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A new software tool is developed for droplet image analysis for the study of wetting of solid substrates. The tool extracts information exclusively from images and does not require the use of any properties of the system. Moreover, its applicability covers both axisymmetric and non-axisymmetric droplets. The developed software processes independently droplet images taken from side and top perspectives. Processing of side-view images is made by polynomial fitting to the droplet shape and provides important 2D geometrical features such as contact angles, length, height, contact point coordinates and contour outline. The analysis of top-view images is achieved through active contours (snakes) and yields droplet dimensions, centroid position and contour outline. The combination of synchronized side and top images provides the reconstruction of the droplet 3D shape by means of slices of circular arc shape, which allows estimation of droplet volume and of the distribution of contact angles along its perimeter. The above is an important feature that has not been delivered by other software tools. Experimental results to support the applicability of the new tool are presented for two distinct substrates having different surface properties (glass and Teflon).

Despite the extensive study of wetting in literature, there are still many unresolved issues regarding forced wetting. One of them refers to the effect of the initial droplet shape on the force required for spreading and sliding along a solid surface; an effect that has not ever been explicitly reported. Previous experimental works, in general, assume initially axisymmetric droplet shapes. In this study, experiments are performed with an innovative device, Kerberos, capable of subjecting sessile droplets at different tilting angles to varying centrifugal forces in order to explore the spreading/sliding behavior of droplets of different volumes and different initial shapes (including non-axisymmetric). A broad validation of the technique is achieved by the repeatability and consistency of measurements. Results for initially axisymmetric and non-axisymmetric droplets concerning contact angles, droplet length, droplet shape and velocity are presented and discussed. Furthermore, detailed results for the critical tangential accelerations required for the inception of spreading and sliding are presented showing the different sensitivity of these parameters on the droplet initial shape. Experimental results are employed to test the applicability of the well-known Furmidge equation for the retention force in the case of initially non-axisymmetric droplets. On this account, the initial length of droplets is found to be a more appropriate length scale in the Furmidge equation than the initial droplet width.

Despite the significant effort spent over the years to study wetting and dewetting phenomena of droplets on solid substrates under applied forces, there are still many unresolved issues. To name a few, a concise theory for the two dimensional contact line is missing. Also, a controversy has recently arisen for the effect of droplet size on the tangential force required for the inception of sliding. It is clear that to resolve such issues and to better understand the underlying processes a new generation of experimental techniques and results are needed. In this spirit, a novel device, Kerberos, is proposed and constructed here. Kerberos offers two major innovative features compared to prior techniques: (a) it allows simultaneous rotation and tilting of the droplet supporting plate (these mechanisms have been employed separately in the past) and (b) three Wi-Fi cameras (viewing the droplet from X–Y–Z directions) follow the rotation/tilting motion. The above features permit on one hand the independent control of normal and tangential forces applied to the droplet and, on the other, allow real time 3D impressions of the droplet shape. Indicative results manifest the wealth of new experimental data accessible by Kerberos which were out of reach by other techniques. The theoretical background of droplet behavior in the new device is discussed, making assumptions that lead to closed form solutions. Such a model is useful for the design of experiments with the new device.

The concept of a small volume emulsification device developed by the authors is further studied in the present work. Following suggestions of previous work, five different piston plates with orthogonal edges are constructed to oscillate inside a small cube-shaped cell. The different pistons yield several combinations of gap width and height between the plates and the walls of the cell. Their performance is assessed by performing emulsification experiments over an extensive range of piston stroke frequency and emulsification time for a specific non-coalescing system (fixed oil fraction and surfactant concentration). The resulting droplet size distributions are estimated by microscopy images analysis and by phase separation observations. The combined information from these techniques is necessary in order to register the broad size range of droplets. A semiquantitative analysis of the flow field in the gap reveals that droplet breakage process starts at the laminar boundary layers developed between the piston and the cell walls and allows characterization of this field. A theoretical analysis for the mechanism of droplet breakup is presented. The analysis of experimental results demonstrates that four of the five tested piston plates are appropriate for emulsification regarding the generated droplet size distribution and the choice among them requires additional criteria, e.g. energy requirements.

