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The captive bubble method is a method for measuring the contact angle between a liquid and a solid, by using drop shape analysis. [1] In this method, a bubble of air is injected beneath a solid, the surface of which is located in the liquid, instead of placing a drop on the solid as in the case of the sessile drop technique. A liquid and a solid are replaced by using drop shape analysis. [2]
The method is particularly suitable for solids with high surface energy, where liquids spread out. Hydrogels, such as those that comprise soft contact lenses, are inaccessible to the standard arrangement; so the captive bubble method is also used in such cases. [3] A contact angle is formed on a smooth, periodically heterogeneous solid surface. Above the solid surface, a liquid drop is submerged in a fluid. The measurement of contact angles usually contributes to the measurement of the surface energy of solids in the industry. Different from other methods of measuring the contact angle, such as the sessile drop technique, the system utilized in the captive bubble method has the fluid bubble attached from below to the solid surface, such that both the liquid bubble and the solid interact with a fluid.
As a system is formed from a solid surface and a drop of liquid, energy minima and maxima are produced by the free energy of the system. When the solid surface is rough or homogeneous, the system (made up of a solid, a liquid, and a fluid) could have multiple minima produced from the free energy at different minima points. One of these minima is the global minimum. The global minimum has the lowest free energy within the system and is defined as the stable equilibrium state. Furthermore, the other minima illustrate the metastable equilibrium states of the system. In between these minima are energy barriers that hinder the motion of energy between the various metastable states in the system. The transition of energy between metastable states is also affected by the availability of external energy to the system, which is associated with the volume of the liquid drop on top of the solid surface. As such, the volume of the liquid may have an impact on the locations of the minima points, which could influence the contact angles created by the solid and the liquid. The contact angles are directly related to whether the solid surface is ideal, or, in other words, whether it is a smooth, heterogeneous surface. [4]
Source: [5]
The measurement of contact angles with the captive bubble method could also be useful in the surface analysis of the reverse osmosis membrane in the study of membrane performances. Through the analysis of contact angles, the properties of membranes, such as roughness, can be determined. The roughness of membranes, which indicates the effective surface area, can further lead to the investigation of the hydrophilic and hydrophobic properties of the surface. Through studies, a higher contact angle may correspond to a more hydrophobic surface in membrane analysis. In the performance of the captive bubble method in membrane analysis, several factors can have an influence on the contact angle, including the bubble volume, liquid types, and tensions.
Source: [6]
In comparison to the use of the captive bubble method in the measurement of contact angles in other cases, the contact angle in the study of the lung surfactant monolayer is kept at a constant 180 degrees, due to the property of the hydrated agar gel on the ceiling of the bubble. The system applied in the study of lung surfactant is designed to be a leak-proof system, ensuring the independence of the surface film of bubbles from other materials and substances like plastic walls, barriers, and outlets. Instead of adding extra tubing or piercing the bubble air-water interface with needles, this closed system is created by adjusting the pressure within the closed sample chamber by adding or removing aqueous media to regulate the bubble size and surface tension of insoluble films at the bubble surface.
Since the bubble volumes are controlled by modifying the pressure in the sample chamber, the surface area and the surface tension of the surfactant film at the bubble surface are reduced as the volume of the bubble decreases.
The bubble shape, in this case, can vary from spherical to oval depending on the surface tension, which can be calculated through the measurement of the height and diameter of bubbles. In addition to measuring the surface tension, bubble formation can also be utilized in the measurement of the adsorption of lung surfactant, which defines how quickly substances build up on the air-liquid interface of pulmonary surfactants to form a film.
There are two methods to measure adsorption with captive bubbles:
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The sessile drop method is another popular way to measure contact angles and is done by placing a two-dimensional drop on a solid surface and controlling the volume of liquid in the drop. The sessile drop method and the captive bubble method are usually interchangeable when performing experiments, as they are both based on the properties of symmetry. Specifically, the axis of symmetry of the drop or bubble makes the contact line of the drop or bubble with the solid surface circular. This creates an observable contact angle corresponding to the contact radius of the drop or bubble.
However, interacting with a rough homogeneous surface in measurements of contact angles, the drop and bubble each present different behaviors in the measuring process, which are related to the volume of liquid and contact angles.
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.
Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, and liquid–gas interfaces. It includes the fields of surface chemistry and surface physics. Some related practical applications are classed as surface engineering. The science encompasses concepts such as heterogeneous catalysis, semiconductor device fabrication, fuel cells, self-assembled monolayers, and adhesives. Surface science is closely related to interface and colloid science. Interfacial chemistry and physics are common subjects for both. The methods are different. In addition, interface and colloid science studies macroscopic phenomena that occur in heterogeneous systems due to peculiarities of interfaces.
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects to float on a water surface without becoming even partly submerged.
Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word "surfactant" is a blend of surface-active agent, coined c. 1950. As they consist of a water-repellent and a water-attracting part, they enable water and oil to mix; they can form foam and facilitate the detachment of dirt.
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of the moment often form a loop or hysteresis curve, where there are different values of one variable depending on the direction of change of another variable. This history dependence is the basis of memory in a hard disk drive and the remanence that retains a record of the Earth's magnetic field magnitude in the past. Hysteresis occurs in ferromagnetic and ferroelectric materials, as well as in the deformation of rubber bands and shape-memory alloys and many other natural phenomena. In natural systems, it is often associated with irreversible thermodynamic change such as phase transitions and with internal friction; and dissipation is a common side effect.
In physics, the meniscus is the curve in the upper surface of a liquid close to the surface of the container or another object, produced by surface tension.
