Aging and memory effects in colloidal glasses

– We study the effect of shear on the aging dynamics of a colloidal suspension of synthetic clay particles. We find that a shear of amplitude γ reduces the relaxation time measured just after the cessation of shear by a factor exp(−γ/γc), with γc ∼ 5%. From this rejuvenation effect, we extract a quantitative strain/temperature correspondence between colloidal systems and molecular or spin glasses. This analogy is furthermore demonstrated by a direct observation of memory effects in suspensions submitted to shearing periods of various strengths. Introduction. – Molecular and spin glasses exhibit very intriguing dynamical properties, which are still poorly understood. One of the most striking features is the aging phenomenon: a slowing down of the microscopic dynamics with the elapsed time. This behavior is generally associated with the existence of many accessible metastable states spanning a broad energy distribution. As time goes on, the material gets trapped in deeper and deeper energy wells for increasing escaping times. As a result, many physical characteristics of these systems (such as rheological [1], or dielectric properties) depend in a non trivial way on their thermal history. In recent years, aging dynamics has been evidenced in very different systems, such as colloidal suspensions [2, 3], dense emulsions, amorphous polymers or weak gels. In these socalled soft glassy materials, temperature can not be varied without drastically modifying the underlying weak interactions between their constituents. It is thus not a convenient parameter to probe such history dependent effects. However, it has been recently suggested that shearing could play an analogous role as temperature by allowing a renewal of the microscopic structure [4, 5, 6]. Quenching of colloidal glasses for instance, can be obtained by applying a temporary strong shearing to the material. This temperature/strain analogy has been extensively developed in different models, and allowed to account for some rheological features of soft glassy materials. () Present address: Physique de la matière condensée, Collège de France (CNRS UMR7125), 11, place Marcelin Berthelot, 75231 PARIS cedex 05