Pb2+ adsorption onto activated carbon was studied as a function of the different liquid medium during batch adsorption process without shaking/agitation of flasks (where adsorbate and adsorbent coexist). As liquid medium deionized water (DW) enhanced with gases bulk nanobubbles (bNBs) was used. The adsorbent material used was activated carbon from potato peels. The maximum Pb2+ adsorption capacity (Qm) was 9 mg/g in the case of adsorption process in DW without agitation, whereas the respective Qm was 94 mg/g by using DW-bNBs as medium without agitation. It was also found that Qm increased to 171 mg/g in the adsorption process with DW-bNBs and agitation (150 rpm). This finding was the start point of a new experimental design where after re-agitation (intense shaking of 225 rpm) of the respective experimental case (94 mg/g), the final Qm was 163 mg/g i.e. very close to the one corresponding to simultaneous adsorption and agitation from the beginning. The results are analyzed and discussed intensively to lead in several important implications on the influence of bNBs to the bulk solute mass transfer. Based on our experimental equilibrium data a modified-Langmuir equation was developed, applied and proposed so as to better simulate the whole (complicated) process.

Foams produced with surfactant concentration below the Critical Micelle Concentration (CMC) are usually moderately stable due to high drainage rates and intense bubbles coarsening/coalescence. Aim of this work is to examine how such low surfactant concentration affects foam destabilization and elucidate the interplay between free drainage and bubble size variation. Foam destabilization experiments are conducted at varying SDS concentrations (below the CMC) where the evolution of liquid fraction and bubbles size is registered simultaneously. Instantaneous volume measurements of the drained liquid and the remaining foam yield the evolution of the global liquid fraction and drainage rate in the foam. Continuous electrical conductance measurements give the local liquid fraction and drainage rate in the foam. Microphotographs allow estimation of bubble size distribution and bubble population at regular time intervals. The present data show that the lifetime of moderately stable foams depend largely on surfactant concentration below the CMC but this effect does not scale linearly with surfactant concentration. Furthermore, measurements are fitted to semi-empirical expressions and are compared to a modified Leonard and Lemlich (L–L) drainage model that has been expanded to incorporate bubble size evolution. The latter is a rough approximation based on certain assumptions but it is a fair approach given the excessive difficulty of detailed numerical calculations. The comparison reveals the significant role of bubble size on the free drainage of moderately stable foams. Interestingly, incorporation of global liquid fraction data into the modified L–L model yields surface shear viscosity values in agreement with literature.

This work studies single bubble–single particle interactions of interest to flotation applications. An experimental device has been developed where a standing bubble is approached at prescribed flow velocity by an aqueous dispersion of particles – much smaller than the bubble. Two separate high-speed cameras are employed to monitor the bubble surface from two different Cartesian directions allowing thus a 3D perspective of particles trajectories and collisions with the bubble. A special feature of the device is that the velocity of the suspension and the size of the bubble can be independently adjusted in a range of values that corresponds to the flotation process. This paper presents experimental trajectories and velocities of particles as they approach and flow past a bubble. A theoretical model has been developed to describe such particle trajectories and velocities. Comparison between experimental observations with model predictions allows a detailed assessment of governing forces and better understanding of their contribution to particle–bubble interactions. It is shown that microhydrodynamic drag has a distinct role in matching experiments with predictions.

A bubble-in-liquid bridge is a unique configuration that resembles the case of two neighboring bubbles in a foam separated by a liquid layer. It calls for a small bubble inside a liquid bridge with the curved free surface of the liquid bridge acting as part of a larger external bubble. This configuration can serve as a prototype to study foam dynamics. A devise exploring this concept has been presented in a previous work (Kostoglou et al., 2011). The present work adds to the theoretical background of the proposed devise. The particular case of zero Bond number is studied theoretically here. Even in its simplest form the particular system exhibits an interesting behavior. It is shown that there are two rupture modes of the liquid bridge as the liquid volume decreases i.e. neck rupture and film rupture. The prevailing one depends on system parameters. The evolution of the bubble size as the liquid volume decreases up to rupture, is extensively studied.

This work presents the results of an experimental parametric study of air-water dispersions in a laboratory scale stirred tank. The quantities monitored are total gas holdup and bubble size distribution. The physical parameters varied, in order to study their effect on the dispersion properties, are liquid viscosity as well as surface tension (static and dynamic) and surface viscoelasticity. Viscosity is varied by adding glycerol whereas surface properties are varied by adding Tween 80. In addition, the effect of the gas flowrate and the stirring rate are studied. Measurements of gas holdup are taken primarily by a non-invasive electrical technique which utilizes ring electrodes flush mounted to the wall. These measurements are verified against simultaneous differential pressure measurements and video images of the instantaneous height of the liquid free surface. Bubble size distributions are estimated analyzing still photographs. In the examined range of parameters, as the concentration of glycerol increases the gas holdup first increases and then decreases going through a peak at 41.6% v/v glycerol (viscosity: 5 mPa s). On the contrary, as the concentration of Tween 80 increases, the gas holdup increases monotonically up to a plateau value at and above 50 mg/l of Tween 80 (static surface tension: 38.3 mN/m). Further analysis of the results indicate no influence of coalescence/breakage phenomena in the observed bubble size distributions.

