  Shear thickening is a type of non-Newtonian behavior in which the stress
required to shear a fluid increases faster than linearly with shear rate. Many
concentrated suspensions of particles exhibit an especially dramatic version,
known as Discontinuous Shear Thickening (DST), in which the stress suddenly
jumps with increasing shear rate and produces solid-like behavior. The best
known example of such counter-intuitive response to applied stresses occurs in
mixtures of cornstarch in water. Over the last several years, this
shear-induced solid-like behavior together with a variety of other unusual
fluid phenomena has generated considerable interest in the physics of densely
packed suspensions. In this review, we discuss the common physical properties
of systems exhibiting shear thickening, and different mechanisms and models
proposed to describe it. We then suggest how these mechanisms may be related
and generalized, and propose a general phase diagram for shear thickening
systems. We also discuss how recent work has related the physics of shear
thickening to that of granular materials and jammed systems. Since DST is
described by models that require only simple generic interactions between
particles, we outline the broader context of other concentrated many-particle
systems such as foams and emulsions, and explain why DST is restricted to the
parameter regime of hard-particle suspensions. Finally, we discuss some of the
outstanding problems and emerging opportunities.
