Prashant Bahadur

Water tribology on graphene

Classical experiments show that the force required to slide liquid drops on surfaces increases with the resting time of the drop, t(rest), and reaches a plateau typically after several minutes. Here we use the centrifugal adhesion balance to show that the lateral force required to slide a water drop on a graphene surface is practically invariant with t(rest). In addition, the drops three-phase contact line adopts a peculiar micrometric serrated form. These observations agree well with current theories that relate the time effect to deformation and molecular re-orientation of the substrate surface. Such molecular re-orientation is non-existent on graphene, which is chemically homogenous. Hence, graphene appears to provide a unique tribological surface test bed for a variety of liquid drop-surface interactions.

Drop retention force as a function of drop size.

The force, f, required to slide a drop past a surface is often considered in the literature as linear with the drop width, w, so that f/w = const. Furthermore, according to the Dussan equation for the case that the advancing and receding contact angles are constant with drop size, one can further simplify the above proportionality to f/V(1/3) = const where V is the drop volume. We show, however, that experimentally f/V(1/3) is usually a decaying function of V (rather than constant). The retention force increases with the time the drop rested on the surface prior to sliding. We show that this rested-time effect is similar for different drop sizes, and thus the change of f/V(1/3) with V occurs irrespective of the rested-time effect which suggests that the two effects are induced by different physical phenomena. The time effect is induced by the unsatisfied normal component of the Young equation which slowly deforms the surface with time, while the size effect is induced by time independent properties. According to the Dussan equation, the change of f/V(1/3) with V is also expressed in contact angle variation. Our results, however, show that contact angle variation that is within the scatter suffices to explain the significant force variation. Thus, it is easier to predict contact angle variation based on force variation rather than predicting force variation based on contact angle variation. A decrease of f/V(1/3) with V appears more common in the system studied compared to an increase.

Drop Retention Force as a Function of Resting Time

The force, f, required to slide a drop on a surface is shown to be a growing function of the time, t, that the drop waited resting on the surface prior to the commencement of sliding. In this first report on the resting time effect, we demonstrate the existence of this phenomenon in different systems, which suggests that this phenomenon is general. We show that d f/d t is never negative. The shorter the resting times, the higher d f/d t is. As the resting time increases, d f/d t decreases toward zero (plateau) as t --> infinity. The increase in the force, Delta f, due to the resting time effect (i.e., f( t --> infinity) - f( t --> 0)) correlates well with the vertical component of the liquid-vapor surface tension, and we attribute this phenomenon to the corrugation of the surface by the drop due to this unsatisfied normal component of Youngs equation.

On the Role of the Three-Phase Contact Line in Surface Deformation

Viscoelastic braking theories developed by Shanahan and de Gennes and by others predict deformation of a solid surface at the solid-liquid-air contact line. This phenomenon has only been observed for soft smooth surfaces and results in a protrusion of the solid surface at the three-phase contact line, in agreement with the theoretical predictions. Despite the large (enough to break chemical bonds) forces associated with it, this deformation was not confirmed experimentally for hard surfaces, especially for hydrophobic ones. In this study we use superhydrophobic surfaces composed of an array of silicon nanostructures whose Young modulus is 4 orders of magnitude higher than that of surfaces in earlier recorded viscoelastic braking experiments. We distinguish between two cases: when a water drop forms an adhesive contact, albeit small, with the apparent contact angle θ < 180° and when the drop-surface adhesion is such that the conditions for placing a resting drop on the surface cannot be reached (i.e., θ = 180°). In the first case we show that there is a surface deformation at the three-phase contact line which is associated with a reduction in the hydrophobicity of the surface. For the second case, however, there cannot be a three-phase contact line associated with a drop in contact with the surface, and indeed, if we force-place a drop on the surface by holding it with a needle, no deformation is detected, nor is there a reduction in the hydrophobic properties of the surface. Yet, if we create a long horizontal three-phase contact line by partially immersing the superhydrophobic substrate in a water bath, we see a localized reduction in the hydrophobic properties of the surface in the region where the three-phase contact line used to be. The SEM scan of that region shows a narrow horizontal stripe where the nanorods are no longer there, and instead there is only a shallow structure that is lower than the nanorods height and resembles fused or removed nanorods. Away from that region, either on the part of the surface which was exposed to bulk water or the part which was exposed to air, no change in the hydrophobic properties of the surface is observed, and the SEM scan confirms that the nanorods seem intact in both regions.

