The fluid disturbance resulting from the surface tension gradient in the interface of the gas-liquid contact systems is usually called the Marangoni effect or surface tension effect. For the liquid thin film, the gas solutes are easier to dissolve into the thinner liquid film than the thicker liquid film [ 1 ], and the concentration in the thinner part would be higher than that in the thicker part.
Therefore, the surface tension gradient is formed in the interface. The Marangoni positive and negative systems can be defined by changes of the gas-liquid contacting area. The interfacial convection resulting from the Marangoni effect is usually called the Marangoni convection. The interfacial instability resulting from the surface tension is normal to the liquid surface, and it is always called the Marangoni instability. The surface tension meter was based on the method for a Wilhelmy plate, as shown in Figure 1.
A Wilhelmy plate is a thin plate, usually in the order of a few square centimeters in area, used to measure equilibrium surface or interfacial tension at a gas-liquid or liquid-liquid interface. The top of the plate was connected to a spring, and the spring was connected to a force sensor.
The equilibrium state is reached instantaneously between the surface tension acting on the plate and the spring force pulling up the plate as the plate comes in contact with the liquid surface.
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According to the Wilhelmy equation, shown in Eq. Mentioned earlier, the flow of interfacial fluid could be provoked by the Marangoni effect. The methods for inducing surface tension gradient include the design of electric field with ionic compounds, concentration difference, temperature difference and surfactant addition to the liquid solution. Therefore, the interfacial disturbance could be produced by solutes transferring across phases. Some studies described the relationship between the Marangoni effect and transport phenomena, which are introduced as follows. The working electrodes were set in the experimental cell to detect the potential energy [ 2 ].
The concentration of the ferrocenyl surfactant was decreased with the decreased potential energy and then the surfactant became the surface active matter. The experimental results showed that the velocity of Marangoni flow decreased with the decreased desorption rate of the ferrocenyl surfactant. The concentration gradient of the desorbed ion resulted from the potential energy, and the surface tension gradient was formed by the concentration gradient. Therefore, the fluid disturbance was provoked in the surface layer of the liquid solution.
In addition, the concept of the solutal Marangoni effect was always used to describe the fluid flow in the liquid surface by vaporizing the volatile materials. For example, see Ref. On the other hand, the studies about the thermal Marangoni effect focused on the convective phenomena for the bulk liquid layer with heating in the bottom. Since the heat source was from the bottom, the liquid density decreased along with the depth of the liquid solution. Therefore, the upward convection occurred due to the temperature gradient.
Furthermore, the temperature of the upward fluid was still higher than the ambient liquid in the surface, and then the surface tension gradient was formed to make the outward flow radial. The local outward flow could be observed all over the liquid surface, and it was the so-called cellular-type flow. The small disturbance analysis was adopted to deduce that the surface tension gradient resulted from the change of temperature, which was large enough to produce the cellular-type flow in the liquid convective cell that was heated from the bottom [ 4 ].
In order to enhance the mass transfer performance for water vapor absorption by the solution in the absorption-refrigeration system, some surface additives were added to the liquid surface to activate the interfacial disturbance. N-hexanol, n-heptanol, n-octanol, 2-ethyl-hexanol, or n-nonanol was used as a surfactant to enhance the performance for water vapor absorption by the aqueous LiCl solution [ 5 ].
For the aqueous LiBr solution, adding the surfactants, except n-hexanol, induced the interfacial disturbance. Besides, the absorption capacities for CO 2 absorbed by water were increased as methanol, ethanol, n-propanol, n-butanol, and n-propanol added onto the liquid surface [ 6 ]. However, the interfacial disturbance could not be observed as n-hexanol and Triton X were added on the surface of water. Based on the thickness of the liquid layer, the Marangoni number was also calculated to assess the critical point for the interfacial disturbance resulting from the spreading liquid.
In addition to adding surfactants on the liquid phase, there are also some studies adding surface additives to the gas phase to discuss the effect of surface active materials on mass transfer performance and the relationship between mass transfer performance and interfacial disturbance. By measuring the surface tension of liquid solution, the surface tension of liquid solution affected by the vapor of 2-ethylhexanol 2EH in the gas phase was demonstrated [ 7 ].
The experimental results showed that the effect of surface additives on surface tension was larger for adding in the gas phase than in the liquid phase. Subsequently, the dynamic theory for the absorption and desorption of 2EH on the surface of the LiBr aqueous solution was discussed [ 8 ]. The simulated results showed that the higher the vapor pressure of 2EH in the gas phase, the better the mass transfer performance for absorbing H 2 O by the LiBr aqueous solution.
Therefore, the mass transfer performance could be enhanced by the interfacial disturbance resulting from adding surface additives in the liquid and gas phases while the operating variables were controlled well. Discussions of the interfacial behaviors resulted from adding surfactants to the gas phase, which were limited in the literature, and the related data were rare.
