeling wall friction.

Another problem I found with these previous research results is related to the lack of consistency in the behavior of the data. In some of the provided plots, there was no clear trend in the data. This makes it hard to find any model. The reason this is happening, at least in my opinion, is that in many of these cases things like particle shape and size (sometimes they used a wide number particle size distribution), or plate roughness were not controlled at all. The plate surface had whatever grain direction the fabrication process provided, the bulk material had any particle shape the manufacturer was able to deliver, and the researchers did not pay attention to any of these details at all.

In order to produce consistent results, especially for problems like wall friction which depend on a lot of variables, a control is needed of these parameters. In my study, spherical particles were used (sphericity above 90%) with a narrow number particle size distribution, and the roughness on the plates was simulated by machining triangular sawtooth (like inverted V shape) grooves on one surface. The height of the groove was varied, but the periodicity remained constant.

To be continued…

Friday, June 24, 2011

What is wall friction? Part 5

Selection of the samples, used as wall material, was made based on the fabrication process of the surface. It would be far easier to work on aluminum than on steel plates, although the action of the particles could cause wearing, but this could be accounted for later on (as a correction factor, maybe). Previous researchers have made similar studies by using plates with a standard finish. There is a table (that you can find on the internet) that lists standard machined finish characteristics, which include the roughness values measured with a profilometer (either mechanical or optical), different values are given in this table (one of these is called RMS, or root mean square) that one could use to establish the roughness of the finish on your plates. There are manufacturing procedures that ensure that the surface would have the corresponding standard finish and, consequently, the corresponding surface roughness.

But the problem with this approach is the lack of consistency in the orientation of the so-called grain. It is an experimental fact that grain (which is the structure above the mean wall surface, such as peaks) plays an influential part in friction. If the flow of the powder is against the grain (picture this as particles finding obstacles as they move along the surface), the wall friction increases. But if the powder flows along the grain (particles move along the peaks, instead of going over the peaks), then wall friction reduces appreciably.

To be continued…

Wednesday, June 22, 2011

What is wall friction? Part 4

Finally, we have come up to one of the many subjects I am hoping to talk about in this blog: modeling of wall friction based on micromechanical parameters such as wall surface roughness, particle shape, particle size distribution, and microfriction values. Wall friction in fact depends on other parameters such as temperature, humidity, particle roughness, etc., but I want to restrict the study to only a few simple and measurable variables.

I hope this is going to be received well by the particle science community since I will be publishing these results pretty soon. One main concern I found during this process is the fact that I used a different tester to measure wall friction. This tester (I am not sure I should talk about it) has the ability to measure static friction, for dynamic friction the procedure has to be changed a little. In both instances the pattern was the same, the measured data had the same trend in the static case as it did in the dynamic case, that problem was solved.

We selected spherical glassbeads as the sample of bulk material because we have a lot of options in the choice of particle size from vendors. Also, the sphericity of these particles (how round they really are) is above 90%. Their particle size distribution by number of particles per size has a small standard deviation (how wide this curve is, I will talk about particle size distribution in another post). We looked at these particles under the microscope and they indeed showed a large amount of sphericity (although it was not measured), the size distribution was measured using laser scattering methods.

To be continued…