Determination of Variables Affecting Surface Tension Using Simple Bubble Method

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Objectives: determine the surface tension of a dishwashing liquid-water solution determine the effect of solution concentration on the surface tension investigate the surface tension as the tube radius varies


A bulk liquid molecule experiences a net zero force due to equal intermolecular forces of attraction in all directions caused by surrounding liquid molecules of the same type. But at surfaces, the liquid molecules are in contact with a different kind of molecule which may cause different forces of attraction compared to that experienced by the bulk molecules. This difference in attraction causes a net force which may not be equal to zero. Depending on the type of molecule in contact with the liquid’s surface and the forces of attraction between these two different molecules, the net force may cause an inward pull or an outward pull. If the cohesive force of the liquid, which tends to hold like molecules together, is greater, the net force is inward. This net inward pull causes the liquid surface to contract. The surface of the liquid behaves as if it were in tension like a structured membrane (Probstein 1994). Surface tension is the state of being stretched tight like an elastic sheet, due to the unequal attraction of the particles in the surface layer to the environment and to the bulk of the liquid (Cheak et al 2004).

A soap bubble has two surfaces- the inner and outer surfaces. These two surfaces enclose a thin liquid film of soap solution. The liquid’s surface tension affects the size of the bubble formed. With a smaller surface tension, the inward pulling forces is smaller such that the force exerted due to pressure inside the bubble may cause it to expand more easily. Soap has an effect of decreasing the surface tension of pure water. Surfactant solutions contain a hydrophilic (water-loving) “head” and a hydrophobic (water-hating) “tail” (KRÜSS). The surfactant molecules accumulate at the surface and thus reducing the surface tension. METHODOLOGY

Five different concentrations of 5%, 25%, 50%, 75% and 100% by weight of the dishwashing liquid-water solution is prepared. The experimental setup is shown below.
Figure 1. From

The free end of the tube is dipped into the solution such that only a thin film of solution is produced making sure that no excess drop is present. The tube is kept vertical so a clamp is used. Next, the syringe is slowly pushed so air is introduced and a bubble forms. Insufflating of the bubble is continued and the maximum pressure reading on the manometer is measured. For each concentration, five tubes of different radius are used. Surface tension is plotted against concentration at different tube radius and against radius of the bubble at different concentrations.


In this experiment, it is assumed that the maximum pressure occurs when a hemispherical bubble is formed or when the radius of the bubble is equal to the radius of the tube. This is the same assumption used in maximum bubble pressure method (KRÜSS). The radius of the tube is taken as the average of its inside and outer radius. From the Young-Laplace equation, the pressure difference between the gas inside and outside of a spherical bubble is given as Equation 1

A plot of surface tension versus radius of the tube (or bubble) at constant concentration is shown in Figure 2. It is observed that there is no direct linear relationship between the surface tension and bubble radius as the data points fluctuates (increases then decreases and vice versa). This is expected since the relationship is between the maximum pressure difference and the radius of the bubble as given by Equation 1. If the maximum pressure difference is plotted against the inverse of the bubble radius as in Figure 3, a more linear relationship is observed with the slope as the surface tension, γ. Figure 2

From Figure 2, it can...
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