Assembling responsive colloids – insight from neutron scattering

Responsive colloids such as thermo- or pH-sensitive microgels are ideal model systems to investigate the relationship between the nature of interparticle interactions and the plethora of self-assembled structures that can form in colloidal suspensions. They allow for a variation of the form, strength and range of the interaction potential almost at will. Moreover, due to their soft nature, we can create dispersions with concentrations far above random close packing. These ultra-dense suspensions exhibit fascinating flow properties and can form a variety of different amorphous and crystalline structures [1]. Particularly interesting are ionic microgels [2]. Due to their large number of internal counterions they possess very large polarisabilities, and we can thus use external electrical ac fields to generate large dipolar contributions to the interparticle interaction potential [3]. This leads to a number of new crystal phases, and we can trigger crystal-crystal phase transitions through the appropriate choice of the field strength [4]. However, while the responsiveness and softness of these particles result in intriguing properties, they also pose considerable problems when attempting to characterize the
response of individual particles to their environment at high packing
fractions, where particles can adapt their shape, de-swell as well as
partially interpenetrate. In my seminar I will demonstrate how we can use small-angle neutron scattering together with appropriate contrast variation experiments, and combined with complementary techniques such as small-angle x-ray scattering and confocal microscopy, in order to gain insight into these systems [2,5].

References
[1] P. Mohanty, D. Paloli, J. Crassous, and P. Schurtenberger, in “Hydrogel Micro- and Nanoparticles”, L. Andrew Lyon and Michael J. Serpe, Eds., Wiley VCH, ISBN 978-3-527-33033-1, pp 369 – 396 (2012).
[2] S. Nöjd, P. Holmqvist, N. Boon, M. Obiols-Rabas P. S. Mohanty, R. Schweins, and P. Schurtenberger, Soft Matter 14, 4150 (2018).
[3] T. Colla, P. S. Mohanty, S. Nöjd, E. Bialik, A. Riede, P. Schurtenberger, and C. N. Likos, ACS Nano 12, 4321 (2018).
[4] P. S. Mohanty, P. Bagheri, S. Nöjd, A. Yethiraj and P. Schurtenberger, Phys. Rev. X 5 (2015) 011030.
[5] P. S. Mohanty, S.Nöjd, K. van Gruijthuijsen, J. J. Crassous, M. Obiols-Rabasa, R. Schweins, A. Stradner, and P. Schurtenberger, Scientific Reports 7, 1487 (2017).