ZAU Controller experienced loss of separation event involving two descending aircraft when failing to comply with the required verbiage during the application of visual separation.

Narrative

Cessna 425 was descending to 11;000 with a good rate down and I stopped the aircraft at 13;000. I wanted to get the A320 under the Cessna 425 because the A320 was overtaking the Cessna 425 by a large difference in speeds. I descended the A320 to 14;000 and called traffic to each airplane. Cessna 425 was the first plane to call the other in sight; so I asked the aircraft if he could maintain visual separation with the A320 when the A320 descends. The Cessna 425 stated he was able to maintain visual separation with the A320; so I cleared the A320 to cross FISSK waypoint at 11;000 feet. I advised the A320 aircraft that Cessna 425 had him in sight and would maintain visual separation from him. Although I thought I gave the Cessna 425 an instruction to 'maintain visual separation from the A320;' upon review of the tapes afterward I forgot to give that instruction. I was trying to abide by my LOA crossings with SBN ATCT and thought I gave that instruction; but apparently never did. On the same token; it appears the rules written for visual separation were written to avoid us using this tool. Perhaps we can take another look at why we need to give so many transmissions to an aircraft to maintain visual separation. In a busy sector like LOOTH; it takes away a valuable tool to move air traffic; and it actually creates more workload for me. Either I leave A320 high for MDW and have to APPREQ the altitude with SBN as well as point out to BEARZ; or I vector the Cessna 425 well out of the way thereby providing a disservice to the pilot.