Physics of colloidal particles with heterogeneously patterned surfaces
- Emanuela Bianchi (Technische Universität Wien, Austria)
- Gerhard Kahl (Vienna University of Technology, Austria)
- Christos N. Likos (University of Vienna, Austria)
- Francesco Sciortino (University of Rome I 'La Sapienza', Italy)
Directional interactions, selective bonding mechanisms and limited valence are nowadays considered among the key ingredients to assemble mesoscopic structures with well defined symmetries and physical properties. Novel colloid-based units are able to assemble into a wealth of structures at the nano- and microscale level via either self- or field driven- organization. These fundamental building blocks are characterized by a heterogeneously patterned surface: a limited number of surface regions (denoted as “patches”) differ from the bare colloidal surface in their interaction properties; via these patches the particles are able to realize the highly directional and selective bonds mentioned above. These particles are commonly referred to as “patchy particles” [1, 2]. The self-organization of patchy units into well defined meso- and macroscopic structures does significantly extend the realm of traditional materials, by virtue of the asymmetry and selectivity in their interaction patterns .
The patterned surface of the colloidal particles is most commonly realized via suitable chemical or physical synthesis processes: fixing magnetic patches on the particle surface [4, 5], decorating colloids via particle stamping or vapour deposition [6, 7], or anchoring double- or single-stranded DNA-chains on the colloidal surface [8, 9] are only a few recent examples of how patchy particles can be synthesized. These examples already serve as ample demonstration that the patchy colloid field is highly interdisciplinary, involving physics, chemistry, chemical engineering and bio-related sciences.
In recent years, considerable progress has been made in tailoring the position, the interaction properties and the number of patches during the experimental synthesis processes. The impressive potentialities offered by the new experimental achievements need, however, information about how to improve the design of patchy particles in order to realise self-assembled structures with the desired properties. Thus, a close cooperation between experimentalists on one side and theoreticians and computer simulators on the other side is of great importance: suitable modelling of the experimental particles can provide guidelines on which features of the patchy units may favor target mesoscopic structures. Patchy particles have meanwhile become a steady topic in many conferences, where – according to the respective scientific orientation of the meeting – physical, chemical or biophysical aspects of these particles are addressed. In an effort to merge the different scientific communities and the diverse approaches related to patchy particles, we take the initiative of proposing to organize a workshop on this highly actual topic. We consider the format of a workshop as an ideal stage to realize the following goals:
• bring together scientists from different fields (physics, chemistry, biophysics);
• bring together experimentalists, theoreticians, and computer simulators;
• offer – in the spirit of a workshop – ideal conditions for extensive discussions on this highly active and rapidly developing field.
Even though considerable progress has been made during the past years in synthesizing patchy particles, experimental studies on the self-assembly processes of such units are so far rather rare because of the small yields of synthesized particles. However, there is no doubt about the great potential of such colloids as self-assembling units of completely new macroscopic structures. Three challenges have to be faced: the fine control on the surface patterns (size, shape, positions and orientation of the patches), the richness of the pattern morphologies (number of patches per particle), and the scalability of the methods (large amount of synthesized particles). Various production methods have been put forward to address these issues, but some limitations still remain. Particle lithography , glancing angle deposition , particle contact stamping , and nano-sphere lithography  are just few examples of experimental top-down techniques to fabricate patchy colloids. Another possible route towards multi-functional patchy colloids is the fabrication of anisotropic colloidal clusters [11, 12] whose components can be selectively modified to induce directional specific bonding. To date, while a fine control on the patch size and position is possible, the number of patches per particle is typically limited.
Great efforts are still devoted to the creation of colloids with rich surface patterns, consisting of many, geometrically arranged, patches. Richer patch decorations, together with higher particle yields, are the present goals of the aforementioned fabrication techniques.
An entirely different class of particles with designed surface functionality is obtained by following the bottom-up route. Instead of using macroscopic, externally-controlled tools to direct the assembly, bottom-up techniques allow to obtain precisely defined nano- and micro-scale units once the appropriate sub-units are chosen. A well known phenomenon is the self-organization into micelles and vesicles when macromolecules with a solvophobic and a solvophilic group are dispersed in a specific solvent. Such aggregates may possibly have patchy surfaces, i.e., the whole surface of the complex aggregate may show extended regions where the solvophobic or solvophilic blocks prevail. Janus-like polymer vesicles, patchy or multi-compartments micelles , block copolymer nanoparticles with microphase separation structures  are some of the most recent successful examples of self-assembly processes that lead to patchy particles. The advantage of bottom-up strategies with respect to the top-down approaches lies in the possibility of producing large amounts of patchy units, once the fine control on the average features of the aggregates is achieved by means of tuning both the microscopic properties of the molecular units and the external conditions.
Theory and Simulations
From the theoretical and the numerical point of view, many model systems have been developed during the past years to include patchy directional interactions in the pair potential between colloidal particles. The first patchy models have been introduced in the 1980s, with the goal of numerically studying associating fluids in a simple and general way [15, 16]. These simple models can be treated within mean field theories: density functional theory and a thermodynamic perturbation theory known as Wertheim theory are able to provide a very good description of the phase behavior of the disordered phases [15, 16]. Nowadays, extensions of such mean field approaches are being developed to include a wide range of possibilities which occur mainly in colloidal or protein systems rather than in molecular and atomic systems, e.g. multiple patch-patch bondings or temperature dependent valencies. Theoretical and numerical investigations of patchy models have played a key role in gaining a deeper understanding of a wide range of physical properties and phenomena, such as the relation between chemical and physical gelation, the dynamics of supercooled liquids and the formation of kinetically arrested states, the stability of the liquid phase with respect to crystal formation, the interplay between locally favored ordered clusters and crystallization. Moreover, patchy systems have significantly contributed to the self-assembly field. On a molecular scale, inspired by the behavior of protein units, a large variety of minimal patchy models, have been designed to aggregate into monodisperse clusters of various symmetries [17, 18, 19] or into specific folded structures . On a macroscopic scale, thanks to the asymmetry in shapes and/or the anisotropy in the interactions, patchy particles have proven to aggregate into a wealth of ordered as well as disordered nano- and macro-scale structures. The relationship between patchiness and equilibrium target structures have been extensively investigated via several coarse-grained patchy models [21, 22].
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