In surface science, surface energy quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energetically favorable than the bulk of the material, otherwise there would be a driving force for surfaces to be created, removing the bulk of the material by sublimation. The surface energy may therefore be defined as the excess energy at the surface of a material compared to the bulk, or it is the work required to build an area of a particular surface. Another way to view the surface energy is to relate it to the work required to cut a bulk sample, creating two surfaces. There is "excess energy" as a result of the now-incomplete, unrealized bonding between the two created surfaces.
An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of the twentieth century. A wide variety of synthetic membranes is known. They can be produced from organic materials such as polymers and liquids, as well as inorganic materials. Most commercially utilized synthetic membranes in industry are made of polymeric structures. They can be classified based on their surface chemistry, bulk structure, morphology, and production method. The chemical and physical properties of synthetic membranes and separated particles as well as separation driving force define a particular membrane separation process. The most commonly used driving forces of a membrane process in industry are pressure and concentration gradient. The respective membrane process is therefore known as filtration. Synthetic membranes utilized in a separation process can be of different geometry and flow configurations. They can also be categorized based on their application and separation regime. The best known synthetic membrane separation processes include water purification, reverse osmosis, dehydrogenation of natural gas, removal of cell particles by microfiltration and ultrafiltration, removal of microorganisms from dairy products, and dialysis.
Wetting is the ability of a liquid to displace gas to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with the first one. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. There are two types of wetting: non-reactive wetting and reactive wetting.
The contact angle is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle between the surface tangent on the liquid–vapor interface and the tangent on the solid–liquid interface at their intersection. It quantifies the wettability of a solid surface by a liquid via the Young equation.
Pulmonary surfactant is a surface-active complex of phospholipids and proteins formed by type II alveolar cells. The proteins and lipids that make up the surfactant have both hydrophilic and hydrophobic regions. By adsorbing to the air-water interface of alveoli, with hydrophilic head groups in the water and the hydrophobic tails facing towards the air, the main lipid component of surfactant, dipalmitoylphosphatidylcholine (DPPC), reduces surface tension.
In materials science, the sessile drop technique is a method used for the characterization of solid surface energies, and in some cases, aspects of liquid surface energies. The main premise of the method is that by placing a droplet of liquid with a known surface energy and contact angle, the surface energy of the solid substrate can be calculated. The liquid used for such experiments is referred to as the probe liquid, and the use of several different probe liquids is required.
In chemistry and materials science, ultrahydrophobic surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the superhydrophobic leaves of the lotus plant. A droplet striking these kinds of surfaces can fully rebound like an elastic ball. Interactions of bouncing drops can be further reduced using special superhydrophobic surfaces that promote symmetry breaking, pancake bouncing or waterbowl bouncing.
In materials science and biology, capillary condensation is the "process by which multilayer adsorption from the vapor [phase] into a porous medium proceeds to the point at which pore spaces become filled with condensed liquid from the vapor [phase]." The unique aspect of capillary condensation is that vapor condensation occurs below the saturation vapor pressure, Psat, of the pure liquid. This result is due to an increased number of van der Waals interactions between vapor phase molecules inside the confined space of a capillary. Once condensation has occurred, a meniscus immediately forms at the liquid-vapor interface which allows for equilibrium below the saturation vapor pressure. Meniscus formation is dependent on the surface tension of the liquid and the shape of the capillary, as shown by the Young-Laplace equation. As with any liquid-vapor interface involving a meniscus, the Kelvin equation provides a relation for the difference between the equilibrium vapor pressure and the saturation vapor pressure. A capillary does not necessarily have to be a tubular, closed shape, but can be any confined space with respect to its surroundings.
In physics, the maximum bubble pressure method, or in short bubble pressure method, is a technique to measure the surface tension of a liquid, with surfactants.
Adsorption is the adhesion of ions or molecules onto the surface of another phase. Adsorption may occur via physisorption and chemisorption. Ions and molecules can adsorb to many types of surfaces including polymer surfaces. A polymer is a large molecule composed of repeating subunits bound together by covalent bonds. In dilute solution, polymers form globule structures. When a polymer adsorbs to a surface that it interacts favorably with, the globule is essentially squashed, and the polymer has a pancake structure.
The du Noüy–Padday method is a minimized version of the du Noüy ring method replacing the large platinum ring with a thin rod that is used to measure equilibrium surface tension or dynamic surface tension at an air–liquid interface. In this method, the rod is oriented perpendicular to the interface, and the force exerted on it is measured. Based on the work of Padday, this method finds wide use in the preparation and monitoring of Langmuir–Blodgett films, ink & coating development, pharmaceutical screening, and academic research.
Jarosław Drelich (1957) is a Polish-born surface engineer and professor of materials science at Michigan Technological University. He also holds an adjunct professorship in the department of Chemical and Materials Engineering at the University of Alberta. He is known primarily for his contributions to the field of wetting phenomena, which include the effect of drop/bubble size on apparent contact angle on a rough surface, in addition to fundamental work on structured hydrophilic/hydrophobic alternating surfaces.
The Zisman plot the graphical method of the Zisman theory or the Zisman method for characterizing the wettability of a solid surface, named for the American chemist and geophysicist, William Albert Zisman (1905–1986). It is a prominent Sessile drop technique used for characterizing liquid-surface interactions based on the contact angle of a single drop of liquid sitting on the solid surface.
Emmie Helena Lucassen-Reynders, last name Reijnders in Dutch spelling, was a Dutch scientist specialising in colloid chemistry and theoretical physics. She worked in both academia and in industry.