Autoradiography studies of the processes of wetting and spreading

Abstract

Wetting and spreading of liquids over solid surfaces are the important stages of many natural and technological processes. Systematic studies in this branch of research covering the diversity of systems with various types of interaction between the liquid and the solid are performed, using direct methods based either on measuring the contact angles or the observation of the movement of wetting perimeter [1][2][3][4]. In modern units, computer-aided processing of the images of droplet or liquid front is performed [5]. Indirect methods such as the rupture of liquid "bridges" [6,7] and capillary rise method [1,8] are used, along with direct methods. In the case of well-wetting liquids, small contact angles on solid surface are measured by studying the light interference in thin menisci near the three-phase contact line (TPCL) [9][10][11].In general, these methods are sufficient to study the regularities of wetting and spreading, if the processes of interaction between the liquid and the solid are of no significance. However, in many cases, the diffusion, the mutual dissolution of liquid and substrate, and chemical reactions greatly affect the statics and dynamics of wetting [1,2]. To analyze the propagation of liquid front limited by physicochemical processes, one needs information on the distribution of substance within the bounds of wetted area, for example, between the grain body and grain boundaries, as well as on the TPLC structure.When discussing the wetting of various solids by surfactant solutions, it is also important to know the surfactant distribution on the solid surface, in particular, near the TPLC.Considerable opportunities to solve such tasks are provided by the tracer technique in combination with autoradiography. The aim of this work is to report results of studying the wetting and spreading processes by autoradiography.The autoradiography is a photographic method of the detection of ionizing radiation of radionuclide preliminarily added to the studied substance [12].Convenient for analysis photographic image (autoradiogram) is formed when a certain radiation dose hits the unit surface area of a photodetector.Autoradiograms are taken upon the direct contact between the studied surface and the detector. Then, they are analyzed visually, photometric measurements are performed, and the electron scanning of negative image or photograph is made. Photometric curves presenting the dependences of the degree of autoradiogram blackening on the labelled substance concentration make it possible to quantitatively estimate the substance distribution over the spreading area of liquid phase. The amount of the substance is proportional to the intensity of autoradiogram blackening. At present, there is a software package that allows one to digitally analyze the autoradiograms [13], thus broadening the method's possibilities. One can determine not only the distribution of liquid phase over the solid surface but also the adsorption of surfactants or impurities on this surface, study the kinetics of formation and the s...