Laminar flow of a suspension of spheres in a gelled fluid through a sudden expansion causes the suspended matter to become organized. Segregation and migration phenomena of mono and bi-disperse suspensions have been highlighted. If different sizes of spheres and bimodal suspensions are used, this phenomenon can be used to form structured materials. Flow conditions are such that gravity effects and inertia are negligible. At rest, the spheres are stable in the gel. Nuclear magnetic resonance imaging was used to reveal the distribution of matter within the suspension downstream of the expansion. The interest of such structures for industrial applications is discussed. (C) 2004 American Institute of Chemical Engineers.