EXPERIMENTAL AND NUMERICAL STUDY OF WALL WETTABILITY ON HYDRODYNAMICS OF LIQUID-LIQUID TWO-PHASE FLOW IN MINIATURE GEOMETRIES

Abstract

Liquid-liquid two-phase flow in micro/mini geometries have attracted scientific communities and industries due to their high-end application in mass transfer, reaction, biomedicals and many other transport processes. The dynamics of such flows are highly influenced by the wettability of the wall contact angle. Phase distribution influences the mass transfer or reaction in miniature geometries. Hence, the present study tried to interpret the fundamental concepts of wettability and its relevance in the context of liquid-liquid two-phase systems. Further spatial variation of wettability may lead to double emulsion. This structure is desirable for targeted drug delivery and encapsulating active ingredient. A systematic investigation of the impact of spatial variation of wettability with in straight and serpentine geometry on each flow pattern is conducted. Three different sets of experiments are performed. In first sets of experiments, the straight glass capillaries are coated with 2%, 10%, 20% (wt/wt) Polytetrafluoroethylene (PTFE) solution giving homogeneous surface with contact angles of 125º, 155º, 169º, respectively. The major flow patterns observed in coated tubes are aqueous slug with film, aqueous slug without film and aqueous droplet. Organic slug with film, organic slug without film and organic droplets are observed in non-coated glass capillary. A theoretical study based on free energy of formation gives a correlation for film formation as a function of Weber number of slugs in terms of contact angle and aspect ratio of slugs. The change in wettability from hydrophilic to hydrophobic enhances the phase inversion. The second set of experiments are performed in a straight capillary with contact angle jump within capillary. The contact angle jump is created by half coating the capillary with different weight % PTFE solution. The contact angle jump of 169º to 13º, 155º to 13º and 125º to 13º are created with 20%, 10%, 2% (wt/wt) PTFE solution. Most flow patterns observed during the experiments are similar to the 1st set of experiments except encapsulated slug flow. The encapsulated slug flow is observed when there is contact angle jump of 169º to 13º. Pressure drop was found to increase with an increase in the wettability of the test section. It is maximum during the formation of encapsulated slug flow due to formation of an additional interface. An analytical model is proposed, which shows the critical value of volume fraction during film formation in terms of phase velocities, physical properties, and wettability. The third set of experiments are performed to study the effect of bends and wettability of bends on the hydrodynamics of liquid-liquid two-phase flow. The bends of the glass channel are replaced with hydrophobic silicone tubing with similar internal and outer diameter. The presence of bends increases the breaking and joining of slugs. The addition of hydrophobic bend produces encapsulated slug also. At last numerical simulations are performed at different contact angles to see the effect of contact angle and phase properties on slug dynamics. Slug velocity is smaller at partial wetting contact angles as compared to fully wetting/non-wetting contact angle. Vorticity inside slug is higher at extreme contact angles (169º/13º), it is very high near interface. At Weber number (0.075<𝑊𝑒𝑠𝑙𝑢𝑔<0.135), slugs have partial developed films. In partial develop film, the contact line velocity is slightly smaller than that of slug velocity, which cause lubrication. At Weber number (𝑊𝑒𝑠𝑙𝑢𝑔>0.135) fully developed film is observed. The film thickness increases with decrease in interfacial tension. The pressure inside slug also increases with increase in interfacial tension.