Tuning the order of colloidal monolayers: assembly of heterogeneously charged colloids close to a patterned substrate

We study the behavior of negatively charged colloids with two positively charged polar caps close to a planar patterned surface. The competition between the different anisotropic components of the particle-particle interaction patterns is able by itself to give rise to a rich assembly scenario: colloids with charged surface patterns form different crystalline domains when adsorbed to a homogeneously charged substrate. Here we consider substrates composed of alternating (negative/neutral, positive/neutral and positive/negative) parallel stripes and, by means of Monte Carlo simulations, we investigate the ordering of the colloids on changing the number of the stripes. We show that the additional competition between the two different lengths scales characterizing the system ($i.e.,$ the particle interaction range and the size of the stripes) gives rise to a plethora of distinct particle arrangements, where some well-defined trends can be observed. By accurately tuning the substrate charged motif it is possible to, $e. g.,$ promote specific particles arrangements, disfavor crystalline domains or induce the formation of extended, open clusters.Comment: 18 pages, 15 figure

Phase diagram of patchy colloids: towards empty liquids

We report theoretical and numerical evaluations of the phase diagram for patchy colloidal particles of new generation. We show that the reduction of the number of bonded nearest neighbours offers the possibility of generating liquid states (i.e. states with temperature $T$ lower than the liquid-gas critical temperature) with a vanishing occupied packing fraction ($\phi$), a case which can not be realized with spherically interacting particles. Theoretical results suggest that such reduction is accompanied by an increase of the region of stability of the liquid phase in the ($T$-$\phi$) plane, possibly favoring the establishment of homogeneous disordered materials at small $\phi$, i.e. stable equilibrium gels.Comment: 4 pages, 4 figures, revised version, accepted in Phys. Rev. Let

Liquid-liquid phase separation driven by charge heterogeneity

Globular proteins as well as recently synthesized colloids engineered with differently charged surface regions have in common a reduced bonding valence and a complex interaction pattern dominated by like-charge attraction and opposite-charge repulsion. While the impact of low functionality on the condensation of the liquid phase has been extensively studied, the combined effect of limited bonding valence and particle charge heterogeneity on the liquid-liquid phase separation has not been investigated yet. We numerically tackle this challenge in a systematic fashion by taking advantage of an efficient coarse-grained model grounded into a robust mean-field description. We consider a relatively simple surface pattern consisting of two charged polar caps and an oppositely charged equatorial belt and investigate how the interplay between geometry and electrostatics affect the critical point parameters. We find that electrostatics has a dramatic effect on the condensation of the liquid phase -- especially in the regime of large polar caps.Comment: 12 pages, 8 figure

Theoretical and numerical study of the phase diagram of patchy colloids: ordered and disordered patch arrangements

We report theoretical and numerical evaluations of the phase diagram for a model of patchy particles. Specifically we study hard-spheres whose surface is decorated by a small number f of identical sites ("sticky spots'') interacting via a short-range square-well attraction. We theoretically evaluate, solving the Wertheim theory, the location of the critical point and the gas-liquid coexistence line for several values of f and compare them to results of Gibbs and Grand Canonical Monte Carlo simulations. We study both ordered and disordered arrangements of the sites on the hard-sphere surface and confirm that patchiness has a strong effect on the phase diagram: the gas-liquid coexistence region in the temperature-density plane is significantly reduced as f decreases. We also theoretically evaluate the locus of specific heat maxima and the percolation line.Comment: preprint, 32 pages, 6 figures, 3 tables, J. Chem. Phys. in pres

Limiting the valence: advancements and new perspectives on patchy colloids, soft functionalized nanoparticles and biomolecules

Limited bonding valence, usually accompanied by well-defined directional interactions and selective bonding mechanisms, is nowadays considered among the key ingredients to create complex structures with tailored properties: even though isotropically interacting units already guarantee access to a vast range of functional materials, anisotropic interactions can provide extra instructions to steer the assembly of specific architectures. The anisotropy of effective interactions gives rise to a wealth of self-assembled structures both in the realm of suitably synthesized nano- and micro-sized building blocks and in nature, where the isotropy of interactions is often a zero-th order description of the complicated reality. In this review, we span a vast range of systems characterized by limited bonding valence, from patchy colloids of new generation to polymer-based functionalized nanoparticles, DNA-based systems and proteins, and describe how the interaction patterns of the single building blocks can be designed to tailor the properties of the target final structures