J. A. Maroto

On the kinetics of heteroaggregation versus electrolyte concentration: comparison between simulation and experiment

Abstract The heterocoagulation kinetics of two latexes bearing surface charges of opposite sign was studied. The doublet formation kinetic constant was used to determine the evolution of the kinetics. It has been adsorbed a non-ionic surfactant onto the anionic latex in order to increase its stability. Aggregation experiments were performed for both latexes bared. Also, measurements were carried out with the anionic latex partially covered. Comparison between these two results showed small differences in the kinetic constant. The study of the heteroaggregation kinetic constant versus ionic concentration showed a trend opposite to that of the homocoagulation case. The experimental results were compared with simulations. The model used for simulations takes into account the interaction forces between particles to perform the random paths for clusters. Comparison showed good agreement. The inclusion of attractive forces was found to be essential for describing processes faster than diffusion limited aggregations.

Use of a Mariotte bottle for the experimental study of the transition from laminar to turbulent flow

A Mariotte bottle is a device that provides a constant effusion velocity for liquids. We discuss a Mariotte bottle that has been designed to study the flow regime and the transition from laminar to turbulent flow. Several straight, smooth, circular glass tubes with different lengths and sections were inserted into the lower part of the bottle so that the rate of flow could be measured in very different experimental conditions by using a precision balance. Reynolds numbers in the interval 1014⩽R⩽6098 were obtained, showing that the flow regime is laminar for R⩽3000. There is a transition in the flow regime for R in the interval 3000–4000 and a turbulent flow regime for higher R. Because the device is very simple and the results obtained are very clear and exemplary, we recommend using this device as a laboratory experiment for physics or engineering students who require a knowledge of fluid mechanics.

INFLUENCE OF MULTIPLE LIGHT SCATTERING ON THE ESTIMATION OF HOMOCOAGULATION AND HETEROCOAGULATION RATE CONSTANTS BY TURBIDITY MEASUREMENTS

Abstract In this work, the homocoagulation and heterocoagulation rates of aqueous dispersions of uniform spherical particles of surfactant-free cationic and anionic latexes were measured with a spectrophotometer. Two polymer colloids with similar particle sizes and different surface charge densities were used. The homocoagulation and heterocoagulation rate constants (K d and K d 12 respectively) were determined as a unction of the total particle number N and the wavelength λ of the spectrophotometer. Constant values of K d and K d 12 were found when N and λ had specific values. The decreasing of the rate constants for lower λ or higher N values was explained as a consequence of the appearance of multiple light scattering at some experimental conditions. It was possible to detect critical N and λ values which prevent this phenomenon. By calculating the effective volume of solid in the cell at the critical wavelength Vef(λc), it is possible to define an experimental parameter NVef which provides a critical value above which the multiple light scattering takes place.

Experimental evaluation of the drag coefficient for smooth spheres by free fall experiments in old mines

Mines of Linares, Jaen, Spain, have been exploited from the age of the Phoenicians, Carthaginians and Romans. These silver and lead mines reached their maximum splendour at the end of the 19th century and at the beginning of the 20th century. Nevertheless, all the mining works finished in the 1970s and the diverse machinery was sold. Only the shafts remain since then which has now permitted carrying out interesting free fall experiments using smooth spheres of both cork and cast iron. The experiments were facilitated by the fact that the tubular shape of the shafts provides excellent transmission of sound, which made feasible the recording of the impact sound of the spheres with water at the bottom of the shafts at distances of up to 200 m. By using these experimental data, we have carried out an evaluation of the drag coefficient for the movement of smooth spheres through the air in the laminar regime with Reynolds number in the interval 103 to 2 × 105. This evaluation was in excellent agreement with the literature data. From the theoretical point of view, the analysis of the free fall movement includes a variety of concepts such as Newtons second law, the drag force, Archimedes principle and the velocity of sound, which makes these experiments very attractive for both physics teachers and physics students at university level. Finally, an easy experiment is proposed in this paper which has permitted an approximate evaluation of the drag coefficient for smooth spheres to be carried out in a laboratory environment.

The approximate determination of the critical temperature of a liquid by measuring surface tension versus the temperature

A classical experience in a physics student laboratory is to determine the surface tension of a liquid versus the temperature and to check the linear appearance of the obtained graph. In this work we show a simple method to estimate the critical temperature of three liquids by using experimental data of surface tension at different temperatures. By a logarithm fitting between surface tension and temperature, the critical temperature can be determined and compared with data from the literature. For two liquids (butanol and nitrobenzene) the comparison is acceptable but the differences are too high for the third liquid (water). By discussing the results it seems to be clear that the difference between the critical temperature of the liquid and the maximum temperature of the surface tension measurements is the determining factor in obtaining acceptable results. From this study it is possible to obtain more information on the liquid characteristics from surface tension measurements that are currently carried out in a student laboratory. Besides, in this paper it is shown how to select the most suitable liquids which provide both acceptable values for the critical temperature and measurements of the surface tension at moderate temperatures. The complementary use of numerical methods permits us to offer a complete experience for the students with a simple laboratory experiment which we recommend for physics students in advanced university courses.

Computational aids for the estimation of the molecular weight of petroleum oils from kinematic viscosity measurements

The ASTM standard D 2502-92 (reapproved 2004) provides a means of calculating the mean molecular weight (relative molecular mass) of petroleum oils from kinematic viscosity measurements. It is applicable to samples with molecular weights in the range from 250 to 700 g/mol and is intended for use with average petroleum fractions. Nevertheless, this estimation was carried out by using a viscosity-molecular weight chart that involves evident interpolation errors. In this work, we have used both of Hirschler’s equations as numerical methods in order to carry out an analytical estimation of the mean molecular weight of petroleum oils from kinematic viscosity measurements. The data provided by the named Hirschler-Maroto equation are in excellent agreement with the ASTM chart for a viscosity index range that covers the majority of commercial oils. Finally, by using the Hirschler-Maroto equation we have designed a PC program in order to make the evaluation of molecular weight of petroleum oils easier for engineers and professionals. The use of this program, which can be downloaded free from the Internet, only requires the insertion of kinematic viscosity data, which remove any graphical or interpolation errors. Nevertheless, the Hirschler-Maroto equation and the PC program must be cautiously used in order to avoid important errors when applied to the higher molecular weight constituents of petroleum.

Evaluation of the Lorentz law by using a Barlow wheel

In this work, an evaluation of the Lorentz law has been carried out by a Barlow wheel. Basically, a Barlow wheel is a primitive dynamo-electric machine (invented by P. Barlow in 1828) which consists of a metallic cogwheel, which moves around a hor- izontal axis. The theoretical analysis includes the Lorentz law and the Petroff law, which characterizes the friction between the axis and the bearing. In the stationary state (when the angular velocity is constant), a simple formula can be obtained which relates the current intensity and the angular velocity. Experimental data ap- proximately confirms this formula, and this device can be used as a simple and classical laboratory experiment in basic electromag- netism courses. Some modern complements were used for the ac- quisition of data, as a light barrier connected to a counter, a voltam- meter and a direct-current power supply Therefore, the Barlow wheel, originally designed for qualitative demonstrations, can be used as a quantitative and useful assembly in a laboratory of Gen- eral Physics.