The insight into structure formation in soft matter from liquid crystals to polymers and colloids relies in many ways on our understanding of how the respective building blocks pack, the best-known examples being hard spheres, rods, and platelets. The insight into packing of soft particles is much less comprehensive, yet during the past decade the body of experimental and theoretical work on systems based on deformable and compressible colloidal particles such as star polymers [1], hydrogel spheres [2], dendrimer micelles [3, 4], and block copolymer micelles [5] reached a level where certain patterns began to appear. For example, the salient features of model dendrimers are cluster crystals [6] as well as the otherwise exotic A15 and sigma phase [7-10].

In view of the rich phenomenology reported, the field is in need of rules, mechanisms, and models relating the phase behavior, rheology, and other aspects of physics involved to the interparticle interaction. Ideally, these rules be simple much like the Onsager theory of nematic phase or the clustering criterion [11]. At the workshop, we wish to make a step ahead towards universal models of soft colloids, aiming at identifying a few working hypotheses to be explored in the future. This is a speculative endeavor, and the CECAM workshop will provide a suitable forum for out-of-the-box discussions of the possible form and scope of these models. The aim of the workshop is twofold

• To compare the experimental and theoretical insight into the structure formation in soft nanocolloidal particles, attempting to outline the generic features of their phase diagram to including orientational, bond-orientational, and other partly ordered mesophases.

• To discuss experimental, numerical, and theoretical methods as well as models applicable to these problems so as to identify the most efficient approach leading to as unified an understanding as possible.

Among the more concrete objectives, we wish that the workshop will address the following open questions:

• How to distinguish between the generic and the specific structural features reported both experimentally and theoretically in various soft nanocolloids? For example, what is it that the different shoulder potentials have in common, and in what respect are they different?

• What is the best working definition of softness in nanocolloidal particles? How to distinguish between particle penetrability and deformability?

• How to experimentally quantify soft nanoparticle shape both in dilute solutions and in condensed phases? What is the shape of soft nanocolloids? Is a large-scale parameter such as the aspect ratio sufficient for the description of shape?

• How do particle softness and anisometry affect the nature and the stability of the bond-ordered phases, and how does bond order interfere or correlate with orientational order in mesophases?

• How is clustering affected by particle anisometry?

• In crystals of penetrable anisometric particles, the interplay of positional and orientational order can be quite pronounced yet different than in hard rods. At small anisometry the orientational order should not matter much and positional order should be the defining feature of the crystals, whereas at large anisometries the orientational order should be dominant. Is the phase sequence similar than in hard rods or not?

• One of the neat recent ideas is the Miller-Batista deformable-droplet model and its implementation in a simulation code [12]. Can it be generalized to non-spherical particles?

• How important is the non-pairwise additivity of interactions in real nanocolloids? What are the most evident signatures of non-pairwise additivity?

The workshop is planned to extend over six half-days. Apart from the invited speakers, we expect some 30 additional participants who can present their results in a poster session. The number of invited speakers has been kept low offering thereby the possibility of two round tables. The topics will be distributed to the participants two months before the workshop so that they will be able to be prepared how to best contribute to the problems to be discussed.