Some thoughts on ``Where are fluid mechanicians?''

I think that the note by Zhigang Suo puts forth a pressing issue. Coming from remotely related areas in physics (quantum mechanics and electromagnetic theory), my own interests and work have recently evolved to topics in both solid mechanics (crystal surfaces) and aspects of fluid mechanics (advection-diffusion and a most recent start on biomembranes). Hence, practically, I cannot see any boundaries between these disciplines.

From an applied mathematics perspective, I have the following comments. Historically, the two communities (fluid and solid mechanics) seemed to use similar mathematical tools (PDEs, asymptotics etc). However, their problems are of course very different. Without having worked in fluid mechanics enough, I tend to believe that solid-mechanics problems were traditionally perceived as ``harder''; especially due to the inclusion of elasticity (non-local effects), and - perhaps later on - the immediate, urgent relevance of the atomistic structure and calculations. (In solid mechanics, it is more urgent to consider what atoms actually do, especially near a crack tip or other defects.) This specialization might have caused divergence in how to think about problems. However, nowadays such boundaries tend to disappear. For instance, atomistic considerations become more and more relevant to fluid mechanics as well. Both disciplines are driven to ever smaller scales, where some convergence of views is possible and plausible. Allow me to add that applied mathematical principles can serve as the common ground, or catalyst, for such a convergence of views. In addition, there are examples of applied mathematicians who have applied techniques traditionally learned in fluid mechanics and gas dynamics (boundary layer theory, shocks etc) to new problems in solid mechanics and materials science. Howard Stone (with Mike Aziz and myself) and Michael Brenner (with Mike Aziz) are two fresh examples that come to mind.