Cuesta, José A.2010-03-102010-03-102006-12Revista Española de Física, 2006, vol. 20, n. 4, p. 13-190213-862Xhttps://hdl.handle.net/10016/72097 págs, 7 figs.-- Número monográfico de la Revista dedicado al Centenario de Ludwig Boltzmann (1844-1906).In spite of the identification between entropy and disorder, there are many phase transitions in which an ordered phase emerges and at the same time entropy increases. In this article it will be shown that this paradox gets resolved by making a literal interpretation of the famous Boltzmann's equation S = k log W. Two examples: freezing of a fluid and demixing of a binary mixture, will illustrate this phenomenon. From them the concept of entropic force or interaction, very useful in polymer or colloid science, will emerge.application/pdfspa© Real Sociedad Española de Físicaresearch articleMatemáticasopen access