Transp Porous Med (2007) 70:335–353 DOI 10.1007/s11242-007-9103-x ORIGINAL PAPER Wetting front instabilities: a three-dimensional experimental investigation Blake P. Tullis · Steven J. Wright Received: 6 September 2006 / Accepted: 12 January 2007 / Published online: 9 March 2007 © Springer Science+Business Media B.V. 2007 Abstract Wetting front instability or fingering experiments were conducted in three- dimensional infiltration columns, featuring stratified fine-over-coarse texture porous media, to investigate the influences of various soil and wetting phase properties on finger diameter and propagation velocity. The system parameter varied in this study included permeability, system flux rate, media size and gradation uniformity, initial moisture content, viscosity, density, and surface tension. The influence of each param- eter is discussed and compared, where applicable, to the finding of previous studies. Finger diameter and velocity data were acquired using a neutron radioscopy based, real-time imaging system. Through the use of the imaging system, a very accurate and reliable experimental data set was produced for three-dimensional fingering events. Keywords Fingering · Wetting front instability · Infiltration · Neutron radiography 1 Introduction Preferential flow paths through porous media can result from fractures, wormholes, or through the development of wetting front instabilities. Wetting front instability refers to the development of a non-planar wetting front as a wetting phase displaces a non- wetting phase in porous media. Wetting front instabilities in porous media typically begin as interfacial disturbances, with the most-unstable wave lengths developing into isolated flow columns commonly referred to as “fingers.” Once fingers form, the origi- B. P. Tullis (B ) Civil & Environmental Eng., Utah State University, 8200 Old Main Hill, Logan, UT 84322-8200, USA e-mail: blake.tullis@usu.edu S. J. Wright Civil & Environmental Eng., University of Michigan, 1351 Beal Ave, 113 EWRE, Ann Arbor, MI 48109-2125, USA e-mail: sjwright@umich.edu