✓ An `uncertainty principle' in granular mechanics.
Below is an excerpt on this issue from our review chapter, "Granular Systems", in The Oxford Handbook of Soft Condensed Matter (Oxford University Press, Oxford, UK, 2015):
"...these calculations are idealised and taking into consideration the redundancy is problematic. In real systems in mechanical equilibrium, exact redundancy is possible only when a grain has exactly two force-carrying contacts, in which case they must be equal, opposite and align along the line between the contacts. For grains with three contacts or more, it is impossible to determine the force directions in general with absolute accuracy and, in particular, whether those coincide exactly at a point. At most one can say that the forces apply very small torques to the grain because the `region of coincidence' (the shaded region in figure 5.10b) is small. Consequently, except for two-contact grains, the equations are never really redundant - they are at most negligible. It follows that the value of $z_c$ depends to some extent on the level of accuracy of the measurement of forces; the more accurate the measurement, the more torque balance equations need to be included and the higher the value of $z_c$.
This sounds strangely like an `uncertainty principle' - the more accurate the measurement the more equations one needs to determine the stresses. This `uncertainty principle' is also a manifestation of the inherent relation between the structure on the granular level and the stress field that the assembly supports ...".
✓ A-thermal (or granular) statistical mechanics appears to give intriguing insight into the nature of reality, time and our perception of these two. I am not referring to entropy and the arrow of time, but rather insight into time's existence in the first place.
✓ My paper, "The unusual problem of upscaling isostaticity theory for granular matter", describing a novel method to coarse-grain to the continuum the sterss equations in solid granular media, has been submitted to the special issue of Granular Matter dedicated to the memory of Bob Behringer
✓ Our paper, "Support of Modified Archimedes' Law Theory in Granular Media", elucidating the different mechanisms governing penetration of solid objects into granular materials, has been submitted to Soft Matter
✓ Our paper, "Force-based three-dimensional model predicts mechanical drivers of cell sorting", elucidating the roles of physical mechanisms in stem cell sorting in the embryo, has been published in Proc. Roy. Soc. B: Biological Sciences
✓ Our paper, "Support of Modified Archimedes's Law Theory in granular media", throwing light on the nonlinear part of the modified Archimedes's law and fitting the entire consitutive force-depth relation in penetration of solid objects into granular materials, has been submitted to Soft Matter
✓ After 6 years of hard labour, our paper, "Stress Tensor for Dense Granular Flow in Plug-Free Regions", in which we derive the rheology of dense plug-free particulate fluids, has been published in Phys. Rev. E
✓ Clara Wanjura has received her masters degree.
✓ Our paper, "Equally probable positive and negative Poisson's ratios in disordered planar systems", showing that disordered assemblies of isostatically linked compliant triangles are equally likely to have negative and positive Poisson's ratios, has been published in Soft Matter
✓ I will be giving 4 lectures at the "Summer School on Soft Matter and Biophysics" on stress field theory in particulate media. Expect lively informal discussions. All welcome.
✓ I now have a paper that I cannot read "Numerical simulation of a spinning sphere moving in granular matter" in Acta Physica Sinica.
✓ I am chairing session K48 of "Athermal Systems and Statistical Mechanics" at the APS meeting, March 5-9, LA, USA. Please join us there.
✓ Our paper, "Archimedes' law explains penetration of solids into granular media: modelling and experimental support", showing that such a process is governed by a simple, albeit universal, Archimedes' law, has been published in Nature Communications
These pages have grown uncontrollably from a modest beginning in 1993, when a home page was a rarity. I cannot pinpoint when exactly over the years was it that I lost the fight against this unstable self-(dis)organised growth. I was probably never in control.
You may notice that the pages are written in the old basic HTML format. This is because none of the current fancy new softwares, which essentially do all the nitty-gritty work, existed. Hence, there are no fancy java scripts, flashing lights and colours, background music, or automatically starting videos. This may be one of the last bastion of the HTML purists, uncluttered by sights or sounds. As my friend Peter once put it: "This is about delegation of responsibility. About trust. About subsidiarity. It's a small-scale mirror of so many of our every day problems."
Anyway, my publication list is updated regularly ... after a fashion. The rest not so much. If your time is short, I recommend to browse my 'Selected publications' and then move to the less frequently updated 'research interests' page. Much of the latter is distilled into limericks at the bottom of this page. You are, of course, welcome to visit all the pages but be warned that there is always the possibility of getting lost in the overgrown meandering paths.
Often, when I finish developing a model or a theory, I write a limerick that summarises it. The weight and rythm are not always great, but they do describe, as accurately as possible in a limerick form, the modelling ideas and the relevant physics. Below are the limericks I wrote so far, each with a link to a paper that the work produced, if it has.