Haptic Perception of Viscosity Wouter M. Bergmann Tiest 1 , Anne C.L. Vrijling 2 , and Astrid M.L. Kappers 1 1 Helmholtz Institute, Utrecht University, The Netherlands {w.m.bergmanntiest,a.m.l.kappers}@uu.nl 2 Royal Dutch Visio, National Foundation for the Visually Impaired and Blind, Huizen, The Netherlands annevrijling@visio.org Abstract. Viscosity is a liquid’s resistance against flow. Using a discrimination experiment, the human ability to distinguish between different viscosities was measured over the range of 200–20,000 mPa·s. Eight blindfolded subjects stirred pairs of different silicone oils using a wooden spatula and had to indicate the “thicker” of the two. The viscosity of the liquids was measured seperately using a rheometer. Weber fractions for discrimination ranged from 0.3 at high viscosi- ties to almost 1 at the lowest viscosity. For the higher viscosities, discrimination could be described as Weber-like, but for the low viscosities, there seemed to be a floor effect for the absolute threshold. The characterisation of the discrimina- tion threshold as a function of viscosity is of fundamental interest in perception research, but also of practical value for designers of haptic devices capable of displaying viscosity. Keywords: Kinaesthesia, Dynamic touch, Thresholds, Liquid, Weber fraction. 1 Introduction The viscosity or “thickness” of a liquid is a property that can be easily perceived hap- tically, as is noticable in the everyday context of stirring in a pot on the stove. But for certain professionals, such as dough-makers or cooks, being able to accurately judge viscosity is of great importance. Medical professionals should be able to correctly judge the viscosity of texture-modified liquids that they prescribe to patients with orpharyn- geal dysphagia. These patients have a delayed swallow initiation and benefit from thick- ened liquids that enter the pharynx slowly [1]. The cited study showed that information on the product packaging was insufficient to reliably characterise the liquid’s viscosity, whereas clinicians could perfectly discriminate between different viscosities by stirring the liquids or by taking a sip. The importance of being able to rely on somesthetic perception is clear. Physically, viscosity is the internal resistance of a liquid against shear force. It is expressed as the ratio between shear stress and shear rate. Shear stress is the amount of force applied in the direction of shear per unit area (in N/m 2 or Pa). Shear rate is the gradient perpendicular to the force of the liquid’s moving speed (in m/s/m or s –1 ). Viscosity is thus expressed in units of Pa·s. Water has a viscosity of 1 mPa·s. The relationship between physical and perceived viscosity has been investigated for silicone A.M.L. Kappers et al. (Eds.): EuroHaptics 2010, Part I, LNCS 6191, pp. 29–34, 2010. © Springer-Verlag Berlin Heidelberg 2010