Pieter Walstra
Wageningen University and Research Centre
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Featured researches published by Pieter Walstra.
Archive | 2005
Eric Dickinson; Pieter Walstra
Food colloids and polymers: stability and mechanical properties , Food colloids and polymers: stability and mechanical properties , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی
Journal of the Chemical Society, Faraday Transactions | 1989
Leon G. B. Bremer; Ton van Vliet; Pieter Walstra
Casein gels consist of particulate disordered structures, formed by aggregation of casein micelles. The aggregation leads to flocs of a fractal nature, which form a gel as soon as the occupy the total volume. A model was developed to describe gel formation and structure usnig the concept of fractal geometry. Parameters derived from this model were compared with experimental results concerning the structure and related properties.The model was in good agreement with the dependence of the permeability or rheological properties, such as the storage modulus of the gels, on the volume fraction of casein, and with the dependence of the turbidity of the gel on wavelength.It is shown that acid casein gels can be described very well as a collection of fractal clusters with a fractal dimensionality, D, of 2.3. Rennet-induced casein gels appeared to lose their fractal behaviour due to microsyneresis. Consequently, the model was not fully valid for the latter gels.
Faraday Discussions | 1995
Ton van Vliet; Pieter Walstra
When gels are used in practice, their large-deformation and fracture characteristics are mostly far more relevant than small-deformation characteristics. In this paper fracture behaviour is discussed of various types of gels, viz. polymer and particle gels, the latter with fairly low and very high volume fraction of particles. First, a general introduction is given on theoretical aspects of fracture mechanics of gels, which involves an essential extension of classical fracture theories. The relationship between large-deformation and fracture behaviour of a gel and its structure proves to be far more complicated than for small-deformation properties. The main reasons for this difference are: (i) the much more important effect of relatively large inhomogeneities on fracture properties and (ii) some very different causes for the strain rate dependence. Not only are the average distance between cross-links and the average stiffness of the strands connecting them of importance, but also the distribution of these parameters. Moreover, inhomogeneities, be it defects (of µm to mm scale) or weak regions (e.g. in composite gels) may have an overriding effect on the fracture properties. To understand the strain rate dependence, one should consider the energies involved as a function of the deformation rate and distinguish between the amount elastically stored during deformation, the amount dissipated due to viscous flow or due to friction processes and the net fracture energy. Moreover crack initiation and fast ‘spontaneous’ crack growth (crack propagation) have to be distinguished. The factors mentioned cause large deformation and fracture properties to be much more strongly dependent on the physical structure of a gel than are the small deformation properties.
Journal of the American Oil Chemists' Society | 2000
W. Kloek; Pieter Walstra; Ton van Vliet
Journal of the American Oil Chemists' Society | 2000
W. Kloek; Pieter Walstra; Ton van Vliet
Journal of Texture Studies | 2005
Wiliam Kloek; Ton van Vliet; Pieter Walstra
ChemInform | 2001
Pieter Walstra; W. Kloek; T. van Vliet
Archive | 2003
Eric Dickinson; Ton van Vliet; Pieter Walstra
Journal of Texture Studies | 2005
Wiliam Kloek; Ton van Vliet; Pieter Walstra
Archive | 1987
Pieter Walstra; H. Luyten; Ton van Vliet