טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentElena Vinnik
SubjectDetachment of Bubbles from Solid Surfaces
DepartmentDepartment of Chemical Engineering
Supervisor Professor Emeritus Marmur Abraham
Full Thesis textFull thesis text - English Version


Abstract

It is well known that gas bubbles tend to attach to solid surfaces in liquid environments. In many applications, bubbles may interfere with the flow over the surface and the effective heat/mass transfer through the surface, and are the most common cause of clogging of the passages/channels. In addition, bubbles may be trapped inside porous media, e.g. during the mixing of powders in liquids. This problem is related to many industrial applications. However, bubble detachment methods and mechanisms have not been sufficiently investigated. Research objectives are to study the effect of pressure reduction and temperature increase on bubble detachment from the solid surfaces, and find the optimal method to do it for various geometries and system properties. This method should allow modifications for a wide range of applications, e.g. detachment from a plane surface, from outside and inside porous materials agglomerates, from the short capillary holes in a plate and from curved surfaces. In this study we report our findings about the limiting equilibrium conditions under which equilibrium cannot be sustained and bubble detachment must take place, and a minimal bubble that may be detached by external pressure reduction and/or temperature changes for various geometries and system properties. We introduce the model that was developed in this study which expands our understanding of bubble detachment and evaluates and predicts remaining and detached bubbles shares. In the reported study, we investigated detachment of the bubbles from: a horizontal plate, a spherical particle, a cylindrical capillary hole inside an infinite plate and a “bagel” hole. The properties of the solid surfaces covered a range of hydrophobicity/hydrophilicity, and of heterogeneity that leads to contact angle hysteresis. The results of the study revealed that the size of a minimal bubble that may be detached from the surface by external pressure reduction and temperature increase is a strong function of the solid geometry and the properties of the solid surface for a given gas-liquid system. A theoretical model that was developed in the study in order to evaluate and predict remaining and detached bubble shares showed the effectiveness of the external pressure reduction and temperature increase on bubble detachment from the examined solid surfaces. The results of the study are expected to be useful for improving processes that suffer from the attached bubbles problem. They can also serve as a guide for experimental demonstration of the proposed method.