We have constructed a novel apparatus to study fundamental interactions between bubbles and particles in drinking-water flotation froths. The high specifications of the apparatus and its modular construction permit experiments on bubble coalescence and bubble-particle adhesion forces to be carried out. We give a description of the apparatus, along with information on the techniques for its use. We observed the effect of a basic constituent of natural waters, humic acid, on bubble coalescence. Intense collision resulted in immediate bubble coalescence (1 ms), whereas less intense collisions prevented, in many cases, bubble coalescence, even at low-concentrated solutions. (C) 2003 Elsevier B.V. All rights reserved.