The spreading of solid powder over a liquid surface is a prevalent phenomenon encountered in many industrial processes such as food and pharmaceutical processes. The driving force for powder spreading over a liquid surface is not clearly understood. The Marangoni effect due to a temperature gradient and the spreading coefficient for solid powder over liquid (lambda(S/L)) have both been proposed as causes for powder spreading over liquids. The proposed lambda(S/L) was based on the same form of the spreading coefficient for a liquid over a solid surface (lambda(L/S)). Whereas lambda(L/S) has a clear thermodynamic definition, the spreading coefficient of solid powder over liquid, lambda(S/L), which was defined by simply interchanging the subscripts of the interfacial energy terms, has not been thoroughly analysed. Our experimental results showed that the spreading behaviour of solid powders over liquids cannot be explained or predicted by lambda(S/L). In this study we focus on problems associated with the lambda(S/L). Through a thermodynamic analysis we conclude that the existing parameter lambda(S/L), is unable to predict the spreading behaviour of solid powder on liquid surface, since the interfacial energy approach does not capture the actual physical process of powder spreading over liquid surface. A closer examination of the powder spreading process reveals the fundamental different between liquid spreading over solid surface and solid powder spreading over liquid. This work shows that further research is required to identify and analyse the physical mechanisms which are responsible for powder particles spreading over liquid surfaces. (C) 2010 Elsevier B.V. All rights reserved.