Claudiu Valentin Suciu

Investigation of a colloidal damper

A novel application of nanotechnology in the field of mechanical engineering, called colloidal damper (CD), is investigated. This device is complementary to the hydraulic damper (HD), having a cylinder-piston construction. Particularly for CD, the hydraulic oil is replaced by a colloidal suspension, which consists of a mesoporous matrix and a lyophobic fluid. In this work, the porous matrix is from silica gel modified by linear chains of n-alkylchlorosilanes and water is considered as an associated working fluid. A design solution from a practical point of view of the CD test rig and the measuring technique of the hysteresis are described. A brief review of the water physical properties relative to the CD concept is presented. Influence of the bonding density, length of the grafted molecule, pore diameter, and particle diameter on the CD hysteresis is investigated for distinctive types and mixtures of silica gels. Temperature variation during functioning is recorded and the CD cycle is interpreted from a thermodynamic standpoint. Variation of the CD dissipated energy and efficiency with pressure, water quantity, and relaxation time is illustrated. Experimental results are justified by the analysis of the water flow into the porous matrix, CD thermodynamics, and the mechanism of the energy dissipation. Our findings agree with the previously published data.

Study of a Novel Squeeze Film Damper and Vibration Generator

A novel squeeze film damper and vibration generator (SFD&VG) is proposed as an option in the vibration control field. The SFD& VG can be used as an active squeeze film damper (ASFD) or as a vibration generator (vibrator), for unidimensional vibration damping or generation. The SFD&VG concept is connected with current research to improve a common industrial process-drilling of deep holes. The SFD&VG is based on the variable area of the lubricant film, which allows the development of a variable force, and a change in fluid film stiffness and damping. The analysis is initiated for an elementary configuration of the SFD&VG-the infinite width Rayleigh step case-and then it is developed for an advanced elliptical SFD&VG. The Reynolds equation is solved for both pure squeeze film effect which provides vibration damping, and pure hydrodynamic wedge effect which provides vibration generation. The the oretical part is continued with the SFD&VG dynamic simulation. The SFD&VG experimental device and vibration measurements, performed for the two defined regimes, ASFD and vibration generator, are presented. Finally, the experimental and theoretical results are briefly compared.

Friction and Wear Properties of Aluminum-Silicon Alloy Impregnated Graphite Composite (ALGR-MMC) under Lubricated Sliding Conditions

In this work, friction and wear properties of aluminum-silicon alloy-impregnated graphite composite (ALGR-MMC) and its component matrices, graphite and aluminum-silicon alloy, in contact with bearing steel are investigated under lubricated sliding conditions. Pin-on-disk type wear experiments are conducted under “dry” sliding; i.e., in moist air with a relative humidity of 50% at 24°C; drop-feed lubrication; i.e., an oil drop with a certain volume selected in the range of 0.005–1 cm3 is deposited on the disk surface before commencing the wear tests; and immersion lubrication; i.e., the pin-on-disk contact is submerged in a 90-cm3 oil bath. Changes in friction and wear are continuously monitored. Tests revealed a reduction of the friction coefficient and wear rate for pins made in graphite with augmentation of the oil-drop volume. For aluminum-silicon alloy, breakdown of the oil film occurred below 0.03 cm3 volume of the drop, and since this was accompanied by a drastic rise of the friction and wear, damage of the counter surface was observed. Oppositely, a gradual decrease of the friction and wear with augmentation of the oil-drop volume was found for ALGR-MMC, without the breakdown of the oil film and/or damage of the counter surface. All tested materials reached the minimum friction and wear under immersion lubrication; however, graphite showed the worst and ALGR-MMC the best anti-friction properties. Since the friction coefficient and the wear rate of ALGR-MMC were much lower than those of the component matrices under all lubrication conditions, one concludes that the proposed composite material can be successfully used under both partially and fully lubricated sliding conditions.

On the Structural Simplification, Compact and Light Design of a Vehicle Suspension, Achieved by Using a Colloidal Cylinder with a Dual Function of Absorber and Compression-Spring

