December 21, 2011 Wednesday 15:30
Cem Yolcu, Carnegie Mellon University
Effective field theory of surface-mediated forces in soft matter

Abstract:Particles adsorbed at soft surfaces such as fluid-fluid interfaces and biological membranes generally deform and constrain the surface in their vicinity. As a consequence of these mechanical effects, the (free) energy of the "modified" surface ends up depending on the spatial arrangement of the particles; forces between the particles arise. Instances of such interactions can be come across in everyday experience between corn flakes floating on your bowl of milk or the froth on your coffee, as well as in the microscopic realm of soft matter physics and surface science between particles (such as proteins or viral capsids) stuck on a biological membrane or colloids adsorbed (for engineering purposes) at the interface between two immiscible liquids. In the microscopic case, forces arising from the constraints of the particles on the thermal fluctuations -- a thermal analog of the quantum electrodynamical Casimir effect -- are expected to be appreciable as well. In the past two decades or so, there have been several attempts to analytically predict the strength and functional form of such interactions. The talk concerns a recent one of these. The approach to view the particles as objects coupling to a fluctuating field has been a fruitful one. However, performing the corresponding partition sum is not particularly easy, as there are multiple regions on the surface occupied by the particles where fluctuations are constrained. I will discuss an approach that avoids this difficulty at the cost of augmenting the surface action/Hamiltonian by local terms, effectively reducing the finite-sized particles to points. However, these "mock" point-particles can still retain all the finite-size information pertaining to the mechanical effects of the real particles on the surface, making the augmented theory effectively the same as the constrained one; hence the name effective field theory. This approach makes the physical situation more transparent, the computations more tractable and allowed us to extend existing predictions on pair forces between asymptotically distant particles by several many-body and intermediate distance corrections.