Fluid surface tension

Despite the fact that most of the alveolar surface is composited of alveolar epithelium, three primary types of cells are present in the alveoli type I alveolar cells, type II alveolar cells, and alveolar macrophages. Type I alveolar cells are also referred to as squamous pulmonary epithelial cells and are the continuous lining of the alveolar sac. Type II alveolar cells are also referred to as septal cells. Type II alveolar cells secrete the alveolar fluid that is necessary to keep the surface moist and to maintain surface tension of the alveolar fluid surface tension is necessary to keep the alveoli from collapsing. Alveolar fluid is a suitable environment for proteins when compared to the low pH and high protease levels associated with the intestine... 

Mass density of fluid Surface tension of fluid... 

Figure 8.2 Solid-fluid surface tension y = yu /e for the Ar-COj system [10] at feT/e = 0.88, where e and cr are the Ar-Ar well-depth and size parameters for a Leimard-Jones (LJ) potential (e/fe = 120°, (T =3.4 A) plotted as a function of the reduced pressure...

It is important to note that, for an overwhelming majority of fluids, surface tension decreases with temperature, and consequently, the inequality 0 in a certain interval of the temperature variation will also be described). 

Formation of a stable nanoelectrospray is dependent on many variables including the fluid surface tension, solvent composition, conductivity of the fluid and the applied voltage and pressure. Flow rate flowing through a nozzle is dependent on these variables as well as the inner diameter of the nozzle. As an example, a nozzle with a 5.5 pm inner diameter and 28 pm outer diameter will spray a solution of 50% methanol with 0.1% acetic acid at a flow rate of lOOnLmin 1 with an applied voltage of 1.4kV and pressure and 0.2 psi. A nozzle with a 2.5 pm inner diameter and 28 pm outer diameter will spray this same solution at a flow rate of 20nLmin 1 with an applied voltage of 1.2 kV and pressure of 0.3 psi. 

The main factors affecting the stability island of an electrospray system is the fluid properties themselves in particular the surface tension and conductivity. The fluid surface tension directly affects the ability of a fluid to atomize an electrospray because it opposes the force applied by the ions at the fluid/air interface. Therefore, increasing the surface tension will increase the required field strength to establish a cone-jet mode. The conductivity of the fluid used has the effect of shifting the electrospray stability island to a narrower range and also to lower flow rates. This is shown in Fig. 32.12. 

In (32.12), H/j, is a dimensionless parameter comparing the viscosity of the fluid used in an electrospray to other fluid parameters such as fluid surface tension, fluid density, dielectric permittivity of the fluid and also the dielectric permittivity of free space. When is much greater than 1 then viscous effects are negligible. 

The effect of increasing the electrospray fluid surface tension tends to increase the electric field strength required to create a stable cone-jet mode. If a fluid, such as distilled water ( 0.074 N m at 20°C), has a sufficiently large surface tension, the ionization potential of the air around the electrospray is reached before any kind of electrospray is created. Therefore a stable cone-jet mode is difficult and almost impossible to create without the use of some sheathing gas around the electrospray system. 

The electrical potential causes the deformation of the fluid drop, and when the applied voltage develops enough force and balances with the fluid surface tension of the polymer solution, the drop is deformed under a cone shape with a semivertical angle of 30°. Beyond this critical value (Rayleigh limit], the electrostatic forces generated by the charge carriers overcome the surface tension and the deformed droplet undergoes a transition zone just before the fiber jet is initiated to the collector screen. By this way, fluid is... 

Thus far, the key dimensionless parameters useful in the design of microfluidic systems for miscible fluids have been discussed. For immiscible fluids, surface tension a also becomes important this parameter affects the dynamics of the free surface. This gives rise to two additional dimensionless quantities the capillary number and the Weber number. The capillary number represents the ratio of viscous forces to the surface tension forces acting across an interface between a liquid and a gas or between two immiscible liquids. It is defined as... 

The expressions for the pressure tensor at a surface can be integrated to give the solid-fluid surface tension, which we here denote as y (also called the spreading pressure) ... 

One also has an exact expression for the hard wall-hard sphere fluid surface tension y, which is [44]... 

Mean particle size Average sand density Average sand bulk density Fluid density Fluid surface tension... 

The following independent variables were investigated feed solids concentration (7 to 46% by volume), slurry feed rate (5 to 10 kg/s), drum speed (7 to 65 rpm), particle size (0.08 to 2.0 mm), and fluid surface tension (35 and 70 mN/m). 

Effect of Fluid Surface Tension on Slurry Hold-up... 

Experimental studies show that for medium particles (dj = 0.5 mm), the cohesive forces (agglomeration) are dominant and lead to higher slurry hold-up as compared with the coarse particles. To reduce these forces, the fluid surface tension, o, was reduced from 70 to 35 mN/m by the addition of Triton X-100 (a nonionic surfactant) to tap water at a concentration of 120 ppm. Figure 19 shows that the effect surface tension on the slurry hold-up was not significant for both particle sizes (dj = 0.5 and 2.0 mm). 

Figure 19. Effect of fluid surface tension on slurry hold-up.

Maximum angle reached during the upswing motion, rad o Fluid surface tension, mN/m... 

Even in the absence of buoyant forces, convection can occur, driven by gradients in the interfacial (surface) tension at the interface of two fluids. Surface tension is affected both by chemical concentration and by temperature. Figure 2 shows how a hot spot can cause convection by locally lowering the surface tension. The cooler fluid has a higher surface tension and draws the warm fluid towards itself If the temperature gradient is perpendicular to the... 

Table 1 provides an overview of a small sampling of liquids of the relevant properties for a few common optofluidic liquids. Note that this represents a non-comprehensive list, other parameters of particular interest include fluid-fluid surface tension, fluid-solid surface energy, contact angle, fluid-solid electroosmotic mobility, compatibility with soft elastomers (or other materials of interest), and numerous others. 

---