Classical suspension (oil damper mounted in parallel with compression helical spring) is replaced by a colloidal suspension, in which case the spring can be omitted. Hence, structural simplification, accompanied by a compact and lighter design can be achieved. Oil is replaced by an ecological mixture of water and water-repellent nanoporous particles of silica (artificial sand). Travel tests using a V8 4.3L auto vehicle equipped with classical and colloidal suspensions were performed. Ride comfort (ISO 2631 method) was evaluated during travel (speed: 5–40 km/h) on a normal road with an asphalt step (height: 37 mm; width: 405 mm), for various values of the tire inflation pressure (150–250 kPa). On normal road without step the travel speed was increased up to 80 km/h. Acceleration at seat, seat-back, and feet surfaces was processed using the commercially available DEICY system for ride comfort evaluation. Spring omission, accompanied by 60 % reduction of the outer diameter, and 30 % reduction of the mass was achieved both for the frontal and rear colloidal suspensions. Results concerning the ride comfort were validated in the case of classical suspensions. Relationship between the travel speed of the vehicle and level of vibration perception was obtained for various values of the tire inflation pressure. Ride comfort decreased at augmentation of the travel speed and the tire inflation pressure. Since the colloidal spring constant was 6 times larger than the constant of the compression helical spring, colloidal suspension provided 1 rank lower ride comfort than the classical suspension. Pitching and rolling movements were not considered during the estimation of the ride comfort. Relation between the lateral acceleration and the rolling attitude angle was experimentally determined. Ride comfort results were explained by taking into account the vehicle behaviour during frontal, rear and superimposed impact excitations, in correlation with the variation against travel speed of the frequency weighting proposed by the ISO 2631. Although the colloidal suspension was found to provide inferior ride comfort than the classical suspension, results obtained so far are encouraging since better performances are to be expected by softening the colloidal spring, and by redesigning the suspension including the stabilizers.

Modeling and Simulation of a Vehicle Suspension with Variable Damping and Elastic Properties versus the Excitation Frequency

Usual vehicle suspensions employ hydro-pneumatic absorbers (e.g., oil, colloidal and air dampers) mounted in parallel with compression helical springs. Although the damping coefficient of the vehicle suspension is changing versus the excitation frequency, conventional design method is based on simplified models that assume for constant damping and elastic properties. In this work, three types of suspensions were considered and modeled as follows: oil damper mounted in parallel with a compression helical spring, for which a Kelvin-Voigt model, consisted of a dashpot and an elastic element connected in parallel is considered, colloidal damper without attached compression helical spring, for which a Maxwell model, consisted of a dashpot and an elastic element connected in series is considered, and colloidal damper mounted in parallel with a compression helical spring, for which a standard linear model, consisted of a Maxwell unit connected in parallel with an elastic element is considered. Firstly, the vibration transmissibility from the rough road to the vehicles body for all these suspensions was determined under the constraint that damping varies versus the excitation frequency. Then, the optimal damping and stiffness ratios were decided in order to minimize the transmissibility of vibration from the rough pavement to the vehicles body. Such results are useful to improve the vehicles ride-comfort.

Analysis of a Torsional Fluid Film Vibrator

A novel hydrodynamic system, called torsional fluid film vibrator (TFFV) is proposed. This device is complementary to the Lanchesters absorber and presents a classical response of a one-degree of freedom linear system with a periodical self-excitation. The fluid film thickness variation produces a variable viscous drag moment, which drives the elastically supported bush in a torsional oscillatory movement. The TFFV concept is connected with current research to improve the drilling technology of deep holes. The Navier-Stokes equations are solved on the particular geometry of this vibrator and the viscous drag moment is explicitly presented. The theoretical part is continued with the TFFV dynamic simulation and the analysis of the influence of the geometrical parameters on the amplitude of the viscous drag moment. Computed structural friction power and the amplitude of vibration agree reasonably well with the experimental measurements conducted on a TFFV test rig.

Molecular Dynamics Study of Wetting on Brushlike Nanopillar and Wavelike Nanorough Surfaces

A Molecular Dynamics technique is proposed to simulate the motion of water nano-droplets on brushlike nanopillar and wavelike nanorough surfaces. Firstly, a brushlike nanopillar structure is obtained by deposition of a hexagonal packing of alkyl linear chains Cn H2n+1 (n = 1–18) on a (0001) type flat surface, consisted of hexagonal packed carbon atoms. Distance between the grafted alkyl chains is selected in the 0.5–1.4 nm range, and the distance between the carbon atoms of the base surface is set to 0.1421nm. Next, the (0001) type flat surface is folded in order to obtain a wavelike nano-roughness. Water cluster is consisted of 729–2197 molecules, and after 25ps it reaches a diameter of 3–5 nm, which corresponds to a liquid phase of 1g/cm3 density, at an equilibrium temperature of 293K. Lennard-Jones potential is used to describe all the interactions into the considered system. By the appropriate input of the Lennard-Jones parameters one controls the hydrophilic level of the base surface. Influences of the intermolecular distance and the length of the grafted alkyls, as well as the influences of the nano-wavelength and the hydrophilic level of the base surface on the contact angle are illustrated. Such results are useful for the appropriate design of ultrahydrophobic nano-surfaces, and for the optimal design of nanoporous materials, able to produce surface dissipation of the mechanical energy.Copyright

