Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 912–921 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jo ur nal ho me p ag e: www.elsevier.com/locate/colsurfa The jamming and unjamming transition in poly(N-isopropylacrylamide) microgel suspensions Ruiwen Shu, Weixiang Sun ∗ , Yang Liu, Tao Wang, Chaoyang Wang, Xinxing Liu, Zhen Tong ∗ Research Institute of Materials Science and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China h i g h l i g h t s • A jamming phase diagram for the microgel suspensions. • Synergetic effect of temperature and concentration. • Nonlinear viscoelasticity by LAOS. g r a p h i c a l a b s t r a c t The jamming and unjamming transition in poly(N-isopropylacrylamide) microgel suspensions were studied. a r t i c l e i n f o Article history: Received 8 May 2013 Received in revised form 8 August 2013 Accepted 12 August 2013 Available online 20 August 2013 Keywords: poly(N-isopropylacrylamide) Microgels Large amplitude oscillatory shear Jamming a b s t r a c t The jamming and unjamming transition in thermo-sensitive poly(N-isopropylacrylamide) (PNIPAm) microgel suspensions was observed by rheology with three independent variables of temperature T, microgel weight fraction w, and the shear stress . The effective volume fraction ϕ eff , depending on both T and w, was adopted to interpret this transition. In the jamming state, the plateau modulus G ′ p fol- lowed the same power law to ϕ eff with varying T and w, indicating that the change of the interaction potential with temperature was insignificant. However, the volume fraction ϕ c at the jamming transition decreased with increasing w because higher concentration of particles led to stronger repulsive interac- tion. The shear-unjamming transition occurred as the shear stress exceeded the yield stress  y . A jamming phase diagram for the microgel suspension was established in a [1/w, R h 3 /(k B ), T] coordinate. The non- linear viscoelasticity was observed by the large amplitude oscillatory shear (LAOS) analysis within the shear-unjamming region, which suggested that the shear-unjamming state under high strain would not be equivalent in rheology to the unjamming state appeared at high temperature or low concentration without shearing. © 2013 Elsevier B.V. All rights reserved. 1. Introduction A variety of amorphous materials, such as sand, toothpaste, mayonnaise, and shaving foam, exhibit a mix of liquid-like and ∗ Corresponding authors. Tel.: +86 20 87112886; fax: +86 20 87110273. E-mail addresses: mswxsun@scut.edu.cn (W. Sun), mcztong@scut.edu.cn (Z. Tong). solid-like behaviors [1,2]. By varying temperature and external stress, these amorphous materials undergo a transition from a freely flow unjamming state to a jamming state. Liu and Nagle [3] presented a jamming phase diagram as a conceptual framework for the jamming transition. They suggested a possible analogy between the effects of temperature and shear stress on the transition. For colloidal suspensions, the control variables for the jamming tran- sition are temperature T, volume fraction ϕ, and external stress . Trappe et al. replaced the temperature axis by k B T/U for attractive 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.08.031