Calorimetric and Scattering Studies on Micellization of Pluronics in Aqueous Solutions: Effect of the Size of Hydrophilic PEO End Blocks, Temperature, and Added Salt

Micellar behavior of five ethylene oxide–propylene oxide (EO–PO) triblock copolymers, called Pluronics, with similar molecular weights of middle block PPO (∼2250 g/mol) and varied percentages of poly(ethylene oxide) (10%, 40%, 50%, 70%, and 80%, referred to as L81, P84, P85, F87, and F88, respectively) was examined by thermal (isothermal titration calorimetry and high-sensitivity differential scanning calorimetry), spectral (UV–vis), and dynamic light scattering (DLS) techniques. Micellization was decreased with increasing hydrophilicity of copolymer but induced in the presence of salt. Critical micelle temperatures (CMTs) of copolymers at different concentrations, with and without sodium chloride, are reported. Viscosity and DLS results reveal that highly hydrophilic copolymers (F87 and F88) did not show significant change in micelle size even at temperatures close to cloud point, whereas micelle growth and sphere-to-rod transition occurred for P84 and P85. Surface tension of solutions in water and salt also show enhanced surface activity and salt-induced micellization. The CMTs for different systems using different methods are compared.

Viscosity of Gas – Inorganic Compounds

Publisher Summary This chapter presents the values of viscosity of gas (organic compounds) in tabular form. For the tabulation, an equation is selected for gas viscosity as a function of temperature. The tabulation is arranged by carbon number such as C, C2, and C3 to provide ease of use in quickly locating the data by using the chemical formula. The compound name and chemical abstracts registry number (CAS No) are also provided in tabular form. Values for regression coefficients are given in the adjacent columns. The temperature range for use is also given in the columns (TMIN and TMAX). The equation should not be used for temperatures outside this range. The next column provides the code for the tabulation. The last three columns provide values for gas viscosity at representative temperatures. The tabulation is applicable to gas at low pressure. Very limited experimental data are available for highly polar and high molecular weight compounds. Thus, the values for these compounds should be considered rough approximations. A comparison of calculated and data values is also shown for a representative compound. The graph shows favorable agreement of equation and data.

Enthalpy of vaporization at boiling point—Inorganic compounds

Publisher Summary This chapter presents the results for the enthalpy of vaporization at the boiling point for inorganic compounds in tabular form. The tabulation is arranged by alphabetical order such as Ag, Al, Ar, and Zr to provide ease of use in quickly locating the data by using the chemical formula. The compound name and chemical abstracts registry number (CAS No) are also provided in columns. The next column provides the normal boiling point and the enthalpy of vaporization at the boiling point. The last column gives the code for the tabulation. In preparing the tabulation, a literature search is conducted to identify data source publications. Both experimental values for the property under consideration and parameter values for estimation of the property are included in the source publications. The publications are screened, and copies of appropriate data are made. These data are then keyed into the computer to provide a database of values for compounds for which experimental data are available. The database also serves as a basis to check the accuracy of the estimation methods. Upon completion of data collection, estimation of values for the remaining compounds is performed. The compilations of CRC, Daubert and Danner, Lange’s Handbook, web sites, and Yaws are used extensively for the enthalpy of vaporization. The Kistiakowsky rule and Riedel method are primarily used for estimates of the enthalpy of vaporization at the boiling point.

Chapter 18 – Radius of gyration—Inorganic compounds

Publisher Summary This chapter presents the results of the radius of gyration for inorganic compounds in tabular format. The tabulation is arranged by carbon number such as C, C2, and C3 to provide ease of use in quickly locating the data by using the chemical formula. The compound name and chemical abstracts registry number (CAS No) are also provided in columns. Values for the radius of gyration are given in the adjacent column. In preparing the tabulation, a literature search is conducted to identify data source publications. Both experimental values for the property under consideration and parameter values for estimation of the property are included in the source publications. The publications are screened, and copies of appropriate data are made. These data are keyed into the computer to provide a database of values for compounds. Upon completion of data collection, estimation of values for the remaining compounds is performed. The compilations of Daubert and Danner and Yaws are used extensively. The radius of gyration is a helpful indication of the size of a compound. It is ascertained from the moment of inertia and molecular weight. Methods of calculation are described by Wikipedia.