Mesler entrainment is the formation of a very large number of very small bubbles by a relatively low velocity drop impacting a liquid surface. The role of the Weber number in Mesler entrainment has received significant attention. However, the effect of the capillary number, which quantifies the relative importance of viscous and surface tension forces, has not been explored. This is due primarily to the fact that virtually all Mesler entrainment research has used a single liquid, water, as the working fluid. This, combined with certain experimental restrictions, makes difficult an independent variation of the Weber and capillary numbers. To address this problem, Mesler entrainment was investigated using two silicone oils, having kinematic viscosities of 0.65 cSt and 10.0 cSt, respectively, revealing the effect of the capillary number on Mesler entrainment, a result which has not been obtained heretofore. The silicone oils give extremely repeatable results when compared to water. (C) 2012 American Institute of Chemical Engineers AIChE J, 2012