ORIGINAL CONTRIBUTION The rheological properties of hydrogenated castor oil crystals Niels De Meirleir & Linda Pellens & Walter Broeckx & Guy van Assche & Wim De Malsche Received: 31 March 2014 /Accepted: 30 May 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract The present study focuses on the rheological performance of a surfactant-rich aqueous suspension containing hydrogenated castor oil (HCO) crystals. HCO can be typically crystallized in five distinct shapes: spherically shaped, irregularly shaped, star- shaped (also called rosettes), short needles, and thick or thin fibers. The effect of the differences in shape on the rheological performance is studied, and the rheolog- ical properties are compared to the behavior of other triacylglycerol’s (TAG) suspensions. A suspension of TAG crystals usually behaves as a colloidal gel wherein a colloidal gel is defined as a network of flocs, with each floc being an aggregate of smaller subunits. All of these surfactant-rich aqueous suspensions of HCO crys- tals behaved according to a colloidal gel in the transient regime, independent of the studied crystal shapes, ex- cept the long thin fibers at a concentration above 0.1 wt% HCO transitioning from a heterogeneous fractal rod network to a homogeneous rod network, shifting from a colloidal gel to a glass. Keywords Hydrogenated castor oil . Fibrous crystals . Triacylglycerol . Rheology modifier Introduction Hydrogenated castor oil (HCO) is a triacylglycerol (TAG) [1, 2] and is a derivative of castor oil [3]. It is commonly used as lubricant and can be found in coatings, carbon paper, cos- metics, polish product, etc. The crystallized form of hydroge- nated castor oil (as described in this work) is less known and scarcely described in the literature, despite its extensive use as a rheology modifier to increase the low-shear viscosity and to prevent phase separation in many household products (e.g., heavy-duty detergents). Other examples of rheology modifiers found in the industry are acrylic polymers [4, 5], alginates cellulose derivatives [6], gums obtained from the endosperm of plant or bacterial origin (for example, guar gum, locust bean gum, and xanthan gum [7]), organo-clays [8], and silica-based compounds [9]. These rheology modifiers are commonly used in food, pharmaceutical, personal care, and household formulations. For all rheology modifiers, a 3D network is formed above the percolation threshold concentration [10]. The mode of interaction within this 3D network strongly influences the rheological properties of the suspension. For example, the rheological properties of chain structures such as flexible polymers (e.g., a solution of polystyrene, polyisoprene, poly- butadiene [11], λ DNA [12], and some hydrocolloids such as gums), semi-flexible polymers (e.g., actin filaments [13, 14]), and rigid rod polymers [15] come from the fact that these fibers behave as in the semi-diluted regime, being hindered in transversal diffusion through entanglement [16]. On the other hand, the rheological properties of colloidal particles like hard spheres (e.g., PMMA particles) [17], soft spheres [16], and microgels [18] originate from hydrodynamic forces and col- loidal interactions. Furthermore, the rheological properties of some suspensions can be influenced by their polymeric as well as their colloidal character. This is observed with N. De Meirleir : W. De Malsche Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium G. van Assche Department of Materials and Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium N. De Meirleir (*) : L. Pellens : W. Broeckx Procter & Gamble Eurocor N.V., Temselaan 100, 1853 Strombeek-Bever, Belgium e-mail: ndemeirl@vub.ac.be Colloid Polym Sci DOI 10.1007/s00396-014-3298-5