A device (100; 200; 300; 400) comprising a sealed cavity (210) containing
n volumes of fluids (80, 87; 220, 230; 320, 332; 420, 422, 430, 432) is
described, where n is an integer and n.gtoreq.2. Each volume of fluid is
substantially immiscible with every contiguous volume of fluid. The
cavity is defined by an interior surface divided into n continuous areas
(60, 170; 260, 270; 360, 362, 370; 460, 462, 470, 472), each continuous
area corresponding to and being in contact with a respective one of the
volumes of fluid. The wettability of each area is such that each volume
of fluid preferentially adheres to the corresponding continuous area
rather than any one of the continuous areas adjacent to the corresponding
area.