Smart and green interfaces: From single bubbles/drops to industrial environmental and biomedical applications

Interfaces can be called Smart and Green (S&G) when tailored such that the required technologies can be implemented with high efficiency, adaptability and selectivity. At the same time they also have to be eco-friendly, i.e. products must be biodegradable, reusable or simply more durable. Bubble and drop interfaces are in many of these smart technologies the fundamental entities and help develop smart products of the everyday life.     

Circulatory bubble dynamics: From physical to biological aspects

Bubbles can form in the body during or after decompression from pressure exposures such as those undergone by scuba divers, astronauts, caisson and tunnel workers. Bubble growth and detachment physics then becomes significant in predicting and controlling the probability of these bubbles causing mechanical problems by blocking vessels, displacing tissues, or inducing an inflammatory cascade if they persist for too long in the body before being dissolved. By contrast to decompression induced bubbles whose site of initial formation and exact composition are debated, there are other instances of bubbles in the bloodstream which are well-defined. Gas emboli unwillingly introduced during surgical procedures and ultrasound microbubbles injected for use as contrast or drug delivery agents are therefore also discussed. After presenting the different ways that bubbles can end up in the human bloodstream, the general mathematical formalism related to the physics of bubble growth and detachment from decompression is reviewed. Bubble behavior in the bloodstream is then discussed, including bubble dissolution in blood, bubble rheology and biological interactions for the different cases of bubble and blood composition considered.     

A New Method for the Characterization of Electrically Conducting Liquid Bridges

An electrical conductance technique is employed in investigating the behavior of constant volume liquid bridges when their length is altered. The liquid bridges are edge-pinned between two vertical, identical rods with a variable separation distance. Rods of different radius, material, and edge geometry are examined as they play a role in the response of the system. It is shown that liquid bridge volume and rod radius are the parameters that mainly influence the conductance signal. A mathematical framework is developed for the identification of the geometrical characteristics of liquid bridges explicitly from conductance data. The role of gravity is discussed in both the experiments and the theoretical analysis. The theoretical predictions obtained show a close agreement with measurements. Copyright 2000 Academic Press.     

A conductance study of reducing volume liquid bridges

This work demonstrates how electrical conductance measurements can be employed for the study of liquid bridge behavior when their volume varies with time while their separation distance remains constant. The liquid bridges are edge pinned between two vertical, identical rods (r-bridges) at varying separation distances. Liquid evaporation is used as a means of reducing the bridge volume in a continuous smooth fashion. A zero-order continuation sequence with respect to Bond number and liquid bridge volume is combined with the shooting method for the solution of the Young-Laplace equation to give the liquid bridge shape as a function of its instantaneous volume. A novel, very efficient computational scheme is developed based on singular perturbation expansion for the solution of the Laplace equation in the liquid bridge to compute its electrical conductance that proved faster by orders of magnitude compared to other alternative approaches. The potential for estimating the liquid bridge characteristics or the evaporation rate by matching the experimental and theoretical results is discussed extensively.     

Modeling local flotation frequency in a turbulent flow field

Despite the significance of turbulent fluid motion for enhancing the flotation rate in several industrial processes, there is no unified approach to the modeling of the flotation rate in a turbulent flow field. Appropriate modeling of the local flotation (bubble-particle attachment) rate is the basic constituent for global modeling and prediction of flotation equipment efficiency. Existing approaches for the local flotation rate are limited to specific set of conditions like high or low turbulence. In addition, the combined effects of buoyant bubble rise and/or particle gravity settling are usually ignored. The situation is even vaguer for the computation of collision and attachment efficiencies which are usually computed using the gravity induced velocities although the dominant mode of flotation is the turbulent one. The scope of this work is clear: the development of a general expression for the flotation rate in a turbulent flow field which will cover in a unified and consistent way all possible sets of the problem parameters. This is achieved by using concepts from statistical approach to homogeneous turbulence and gas kinetic theory.     

Electrical conductance study of theta-liquid bridges

This work investigates the behavior of small liquid bridges that are formed between two horizontal supporting surfaces, aligned at the vertical direction. The contact lines of the liquid bridges are not edge-pinned but free to move across the supporting surfaces with the contact angle as a parameter (theta-bridges). An a.c. electrical conductance technique coupled with high resolution optical images is used to characterize the geometrical details of constant volume liquid bridges when their length is increased gradually until rupture. A mathematical framework is developed for the identification of the geometrical characteristics of theta-liquid bridges explicitly from conductance data. Theoretical predictions show good agreement with measurements for most of the bridge lengths (separation distance between supports) except close to the rupture point where the bridge is highly stretched. It is further shown that for short and moderate separation distances the present model can be used with confidence to determine the bridge volume and neck radius from the electrical signal.     

Dynamic surface activity of phenylalanine glycerol-ether surfactant solutions measured by a differential maximum bubble pressure tensiometer

A refined differential maximum bubble pressure tensiometer was used for measuring the dynamic surface tension at various concentrations of a nonconventional surfactant, a member of a new homologous series of phenylalanine glycerol-ether amphiphiles, with 10 carbon atoms to the hydrophobic alkyl chain (C(10)-PhGE). The effective bubble formation frequency for the examined surfactant concentrations was varied from 2 bubbles per second to 1 bubble per 20 s. The variation of equilibrium surface tension with concentration as well as the critical micelle concentration were determined by a Wilhelmy plate technique. Comparisons between dynamic and equilibrium surface tension values demonstrate that, under the employed surface deformation rates, the equilibrium surface tension is a misleading indicator of surface activity. This is also supported by simple surface rheology considerations. Results based on a diffusion-controlled kinetic analysis provide further evidence on the strong dependence of surface activity on the particular time scale of deformation.