Instituto De Fisica

The effective solid-liquid interfacial tension (SL-IFT) between pure liquids and rough solid surfaces is studied through coarse-grained simulations. Using the dissipative particle dynamics method, we design solid-liquid interfaces, confining a pure liquid between two explicit solid surfaces with different roughness degrees. The roughness of the solid phase was characterized by Wenzel's roughness factor and the effective SL-IFT (′) is reported as a function of it also. Two solid-liquid systems differentiated from each other by their solidliquid repulsion strength are studied to measure the effects caused by the surface roughness on the calculation of ′. We found that the roughness produces changes in the structure of the liquid, which is observed in the first layer of liquid near the solid. These changes are responsible for the effective SL-IFT increase as surface roughness increases. Although there is a predominance of surface roughness in the calculation of ′ , it is found that the effective SL-IFT is directly proportional to the magnitude of the solid-liquid repulsion strength. The insights provided by these simulations suggest that the increase of Wenzel's roughness factor

This study evaluated the association of surface degradation and formation of Streptococcus mutans (S. mutans) biofilm in resin-based composites (RBCs) after storage in different acidic liquids. Methods: To evaluate microhardness and surface micromorphology, hybrid and nanohybrid RBC discs were stored in artificial gastric acid, cola drink, orange juice, artificial saliva, and distilled water for three intervals of 15 min per day for 7, 15, and 30 days. After 30 days of storage, surface roughness was analyzed, and the RBC discs were placed in a biofilm reactor inoculated with S. mutans to evaluate surface biofilm formation. Results: As compared with nanohybrid RBCs, roughness and surface microhardness values were significantly lower (P < 0.05) for hybrid RBCs stored in artificial gastric acid, followed by specimens stored in cola drink and orange juice. Artificial gastric acid caused greater surface degradation, which increased the biomass of S. mutans on the surface of both RBC types. Conclusion: Surface degradation of hybrid and nanohybrid RBCs correlated with the pH of the liquid, while S. mutans biofilm formation was associated with increased surface roughness in hybrid RBCs.

Hexagonal boron nitride (h-BN) is a 2D material with excellent properties, such as large band gap, high thermal and chemical stability, transparency, and high resistance to oxidation and corrosion. These properties make h-BN a suitable candidate to be used in the development of advanced coatings. However, as for other nanomaterials, tailor and control the properties of h-BN is a fundamental key for their application into several fields. Here, the wetting properties of h-BN when is exfoliated by ultrasonic cavitation in different solvents including isopropyl alcohol (IPA), dioxane (Dx), N-methyl pyrrolidone (NMP) and dimethyl formamide (DMF) were investigated. The wetting properties of the different h-BN materials were determined by measuring the water contact angle (WCA) of h-BN thin films deposited on silicon dioxide, different contact angles were observed for each sample, the different WCA values are explained by the differences in the structure and roughness of the thin film surf...

We present an experimental study of the spatial correlations of a quasi-two-dimensional dissipative gas kept in a non-static steady state via vertical shaking. From high temporal resolution images we obtain the Pair Distribution Function (PDF) for granular species with different restitution coefficients. Effective potentials for the interparticle interaction are extracted using the Ornstein-Zernike equation with the Percus-Yevick closure. From both the PDFs and the corresponding effective potentials, we find a clear increase of the spatial correlation at contact with the decreasing values of the restitution coefficient. PACS. 45.70.-n Granular systems -61.20.Ne Structure of simple liquids

Compaction from a random-loose-packed to a random-close-packed phase is observed when monodisperse granular beds are shaken, but beyond this packing, the system freezes up in a jammed structure. Here we report a technique to grow large hard-sphere granular crystals, with perfect stacking and no defects by means of a ''gas phase'' epitaxial procedure. We study the growth mechanism and provide evidence that the observed granular crystallization is driven by gravity and energy dissipation.

We present a novel study on the effect of a magnetic field applied on a binary mixture doped with magnetic nanoparticles close to its demixing transition. Turbidity measurements in the Faraday configuration show that the effect of applying an external field produces changes in the critical opalescence of the mixture that allow us to track an aggregation produced by critical Casimir forces and a reversible aggregation due to the formation of chain-like flocks in response to the external magnetic field. The observation of a crossover of the aggregation curves through optical signals is interpreted as the evolution from low to high power dispersion nuclei due to an increase in the radius of the condensation seed brought about by Casimir or magnetic interactions. Finally, evidence of an enhanced magnetocaloric effect due to the coupling between mixing and ordering phase transitions is presented which opens up a nonsolid state approach of designing refrigerating cycles and devices.

We study the propagation of an elastic excitation through a magnetorheological slurry made of hydrogen-reduced iron particles suspended in glycerine. Two different low-frequency longitudinal modes are observed, the second of them appearing as soon as a magnetic field is applied to the fluid. The first mode travels through glycerine channels within the suspension, whereas the second one propagates through the fibrillated structure formed by the iron particles. This second mode is very strong in amplitude and travels at lower speeds, depending on the magnetic field intensity.

We present results from a detailed simulation of a quasi-2D dissipative granular gas, kept in a non-condensed steady state via vertical shaking over a rough substrate. This gas shows a weak power-law decay in the tails of its Pair Distribution Functions (PDF's), indicating fractality and therefore a tendency to form clusters over several size scales. This clustering depends monotonically on the dissipation coefficient, and disappears when the sphere-sphere collisions are conservative. Clustering is also sensitive to the packing fraction. This gas also displays the standard nonequilibrium characteristics of similar systems, including non-Maxwellian velocity distributions. The diffusion coefficients are calculated over all the conditions of the simulations, and it is found that diluted gases are more diffusive for smaller restitution coefficients.