Mentioned earlier, the surface tension was affected by adding surfactant to the liquid and gas phases, leading to the influenced mass transfer performance by the interfacial disturbance resulting from the surface tension gradient. Therefore, the surfactant was added in the gas and liquid phases to discuss the effect of surfactant on mass transfer performance.
Besides, the mass transfer performance with and without surfactant addition to the working solution in the packed-bed absorber was also compared. Not only was the relationship between mass transfer process and interfacial phenomena described but also the enhancement of mass transfer performance for the absorption system was demonstrated in this study.
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Table 1 shows some literature related to mass transfer equipment with continuous liquid phase. These mass transfer equipment include packed-bed absorber, packed-bed or tray distillation column, falling film absorber, concentric absorption system, and bubble absorber. Except for bubble absorber [ 9 , 10 ], a continuous liquid phase was presented as a solution film in the mass transfer equipment for all others. As mentioned by Wu [ 11 ], the Marangoni effect could be triggered in mass transfer systems with continuous liquid phases.
Therefore, mass transfer behaviors that occurred in the solution film are discussed in this article. In order to discuss the spontaneous Marangoni effect in the absorption process, an absorber packed closely with cylindrical packing was designed [ 11 ]. The solution flow rate was controlled under the state of laminar flow. Since the surface tension of water vapor is larger than that of TEG solution, the spontaneous Marangoni effect is triggered by absorbing water vapor in the solution film. Although the mass transfer performance could be enhanced by adding a promoter in capturing CO 2 by potassium carbonate KCO 3 , the pressure drop and holdup increased in the packed absorption column.
In addition to adding surface additives, the Marangoni instability could also be produced by the temperature dependence of the surface tension, such as nonlinear model of the instability in gas absorption was developed [ 13 ] to discuss the performance for carbon dioxide absorbed by water. Recently, the structured packings with different thickness and channel angles were designed [ 14 ] to study effect of packings and surface additives on the performance of water vapor absorbed by LiCl film.
The criteria for determining the positive or negative driving force for the packed-bed distillation column were based on the packings; however, the criteria for determining the positive or negative Marangoni effect was decided by the mixture. In addition to the packed-bed absorber, the falling film or wetted wall column was also applied for the absorption process widely.
Furthermore, the flat copper plate and the copper plate covered with a copper wire screen were also tested to observe the Marangoni convection resulting from adding 2EH to the solution film [ 20 ]. In contrast with water vapor absorbed by aqueous lithium bromide solution, carbon dioxide absorbed by aqueous monoethanolamine MEA solution could be regarded as a chemical absorption process. Since the surface tension of the absorbent solution was changed by a chemical absorption process, the Marangoni effect was always accompanied with this process.
Lube thin. BA represents raising the temperature at constant pressure, whereas CA causes boiling by reducing the pressure at a constant temperature. The spinodal represent the thermodynamic limit of instability. The size increases very rapidly as the supersaturation decreases. The work of formation of the nucleus correspondingly decreases, so that bubble nucleation becomes easier as the contact angle gets larger. Contact angles are measured through the liquid phase or more generally, through the denser phase.
The advancing and receding angles are shown. The Pyrex surface is close to complete coverage at ppm. It can be seen that by stopping the etch at various times 9— 81 m are shown a range of half angles can be produced. It is possible to produce essentially straight-sided conical pits, steeply curved pits or round-ended pits, but these geo-metries cannot be achieved independently of the half angle. By using a controlled, collimated radiation source, it is possible in principle to produce pits where the axis intersects the polymer surface at different angles, giving openings of different sizes and geometries e.
On the right b , a small-mouthed re-entrant cavity. In each case, the opening radius is a. The part ACB is the normal superheat limit. As the temperature is increased, so the time required for explosive boiling of liquid drops rapidly decreases. In going from If the superheated liquid droplets are exposed to radiation, very much lower superheats are needed to cause bubble nucleation. Temperatures as low as The equilibrium is stable for a thermodynamically closed bubble but unstable for an open bubble. When an excess phase is present Winsor I case , the chemical potential of the oil in the fully equilibrated droplet is the same as in the excess oil phase.
Redrawn from A. Kabalnov, B. Lindman, U. Olsson, L. Piculell, K. A technician draws blood into a small-diameter tube just by touching it to a drop on a pricked finger. A premature infant struggles to inflate her lungs. What is the common thread? All these activities are dominated by the attractive forces between atoms and molecules in liquids—both within a liquid and between the liquid and its surroundings. Attractive forces between molecules of the same type are called cohesive forces. Liquids can, for example, be held in open containers because cohesive forces hold the molecules together.
