There is a coworker who regularly ignores my emails, even though his technical input is critical to my portion of the design. I usually have to send him a "bump" email after a week to remind him that I am still waiting, but now even the bump emails get no response. This has gone on for a few months. I really do prefer getting a response in email form instead of talking face to face since there is traceability. In my opinion, he should be able to come up with the answers within a couple short minutes. If that is not the case, it would be nice to get some sort of "I am working on this and will get you a response by ..." acknowledgement.
What would be the correct action here to end this practice?