Energy Dissipation During Liquid Adsorption/Desorption In/From Liquid-Repellent Nanochannels

Ability of viscous fluids, flowing in narrow interstices, to dissipate the mechanical energy of shock and vibration is well known. In recent years, connected to the nano-technological development, solid-liquid interfaces have been used to dissipate surface energies, in systems where the solid is liquid-repellent; such interfaces are able to store, release or transform the energy. Thus, the contact angle hysteresis can be applied to dissipate the mechanical energy, and this kind of energy loss, in which not the viscosity but the surface tension of the liquid plays the main role, is called surface dissipation. In fact a liquid nano-porosimeter that exhibits nano-damping ability, when applied to mechanical systems is called colloidal damper. Concretely, during the cyclical adsorption/desorption of the liquid (e.g., water or aqueous solutions) in/from the liquid-repellent nanochannels (e.g., modified nanoporous silica gel) the energy is dissipated. Such absorber is convenient from the ecological standpoint since it is oil-free and since both the silica gel (artificial sand with controlled architecture) and the liquid are environment-friendly. Connected to this attractive kind of energy loss, one of the problems awaiting solution is that a theoretical model of the surface dissipation remains to be developed and validated by tests. Accordingly, in this work, based on a detailed discussion of the mechanism of surface dissipation one reveals that the parameters which determine the magnitude of the energy loss are the silica gel mass, the liquid and solid surface tensions, and an integral function (specific pore surface) which is related to the nano-architecture of the liquid-repellent coating, to the silica gel pore architecture and to the maximum applied pressure. Silica gel particles are supposed to be obtained through the aggregation of nano-particles, producing rough nanochannels of variable radius, and normal distribution fits quite well the measured pores size distributions. Heterogeneous molecules of the liquid-repellent coating have a methyl group as head, and a body consisted of methylene groups; they produce a nanopillar structure on the silica gel surface. Maximization of the surface dissipation for imposed working liquid or imposed coating molecule is discussed. Test rig is a compression-decompression chamber used to validate the theoretical findings. Results obtained are useful in general for the appropriate design of liquid-repellent nanochannels with technological applications, and in particular for the absorber optimum design under imposed requirements.Copyright

Investigation of the Water Flow Into a Mesoporous Matrix From Hydrophobized Silica Gel

In this work a generalized hydrodynamic theory for the water flow into a mesoporous matrix from hydrophobized silica gel is suggested. Although we examine a fluid dynamics problem, i.e., the motion of the water-gas-solid contact line, motivation for such research derives from the investigation of a novel principle of mechanical energy dissipation, called colloidal damper. Similar to hydraulic damper, this absorber has a cylinder-piston structure, but oil is replaced by a colloid consisted of a mesoporous matrix and a lyophobic liquid. Here, the mesoporous matrix is from silica gel modified by linear chains of alkyldimethylchlorosilanes and water is the associated lyophobic liquid. Mainly, the colloidal damper energy loss can be explained by the dynamic contact angle hysteresis in advancing (liquid displaces gas) and receding (gas displaces liquid); such hysteresis occurs due to the geometrical and chemical heterogeneities of the solid surface. Measuring technique of the hysteresis loop is described. From experimental data one calculates the dissipated energy, damper efficiency and the damping coefficient versus the length of the grafted molecule on the silica gel surface. Experimental results are justified by the flow analysis. Generalized hydrodynamic theory means here that the basic structure of Navier-Stokes equations is kept, but in order to include the relation between macroscopic flow and molecular interactions, slip is allowed on the solid wall. Nano-pillar architecture of the silica gel hydrophobic coating is described. During adsorption, water penetrates the pore space by maintaining contact with the top of the coating molecules (region of -CH3 groups); after that, water is forced into and partially or totally fills the space between molecules (region of -CH2 groups); in such circumstances, at the release of the external pressure, desorption occurs. Mechanism of energy dissipation is discussed. Results obtained are useful for the appropriate design of the hydrophobic coating of a mesoporous matrix which is destined to colloidal damper use.Copyright

Rheological Model for a Nanoporous-Elasto-Hydrodynamic Composite Material

This work proposes a rheological model for a nanoporous-elasto-hydrodynamic composite material (NPEHDCM), which can be obtained by mixing a colloid, consisted of water and water-repellent nanoporous silica micro-particles, with an adequate jellification agent. Hydrogel is modeled as a biphasic mixture consisted of a nanoporous hydrophilic isotropic and linear elastic solid matrix, and a liquid phase (water). At dynamic pressurization, water molecules exude from the hydrogel matrix and forcedly penetrate the nanopores of hydrophobic silica particles. Based on the proposed rheological model, the NPEHDCM can be suitably designed.