A new device is proposed for registering thin film drainage around a single foam bubble based on the increase of the electrical resistance of the draining film. Initially, a small bubble is expanded inside an electrically conductive liquid bridge that is formed between two vertically aligned metallic electrodes. Then the liquid of the bridge is withdrawn at a pre-selected flow rate until rupture of the bridge/bubble system while monitoring its electrical resistance across the electrodes. The present work reports results for liquid withdrawal rates between 20 and 200 μl/h which correspond to total drainage time between ∼3 and ∼25 min. Experiments are performed with aqueous solutions of SDS between 50 and 2000 ppm. A simple theoretical model is used to describe the evolution of the resistance of the global liquid bridge/bubble system. The model assumes a perfectly spherical bubble which affects the shape of the liquid bridge only as a volume constraint. A simple approach is employed for the computation of the electrical resistance of the system during the last stages of drainage where the thin film around the bubble gets gradually thinner and stretches until rupture. A detailed parametric study of the model is presented. The comparison between experimental and model results demonstrates the potential of the new device for the assessment of foam stability with respect to coalescence.

This work measures the variation of local water fraction during emulsification of hexane-in-water by an electrical conductance technique employing multiple non-intrusive ring electrodes. The emulsifier, C 10 E 5 , was initially dissolved only in water although it is highly soluble in both phases. This gives birth to interesting emulsification kinetics. Experiments were conducted at four emulsifier concentrations (from 1 × 10 −4 to 1 × 10 −3 M) and three hexane-to-water ratios (from 40/60 to 5/95, v/v). Emulsification was conducted by intense mixing with a Rushton turbine. CFD calculations were employed to estimate the distribution of turbulent dissipation rate in the mixing vessel. Furthermore, measurements of interfacial tension (static and dynamic) and interfacial dilatational elasticity and viscosity were conducted. An effort was made to explain the observed phase's distribution during emulsification by invoking arguments with respect to interfacial properties and turbulent dispersivity.

This study deals with the electric double layer repulsive force between two colloidal flat plates, approaching at constant velocity and undergoing surface charge relaxation. In particular, closed form solutions are obtained for the case of low surface potential and ion exchange between surface and Stern layer. A closed form solution is also presented for the case of low potential and finite rate of surface charge dissociation using a perturbation expansion. In the former case, the force between the surfaces upon contact depends on the approach velocity through a relation derived in the present work, whereas in the latter the force at contact always diverges to infinity. Asymptotic results for the force upon contact are also obtained for the general nonlinear case, i.e. for any level of potential. Using these results, an optimized numerical code is developed for the solution of the nonlinear problem. The repulsive force profiles are computed for several cases and discussed.

A mathematical framework is developed for measuring the dilatational rheological properties of gas-liquid interfaces by analyzing small amplitude radial oscillations of an air bubble blown into a liquid substrate. Contrary to earlier analyses which customarily require the bubble radius change during the oscillation, the present study is based on the readily measurable quantities of bubble pressure change during the oscillation and phase lag between the bubble pressure and the relative displacement of the oscillation generating device. To expose the extensive interrelationships among the various experimental parameters that have a profound effect on bubble dynamics, the analysis is focused on the influence of the variable gas volumes at the sides of the differential pressure transducer, the gas phase compressibility, the dynamic behavior of the transducer membrane and the bubble surface being only a segment of a sphere, as they play a key role in the response of the system. A linear viscoelastic surface behavior is assumed which includes intrinsic surface viscosities and Gibbs elasticities. New data are deduced from previous measurements on the dilatational surface properties of stearic acid monolayers which, being practically insoluble in water, offer an ideal experimental system without the complications due to bulk-interface interactions. For a fixed frequency of oscillation, a virtually constant dilatational surface viscosity is determined throughout the employed concentration range. Yet, surface viscosity is found to slightly increase when the oscillation frequency increases. In addition, for the range of the examined parameters, no firm evidence of a dilatational surface elasticity is observed.