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opened the discussion on the paper by Ploehn and Goodwin: In discussing table 2, the authors state 'the steady shear viscosity was found to be a power-law function of shear rate at each particle volume fraction', and also, 'Above a critical volume fraction,. . . the viscosity curves exhibited apparent yield stresses.' Judging from the curves in fig. 2 of their paper, neither of these mutually exclusive statements is valid. Do the authors have additional data which validate their claim that a yield stress is present? Dr H. J. Ploehn (Texas A & M University) responded: The curves in fig. 2 of our paper show that, for the 'solid-like' samples, the viscosity increases sharply as the shear stress decreases. Although we described this as an apparent yield stress, it is not a true yield stress, since the slopes of the curves do not become infinite. Our use of the term 'apparent' is related to the time dependence that enters at low stresses: we also measured viscosity using a stress-controlled Bohlin CS rheometer, and we always found that there was a stress below which the sample would not flow over reasonable experimental timescales (e.g. 10 min). This is the apparent yield stress. We believe that if one is willing to wait long enough, the samples would flow at stresses below the apparent yield stress. The timescale for the transient yielding process is (i) probably inversely proportional to the driving force, i.e. the imposed stress, and (ii) related to the timescale for collective or mutual diffusion of particles. Therefore one would have to be very patient for high volume fractions and low stresses.Dr Th. F. Tadros ( I C I Jealotts Hill, Bracknell) addressed Dr Ploehn: The layer thicknesses you obtain are relatively small for such high-molecular-weight polymers. It seems to me that the graft density is small and hence the polymers are coiling on the particle surface. This may explain the small thickness observed and also the difference in thickness obtained using small and large particles. If coiling occurs, there would also be an effect on the radius of curvature of the particle.Dr Ploehn replied: I certainly agree with the physical picture that you have described. Unfortunately, our speculation is based upon incomplete data: measurements of adsorbed amounts and graft densities would be desirable.The DLS and electrophoresis data presented in the paper imply that grafted and adsorbed APE0 layers have similar structures. The rheological data indicate that the flow properties of the concentrated suspensions are quite different from the rheology of suspensions stabilised by dense layers of low-molecular-weight stabilisers. The best explanation for these observations is that the polymer layers are diffuse, relatively thick compared to layers of low-molecular-weight stabilisers and interact via a long-range 'soft' repulsion.Theories for adsorption of high-molecular-weight polymer predict that the fraction of chain ends near an adsorbing surface is small. If the grafting mechanism is adsorption followed by reaction, then w...