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Does the PDE $u_t = -\Delta^2 u -\Delta u + f$ have a physical use or meaning? I am asking specifically about the the Laplace term after the biLaplace term.. is it unusual or "unnecessary" in some way to have both terms at the same time?

Thank you

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    In the stationary case $u_t=0$ you can find a lot of information (physical interpretation for example) in this book: http://www.amazon.com/Polyharmonic-Boundary-Value-Problems-Mathematics/dp/3642122442/ref=sr_1_1?ie=UTF8&qid=1356369525&sr=8-1&keywords=polyharmonic2012-12-24

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This is the linearization of th Cahn-Hilliard equation about a constant state. The equation is used to model phase separation in binary mixtures and alloys. The quantity $u$ then is a volume fraction.