Previous research in CSCW has highlighted the importance of proximity in informal interactions. Nowadays, most current tools aimed at supporting informal interactions are focused on providing artificial proximity in traditional office environments in which workers spend most of their time "behind their desk." However, in other working environments, such as hospitals, users experience a high level of mobility that enables them to establish co-located interactions in order to collaborate and coordinate their activities with colleagues, involving the exchange and analysis of documents distributed in space or time. These forms of interaction pose new challenges for the design of pervasive computing environments aimed at providing collaboration support as people work on-the-move, seamlessly integrating heterogeneous resources and devices scattered around the premises. In order to address these concerns, we proposed and developed the concept of On-the-move Collaborative Environments (OCEs), a design concept integrating an ensemble of specialized services to provide support for informal interaction for mobile work. Further, we informed OCE's conceptualization with insights obtained from a workplace study conducted in a hospital, and from projections of envisioned support identified by means of scenarios of use. To exemplify the design principles proposed, we designed and implemented SOLAR, an OCE system aimed at supporting co-located collaboration, proximity-based application-sharing, and the remote control of heterogeneous devices.