Automation of an atomic force microscope via Arduino.

The Atomic Force Microscopy is a very versatile technique that allows to characterize surfaces by acquiring topographies with sub-nanometer resolution. This technique often overcomes the problems and capabilities of electron microscopy when characterizing few nanometers thin coatings over solid substrates. They are expensive, in the half million dollar range for standard units, and therefore it is often difficult to upgrade to new units with improved characteristics. One of these improvements, motorization and automation of the measurements is very interesting to sample different parts of a substrate in an unattended way. Here we report a low cost upgrade under 60 $ to a Dimension 3000 AFM based on a control unit using an Arduino Leonardo. It enables to acquire dozens or hundreds of images automatically by mimicking keyboard shortcuts and interfacing the AFM PCI card.

An experimental/theoretical method to measure the capacitive compactness of an aqueous electrolyte surrounding a spherical charged colloid.

The capacitive compactness has been introduced very recently [G. I. Guerrero-García et al., Phys. Chem. Chem. Phys. 20, 262-275 (2018)] as a robust and accurate measure to quantify the thickness, or spatial extension, of the electrical double layer next to either an infinite charged electrode or a spherical macroion. We propose here an experimental/theoretical scheme to determine the capacitive compactness of a spherical electrical double layer that relies on the calculation of the electrokinetic charge and the associated mean electrostatic potential at the macroparticle's surface. This is achieved by numerically solving the non-linear Poisson-Boltzmann equation of point ions around a colloidal sphere and matching the corresponding theoretical mobility, predicted by the O'Brien and White theory [J. Chem. Soc., Faraday Trans. 2 74, 1607-1626 (1978)], with experimental measurements of the electrophoretic mobility under the same conditions. This novel method is used to calculate the capacitive compactness of NaCl and CaCl2 electrolytes surrounding a negatively charged polystyrene particle as a function of the salt concentration.

Impact of the collective diffusion of charged nanoparticles in the convective/capillary deposition directed by receding contact lines.

The motion of electrically charged particles under crowding conditions and subjected to evaporation-driven capillary flow might be ruled by collective diffusion. The concentration gradient developed inside an evaporating drop of colloidal suspension may reduce by diffusion the number of particles transported toward the contact line by convection. Unlike self-diffusion coefficient, the cooperative diffusion coefficient of interacting particles becomes more pronounced in crowded environments. In this work, we examined experimentally the role of the collective diffusion of charge-stabilized nanoparticles in colloidal patterning. To decouple the sustained evaporation from the contact line motion, we conducted evaporating menisci experiments with driven receding contact lines at low capillary number. This allowed us to explore convective assembly at fixed and low bulk concentration, which enabled to develop high concentration gradients. At fixed velocity of receding contact line, we explored a variety of substrate-particle systems where the particle-particle electrostatic interaction was changed (via p H) as well as the substrate receding contact angle and the relative humidity. We found that the particle deposition directed by receding contact lines may be controlled by the interplay between evaporative convection and collective diffusion, particularly at low particle concentration.