The study of fluid surfaces plays an important role in understanding the interfaces encountered in biological systems, as it allows for the investigation of the basic characteristics such as the formation, stability and permeability. Moreover, the adhesion and the fusion of biological membranes can be better understood by the experimental investigations of drops and bubbles formation in controlled dynamical processes. These studies have the potential to generate novel and value information for medical applications in the diagnosis and therapy using microfluidic-based biosensors and controlled drug-delivery micro-devices. In this paper, the dynamics of fluid interfaces have been studied experimentally and a method for determining the surface/interfacial tension is proposed. The analysis started with the investigation of the soap bubble formation and break-up. The rupture was triggered manually, by pinching the tip with a needle. The burst was recorded with high-speed cameras and the burst speed was determined. Furthermore, the thickness of the fluid membrane was approximated and the surface tension was calculated using the Culick-Taylor's law. The obtained values for the surface tension were in the same order of magnitude with that from the literature, thus, considering that the employed method can lead to adequate results. Subsequently, a set-up was created to automatically generate fluid bubbles, at different imposed flow rates. The spontaneous burst was analyzed for three different liquids: soap solution, vegetable oil and polyacrylamide. The phenomenon is characterized by the Ohnesorge number, which takes into account the influence of viscous forces in relation to the inertial and surface tension forces. For the soap bubbles, the obtained thickness of the membrane was in the range of (300-500) nm. The calculated surface tension was found to be 0.038 N/m. In the case of automatically generated fluid bubbles, the lowest Ohnesorge number was obtained for soap bubbles and the highest for oil bubbles. Moreover, soap bubbles had the highest break-up speed, while vegetable oil and polyacrylamide had lower and similar break-up speeds. The experimental study described in this paper is an alternative method for the identification of material parameters, such as density and surface tension, in a dynamical process. Numerical simulations are reported from the viewpoint of servo time constant performance.
Experimental study on the formation and break-up of fluid bubbles
