Project proposes a petaflop-scalable computational infrastructure for the direct simulation of particulate flows, in particular the simulation of spatio-temporal dynamics of platelet aggregation. Better understanding of microcirculation of blood and platelet rheology will impact clinical needs in thrombosis risk assessment, anti-coagulation therapy, and stroke research. The proposed method comprises two algorithmic components: (1) integral equation solvers for Stokesian flows with dynamic interfaces; and (2) scalable fast multipole algorithms. Why do we need petaflop-scale computing power to tackle this problem? One microliter of blood contains millions of red blood cells(RBCs) and a few hundred thousand platelets. Discretizations with O(100 points/cell and O(1000) time steps result in more than a trillion space-time unknowns. Solving problems of such size will require 50K-core machines. Computational tools that achieve such scalability, will enable direct numerical simulation of several microliters of blood, once million-core computing platforms are available.