1. Department of Physics and Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA


2. Department of Chemistry, Columbia University, New York, New York 10027, USA


3. Department of Physics, University of North Texas, Denton, Texas 76203, USA


4. Present addresses: Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden (J.M.); TU Eindhoven, Institute for Complex Molecular Systems and Department of Mechanical Engineering, PO Box 513, 5600 MB Eindhoven, The Netherlands (H.M.W.); School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA (A.F.-N.); Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan (K.M.).

Correspondence to: Johan Mattsson^1,4 Correspondence and requests for materials should be addressed to J.M. (Email: johanm@chalmers.se).

Glass formation in colloidal suspensions has many of the hallmarks of glass formation in molecular materials^{1, 2, 3, 4, 5}. For hard-sphere colloids, which interact only as a result of excluded volume, phase behaviour is controlled by volume fraction, ; an increase in drives the system towards its glassy state, analogously to a decrease in temperature, T, in molecular systems. When increases above *  0.53, the viscosity starts to increase significantly, and the system eventually moves out of equilibrium at the glass transition, _g  0.58, where particle crowding greatly restricts structural relaxation^{1, 2, 3, 4}. The large particle size makes it possible to study both structure and dynamics with light scattering¹ and imaging^{3, 4}; colloidal suspensions have therefore provided considerable insight into the glass transition. However, hard-sphere colloidal suspensions do not exhibit the same diversity of behaviour as molecular glasses. This is highlighted by the wide variation in behaviour observed for the viscosity or structural relaxation time, , when the glassy state is approached in supercooled molecular liquids⁵. This variation is characterized by the unifying concept of fragility⁵, which has spurred the search for a 'universal' description of dynamic arrest in glass-forming liquids. For 'fragile' liquids, is highly sensitive to changes in T, whereas non-fragile, or 'strong', liquids show a much lower T sensitivity. In contrast, hard-sphere colloidal suspensions are restricted to fragile behaviour, as determined by their  dependence^{1, 6}, ultimately limiting their utility in the study of the glass transition. Here we show that deformable colloidal particles, when studied through their concentration dependence at fixed temperature, do exhibit the same variation in fragility as that observed in the T dependence of molecular liquids at fixed volume. Their fragility is dictated by elastic properties on the scale of individual colloidal particles. Furthermore, we find an equivalent effect in molecular systems, where elasticity directly reflects fragility. Colloidal suspensions may thus provide new insight into glass formation in molecular systems.