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When a drop of a colloidal solution of nanoparticles dries on a surface, it leaves behind coffee-stain-like rings of material with lace-like patterns or clumps of particles in the interior. These non-uniform mass distributions are manifestations of far-from-equilibrium effects, such as fluid flows and solvent fluctuations during late-stage drying. However, recently a strikingly different drying regime promising highly uniform, long-range-ordered nanocrystal monolayers has been found. Here we make direct, real-time and real-space observations of nanocrystal self-assembly to reveal the mechanism. We show how the morphology of drop-deposited nanoparticle films is controlled by evaporation kinetics and particle interactions with the liquid-air interface. In the presence of an attractive particle-interface interaction, rapid early-stage evaporation dynamically produces a two-dimensional solution of nanoparticles at the liquid-air interface, from which nanoparticle islands nucleate and grow. This self-assembly mechanism produces monolayers with exceptional long-range ordering that are compact over macroscopic areas, despite the far-from-equilibrium evaporation process. This new drop-drying regime is simple, robust and scalable, is insensitive to the substrate material and topography, and has a strong preference for forming monolayer films. As such, it stands out as an excellent candidate for the fabrication of technologically important ultra thin film materials for sensors, optical devices and magnetic storage media.

A theory of condensation and resolubilization of a dilute DNA solution with growing concentration of multivalent cations, N is suggested. It is based on a new theory of screening of a macroion by multivalent cations, which shows that due to strong cation correlations at the surface of DNA the net charge of DNA changes sign at some small concentration of cations N0. DNA condensation takes place in the vicinity of N0, where absolute value of the DNA net charge is small and the correlation induced short range attraction dominates the Coulomb repulsion. At N > N0 positive DNA should move in the oppisite direction in an electrophoresis experiment. From comparison of our theory with experimental values of condensation and resolubilization thresholds for DNA solution containing Spe 4+ , we obtain that N0 = 3.2 mM and that the energy of DNA condensation per nucleotide is 0.07 kBT .

Screening of a strongly charged macroion by oppositely charged colloidal particles, micelles, or short polyelectrolytes is considered. Due to strong lateral repulsion such multivalent counterions form a strongly correlated liquid at the surface of the macroion. This liquid provides correlation induced attraction of multivalent counterions to the macroion surface. As a result even a moderate concentration of multivalent counterions in the solution inverts the sign of the net macroion charge. We show that at high concentration of monovalent salt the absolute value of inverted charge can be larger than the bare one. This giant inversion of charge can be observed in electrophoresis.PACS numbers: 87.14Gg, 87.16.Dg, 87.15.Tt Charge inversion is a phenomenon in which a charged particle (a macroion) strongly binds so many counterions in a water solution, that its net charge changes sign. As shown below the binding energy of counterion with large charge Z is larger than k B T , so that this net charge is easily observable; for instance, it is the net charge that determines linear transport properties, such as particle drift in a weak field electrophoresis. Charge inversion has been observed 1 in polyelectrolyte-micelle system and is possible for a variety of other systems, ranging from solid surface of mica or lipid membranes, to DNA or actin.Charge inversion is of special interest for delivery of genes to the living cell for the purpose of gene therapy. The problem is that both bare DNA and a cell surface are negatively charged and repel each other. The goal is to screen DNA in such a way that the resulting complex is positive 2 .Theoretically, charge inversion can be also thought of as an over-screening. Indeed, the simplest screening atmosphere, familiar from linear Debye-Hückel theory, compensates at any finite distance only a part of the macroion charge. It can be proven that this property holds also in non-linear Poisson-Boltzmann (PB) theory. The statement that the net charge preserves sign of the bare charge agrees with the common sense. One can think that this statement is even more universal than results of PB equation. It was shown 3-5 , however, that this presumption of common sense fails for screening by Z-valent counterions (Z-ions), such as charged colloidal particles, micelles, or short polyelectrolytes, because there are strong lateral correlations between them when they are bound to the surface of a macroion. These correlations are beyond the mean field PB theory, and charge inversion is their most spectacular manifestation.Charge inversion has attracted a significant attention in the last couple of years 6 . Our goal in the present paper is to provide a simple physical explanation of charge inversion and to show that in the most practical case, when both Z-ions and monovalent salt, such as NaCl, are present, not only charge sign may flip, but the inverted charge can become even larger in absolute value than the bare charge, thus giving rise to giant charge inversion.Let us demonstrate the role of la...

Screening of a macroion such as a charged solid particle, a charged membrane, double helix DNA or actin by multivalent counterions is considered. Small colloidal particles, charged micelles, short or long polyelectrolytes can play the role of multivalent counterions. Due to strong lateral repulsion at the surface of macroion such multivalent counterions form a strongly correlated liquid, with the short range order resembling that of a Wigner crystal. These correlations create additional binding of multivalent counterions to the macroion surface with binding energy larger than kBT . As a result even for a moderate concentration of multivalent counterions in the solution, their total charge at the surface of macroion exceeds the bare macroion charge in absolute value. Therefore, the net charge of the macroion inverts its sign. In the presence of a high concentration of monovalent salt the absolute value of inverted charge can be larger than the bare one. This strong inversion of charge can be observed by electrophoresis or by direct counting of multivalent counterions.