An array of honeycomb substrates comprises honeycomb substrates, a plurality of which have, for each substrate, substrate cells extending from a first end of the respective substrate to a second end and substrate sides extending from the first end to the second end. The substrates of the plurality are arranged in an array with sides of respective substrates facing one another and cells of respective substrates extending in a common direction. One or more channels are defined by facing substrate sides of two or more substrates of the plurality, and the one or more channels extend in a direction perpendicular to the common direction.