The rise dynamics of a large spherical particle, in a granular bed under vertical vibrations, is experimentally studied. An inductive device has been designed to track the particle while it climbs through the bed under different conditions. A simple model based in energy considerations is presented to explain our experimental data, drawing important conclusions about the physics of the phenomenon.

The rise dynamics of a large particle, in a granular bed under vertical vibrations, is experimentally studied with an inductive device designed to track the particle while it climbs through the granulate under different conditions. A model based on energy considerations is presented to explain our experimental data, drawing the important conclusion that it is the inertia of the particle, assisted by Reynolds dilatancy, the driven force behind its ascension mechanism. The ascension reveals a friction profile within the column which remains unchanged for different accelerations.

We present a new tool for the textural study of inaccessible outcrops of pyroclastic and epiclastic deposits. The new method, called Laser Remote Optical Granulometry (LROG), is based on high resolution tele-photography and stereologic techniques. LROG consists on taking several pictures of the outcrop with a high resolution CCD camera coupled to a small aperture telescope that can be placed several tenths of meters away. The scale of the image is obtained projecting an equilateral triangle with known size on the outcrop by means of three laser beams. The LROG allows the measurement of clasts less than 1 mm in size from a distance of 80 to 100 meters, and can reach much better resolution when operated closer to the outcrop. Perspective distortion can be corrected with the equilateral triangle projected by the lasers. To get high resolution images and remove the effects of air turbulence, hundreds of frames of the same field are captured in rapid sequence and then stacked and averaged with image processing algorithms developed for astronomical imaging. The LROG was validated on the pyroclastic deposits of the Joya Honda maar (San Luis Potosi, Mexico). The LROG provided precise granulometric analysis and vertical granulometric profiles of this pyroclastic sequence, useful to recognize the eruptive history of the volcano. This method can be used for the analysis of any kind of sedimentary deposits in the granulometric range of clasts greater than fine sand. We are improving the LROG to obtain other useful textural information like clast shape and apparent fabric. This method, implemented on a robotic probe could be a promising tool to carry out detailed stratigraphic and sedimentological study of Martian sedimentary successions.

We present results from a detailed simulation of a quasi-2D dissipative granular gas, kept in a non-condensed steady state via vertical shaking over a rough substrate. This gas shows a weak power-law decay in the tails of its Pair Distribution Functions (PDF's), indicating fractality and therefore a tendency to form clusters over several size scales. This clustering depends monotonically on the dissipation coefficient, and disappears when the sphere-sphere collisions are conservative. Clustering is also sensitive to the packing fraction. This gas also displays the standard nonequilibrium characteristics of similar systems, including non-Maxwellian velocity distributions. The diffusion coefficients are calculated over all the conditions of the simulations, and it is found that diluted gases are more diffusive for smaller restitution coefficients.

We present an experimental study of the spatial correlations of a quasi-two-dimensional dissipative gas kept in a non-static steady state via vertical shaking. From high temporal resolution images we obtain the Pair Distribution Function (PDF) for granular species with different restitution coefficients. Effective potentials for the interparticle interaction are extracted using the Ornstein-Zernike equation with the Percus-Yevick closure. From both the PDFs and the corresponding effective potentials, we find a clear increase of the spatial correlation at contact with the decreasing values of the restitution coefficient.

A machine vision has been devised for determination of the size distribution of spherically shaped pellets in granular beds under static and dynamic conditions. This machine vision involved establishing the optimal distance between the illumination source and the top layer of the granular bed, as well as the development of the data image acquisition software and control systems, I/O interface hardware, and electromechanical system. The size distribution of pellets given by the whole system gives an error lower than 5% for pellets. The size distribution histogram or the Sauter diameter of the pellets given by the vision system is used as the input signal for controlling an electromechanical scanning device, which can be reliably used for automation tasks. The system software was developed for open access on a commercially widely used platform. As the machine vision is robust, it can be used in industrial environments and it has the potential to contribute in improving and optimizing pelletizing industrial processes. This machine vision is reliable, flexible, user friendly, inexpensive, and easy to implement.

We study the formation of crystalline clusters for a two-dimensional (2D) sinusoidally vibrated granular gas, with maximum vertical acceleration smaller than gravity, using fully 3D simulations. It is found that this phenomenon arises from the spontaneous segregation of the granulate into two dynamical modes: one of grains that bounce in synchrony with the motion of the sustaining plate (“bouncers”) and another of grains that cease to bounce and simply rolls on the plate, without ever loosing contact with it (“rollers”). These two dynamical categories are quite robust with respect to perturbations. The populations for bouncers and rollers depend on the preparation of the granulate and can be made to take arbitrary values in all the range of accelerations where both dynamical modes are present. It is found that the dynamical mode with the largest population coalesces in clusters under the influence of the other mode, whose grains act as a higher pressure gas that compresses the clusters. In this way it is possible to produce clusters of rollers or clusters of bouncers. A gas made of grains from only one dynamical class shows only weak density fluctuations. When the occupation fractions for both modes are similar, one observes segregation and clusters of both types. The clustering of the gas is monitored using both the average coordination number and the local hexatic order parameter ψ6. Energy flows in the plane are monitored, and it is shown that roller-bouncer collisions increase horizontal kinetic energy, while all other types of collisions reduce this energy. We find that friction with the substrate is the main sink of horizontal energy for these granular gases.

We present a simple experimental technique to determine size distributions of metallic polydispersions. The particles are first suspended in a viscous fluid-like glycerol and then their sedimentation is followed by measuring the effective dielectric constant in a cylindrical cell at a fixed frequency. Thereafter, an inversion procedure of the data, based on the Maxwell-Garnett effective medium theory and Stokes law, is used to directly obtain the size distribution. The technique is applied to three different stainless steel dispersions and compares very well with a traditional sizing method based in microphotography.