Russel:

Particle size has significant effect on rheology and I have done some works and published few papers last two years. What you are talking that is based on physical property; not rheology.

Particles affect the way that a fluid moves in a shear field so they a have a huge influence. See:

Rayment, P.; Ross-Murphy, S. B.; Ellis, P. R. Effect of size and shape

of particulate inclusions on the rheology of guar galactomannan

solutions. Carbohydr. Polym. 2000, 43, 1–9.

One thing to be very careful about is how you measure the viscosity. Particles cannot be larger than about 1/10th the size of the gap in a parallel plate, cone and plate or cuette geometry rheometer. They interfer with accurate measurement.

Thanks Susan. Recently we published one paper (2014) in J Food Engg and Food Research International (2013) on the effect of particle size on rheology and in fact it is very interesting study. If you will go through our paper based on oscillatory rheology, you can observe a critical particle size where the property changes considerably. My group is working intensively using Rheometry and SEM.

Thanks for your comments.

Food rheology has a lot of similarities to polymer composite rheology. You may want to see whether you can get any directional help by reading certain sections of the book Aroon V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Netherlands (1999). There is a chapter on constitutive theories and equations of suspensions where the effects of shape, concentration, dimensions, and size distributions of the particles have been discussed.

Recently we also published the effect of incorporation of different silica particles on textural properties of gelatine gels. http://www.ncbi.nlm.nih.gov/pubmed/24824423

Several distinct phenomena

Changing primary particle size of non-agglomerated particles at constant mass fraction and interaction with the continuous phase

interaction with dissolved components

effect of agglomeration

depletion effects.

Somebody else may extent this list

Thank you Titus; my group is working in this area and we will explore those factors.

I would recommend the recent text "Colloidal suspension rheology" by Jan Mewis and Norman Wagner which covers rheological effects of particles in fluids very systematically and refers to pertinent specialist literature. If you are dealing with highly concentrated and/or non-colloidal particle suspensions, consider also the books and articles by Philippe Coussot.

Thank you all for your excellent comments. Working with food fiber dispersion, I faced lot of problems or I may say interesting observation which is completely different from polymer rheology as I also wroked on polymer rheology. First food is a complex material composed of protein and starch which contribute a lot and each composition varies with the particle size. Furthermore, the fibers (soluble and insoluble) have their own characteristics especially water holding capacity and volume fraction. Recently, I submitted couple of papers on the subject and reviewers have difficulty to understand as always they are comparing with polymeric system.  

An interesting way of characterizing these types of samples is the use of DWS microrheology; this novel technique measures the motion of the particles in your sample and calculates the frequency depended rheological properties G' and G''. A great advantage of DWS microrheology is that the measurement are contact less via a laser beam such that the sample can be sealed in a glass cuvette. This allows the study of sensitive gel-type of samples over an extended time. For more information and applications please refer to: http://www.lsinstruments.ch/technology/diffusing_wave_spectroscopy_dws/microrheology/