Surface tension as a controlled variable in mechanical dishwashing

Abstract

931Using a continuous, in-line, nondestructive technique based on the maximum bubble pressure method, the surface tension of the wash water in a mechanical dishwashing machine has been monitored. This technique has enabled surface tension to be used as a controlled variable, providing feedback to regulate the amount of surfactant added. Assuming the mechanical force of the water spray is adequate to remove bound soil from the dish surface, the food soil may be titrated against surfactant, providing an indirect indication as to when the dishes are clean. This technique also could be applied to a recirculated wash solution for in-place cleaning.In spite of its significance to a wide variety of processing operations, surface tension has remained one of the most neglected physical parameters to be used as a control variable. This situation can be attributed to a number of factors. The forces involved in surface tension are relatively small, and variations in some of the other variables may interfere with its measurement. The surface tension of a pure liquid can be markedly affected by small quantities of solutes. Consequently, contamination of test equipment or test solution may alter the result.Of the techniques available for the measurement of surface tension, the torsion balance, drop weight and capillary rise method all use delicate equipment which needs to be scrupulously clean prior to each determination. In contrast, the maximum bubble pressure method uses a relatively simple and robust apparatus. Furthermore, a new surface is created with each bubble, thereby eliminating problems due to surface contamination.Miller and Meyer (1) review some of these methods and describe an automated instrument for determining surface tension. Their method, based on the maximum bubble pressure technique, requires a known constant hydrostatic pressure and cannot be used when the liquid level varies {as in a mechanical dishwasher}.Smith and Schlein {2} describe a differential technique based on the maximum bubble pressure method: two capillary tubes of known different diameters are immersed in the test solution and a gas is bubbled through them into the liquid. The theory of this method is discussed in depth by Sugden {3) and Cuny and Wolf (4}. The surface tension {y) can be calculated from the equation: y = AAP + Bos + C ofl/y Where: A = g~m/2 (1/r~ -1/r~} B = g{[(r2-rJ/3-(tl -t2)/2]l(1/r,lira)} C = g2 [(r~-r~)/24 (l/r1 -l/r2)]rl and r~ are the radii of the narrow and wide tubes t~ and t2 are the depths of their immersion 6P is the difference of the maximum bubble pressure between the two tubes Qs is the density of the solution 0m is the density of the manometer fluid.Cuny & Wolf showed that the third term 'C' contributes less than 0.07% to the equation and can be neglected.If the tubes are arranged such that:(tl -t~}/2 --(r2 -rl}/3 then the second term becomes zero and also can be ignored.The maximum difference between the back pressures of the two 'bubbler' tubes is, therefore, a function of surface tension only, being unaffe...