8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Wed 711:10-14:00P5.43Interactions between heavy colloids induced by softcross-linked polymer substrateLorenzo Di Michele, 1 Taiki Yanagishima, 1 Anthony R. Brewer, 1 Jurij Kotar, 1 SethFraden, 2 and Erika Eiser 31 University of Cambridge, Cavendish Laboratory, JJ Thomson Av CB3 0HE, Cambridge,United Kingdom2 Brandeis University, Waltham, United States of America3 University of Cambridge, Cambridge, United KingdomWe studied aggregation and surface-mediated interactions between silica colloids (1.16µmdiameter) sedimented on soft cross-linked polyacrylamide gels. Investigation of new interactionsbetween colloids in confined geometries, with the target of inducing controlled self-assembly ofordered phases, is crucial for applications such as engineering of photonic crystals and porousmaterials. Polymer gels are often used as substrates for testing the response of cellular biologicalprocesses to variations in the mechanical properties of the environment [1]. Nonetheless, acomplete understanding of interactions between inanimate micron-size objects confined by thesesubstrates is, in our opinion, still lacking. We classify gels with bulk elastic shear modulus G’between 55 and 522 Pa as ”soft” and those with G ′ = 12 Pa as ”ultra-soft”. Colloids sedimentedon soft substrates only formed small aggregates while those confined to ultra-soft gels showedlarge scale aggregation in both amorphous and ordered clusters. Combining artifact-correctedvideo microscopy and optical trapping we measured colloids pair-potentials and colloid-surfaceinteraction potentials. In the pair-potentials we found a short-range attraction that become strongeron the ultra-soft gels and can explain the aggregation. The colloid-colloid potential is related to acolloid-surface attraction that is again stronger on the ultra soft gels. We suggest an explanationof these effects as a combination of depletion-like attraction and polymer-colloid adhesionsubsequent to a partial inclusion of the beads into the upper layers of the gel [2]. Nonetheless, wehypothesized that elastic energy stored in the substrates as a consequence of the indentation dueto heavy colloids could partially account for the large-scale aggregation observed for ultra-softsubstrates. In order to check this hypothesis overcoming experimental limitations we performeda set of Monte Carlo simulations. The results confirmed that elasticity mediated interaction caninduce aggregation when soft enough substrates are used.[1] Discher et Al. , Science 310 1139-1143 (2005).[2] Chen and Ma, J. Phys. Chem. B 109 17617-17622 (2005).43