Attractive forces between molecules of different types are called adhesive forces. Such forces cause liquid drops to cling to window panes, for example. In this section we examine effects directly attributable to cohesive and adhesive forces in liquids. Surface Tension Cohesive forces between molecules cause the surface of a liquid to contract to the smallest possible surface area. This general effect is called surface tension. Molecules on the surface are pulled inward by cohesive forces, reducing the surface area.
Molecules inside the liquid experience zero net force, since they have neighbors on all sides. Cohesive forces between molecules cause the surface of a liquid to contract to the smallest possible surface area. Forces between atoms and molecules underlie the macroscopic effect called surface tension. These attractive forces pull the molecules closer together and tend to minimize the surface area. This is another example of a submicroscopic explanation for a macroscopic phenomenon.
The model of a liquid surface acting like a stretched elastic sheet can effectively explain surface tension effects. For example, some insects can walk on water as opposed to floating in it as we would walk on a trampoline—they dent the surface as shown in [link] a. The iron needle cannot, and does not, float, because its density is greater than that of water. Rather, its weight is supported by forces in the stretched surface that try to make the surface smaller or flatter.
If the needle were placed point down on the surface, its weight acting on a smaller area would break the surface, and it would sink. Surface tension supporting the weight of an insect and an iron needle, both of which rest on the surface without penetrating it. They are not floating; rather, they are supported by the surface of the liquid. Surface tension is proportional to the strength of the cohesive force, which varies with the type of liquid.
Surface tension is defined to be the force F per unit length exerted by a stretched liquid membrane:. The liquid film exerts a force on the movable wire in an attempt to reduce its surface area. The magnitude of this force depends on the surface tension of the liquid and can be measured accurately. Surface tension is the reason why liquids form bubbles and droplets. The inward surface tension force causes bubbles to be approximately spherical and raises the pressure of the gas trapped inside relative to atmospheric pressure outside.
It can be shown that the gauge pressure inside a spherical bubble is given by. Thus the pressure inside a bubble is greatest when the bubble is the smallest. Another bit of evidence for this is illustrated in [link]. When air is allowed to flow between two balloons of unequal size, the smaller balloon tends to collapse, filling the larger balloon. With the valve closed, two balloons of different sizes are attached to each end of a tube.
Upon opening the valve, the smaller balloon decreases in size with the air moving to fill the larger balloon. The pressure in a spherical balloon is inversely proportional to its radius, so that the smaller balloon has a greater internal pressure than the larger balloon, resulting in this flow. Convert this pressure to mm Hg. The radius is given and the surface tension can be found in [link] , and so can be found directly from the equation.
Substituting and into the equation , we obtain. Note that if a hole were to be made in the bubble, the air would be forced out, the bubble would decrease in radius, and the pressure inside would increase to atmospheric pressure mm Hg. Our lungs contain hundreds of millions of mucus-lined sacs called alveoli , which are very similar in size, and about 0.
You can exhale without muscle action by allowing surface tension to contract these sacs. Medical patients whose breathing is aided by a positive pressure respirator have air blown into the lungs, but are generally allowed to exhale on their own. Even if there is paralysis, surface tension in the alveoli will expel air from the lungs. Since pressure increases as the radii of the alveoli decrease, an occasional deep cleansing breath is needed to fully reinflate the alveoli.
Respirators are programmed to do this and we find it natural, as do our companion dogs and cats, to take a cleansing breath before settling into a nap. The tension in the walls of the alveoli results from the membrane tissue and a liquid on the walls of the alveoli containing a long lipoprotein that acts as a surfactant a surface-tension reducing substance.
The need for the surfactant results from the tendency of small alveoli to collapse and the air to fill into the larger alveoli making them even larger as demonstrated in [link]. During inhalation, the lipoprotein molecules are pulled apart and the wall tension increases as the radius increases increased surface tension.
During exhalation, the molecules slide back together and the surface tension decreases, helping to prevent a collapse of the alveoli. This tension change is a unique property of these surfactants, and is not shared by detergents which simply lower surface tension. If water gets into the lungs, the surface tension is too great and you cannot inhale. This is a severe problem in resuscitating drowning victims.
A similar problem occurs in newborn infants who are born without this surfactant—their lungs are very difficult to inflate. This condition is known as hyaline membrane disease and is a leading cause of death for infants, particularly in premature births.
Surface Tension and Adsorption Studies by Drop Profile Analysis Tensiometry
Emphysema produces the opposite problem with alveoli. Alveolar walls of emphysema victims deteriorate, and the sacs combine to form larger sacs. Because pressure produced by surface tension decreases with increasing radius, these larger sacs produce smaller pressure, reducing the ability of emphysema victims to exhale.
A common test for emphysema is to measure the pressure and volume of air that can be exhaled. In order for this activity to work, the needle needs to be very clean as even the oil from your fingers can be sufficient to affect the surface properties of the needle.