Absorption refers to the process of a liquid being taken up by a solid material. This occurs when the liquid is able to penetrate the small pores and irregularities on the surface of the solid. The absorption of the liquid by the solid can also contribute to the wetting behavior, as the liquid is drawn into the spaces between the solid particles and adheres to the surface. The degree of absorption of a liquid by a solid depends on several factors, including the nature of the solid surface, the chemical properties of the liquid, and the temperature and pressure of the system. In general, materials that have a high surface energy and low contact angle with the liquid will absorb more of the liquid than materials with lower surface energy and higher contact angle. Additionally, liquids that are more polar or have higher surface tension will tend to wet surfaces more readily and be absorbed more readily. DROPimage Advanced software is ideal for measuring the rate of absorption in porous solids.
The Acid-Base Tool is used to measure surface energy using three different test liquids. Van Oss, et al, have shown that the contribution due to acid-base interactions can be expressed in terms of the product of their electron donor and electron acceptor components by using three liquids, one apolar and two polar. Recommended test liquids are methylene iodide for the apolar liquid and a polar liquid pair of water and glycerol. This tool can be found in DROPimage Pro and DROPimage Advanced software.
Adhesion (also called dispersive or adsorptive adhesion) describes the force that holds dissimilar molecules together. (See image under wettability section.) In surface science the Work of Adhesion is the interactive force between the liquid and solid phases and we use the Young-Dupree equation as follows:
In surface science, the term "adhesion" almost always refers to dispersive adhesion. In a typical solid-liquid-gas system (such as a drop of liquid on a solid surrounded by air) the contact angle is used to quantify adhesiveness. In the cases where the contact angle is low, more adhesion is present. This is due to a larger surface area between the liquid and solid and results in higher surface energy. The Work of Adhesion explains the interactive force between the liquid and solid phases and the Young-Dupree equation is used to calculate the Work of Adhesion. The contact angle of the three-phase system is a function not only of dispersive adhesion (interaction between the molecules in the liquid and the molecules in the solid) but also cohesion (interaction between the liquid molecules themselves). Strong adhesion and weak cohesion results in a high degree of wetting, a lyophilic condition with low measured contact angles. Conversely, weak adhesion and strong cohesion results in lyophobic conditions with high measured contact angles and poor wetting.
Adsorption is a process in which a solid material, known as the adsorbent, attracts and binds molecules or particles a liquid, known as the adsorbate, onto its surface. The adsorbate molecules adhere to the surface of the adsorbent by weak van der Waals forces or chemical bonds, and can accumulate in a monolayer or multiple layers on the surface. Adsorption occurs when the surface of the adsorbent material has a high affinity for the adsorbate molecules, which can be due to several factors such as the surface chemistry, porosity, and specific surface area of the material. The adsorbate molecules can be physically adsorbed, which involves weak intermolecular forces, or chemically adsorbed, which involves chemical bonding between the adsorbent and adsorbate molecules. The adsorption coefficient is the speed with which molecules of a surfactant are adsorbed at the solid surface.
Advancing and Receding Contact Angles are two important metrics used to describe the wetting behavior of a liquid on a solid surface. The advancing contact angle is the angle between the solid surface and the tangent to the liquid-vapor interface at the point where the liquid first comes into contact with the solid. This angle is measured while increasing the volume of the liquid drop on the solid surface or by tilting the solid. The advancing contact angle represents the maximum angle at which the liquid can spread on the solid surface, indicating the degree of wetting of the surface by the liquid. The receding contact angle, on the other hand, is the angle between the solid surface and the tangent to the liquid-vapor interface at the point where the liquid is just about to break contact with the solid surface. This angle is measured while decreasing the volume of the liquid drop on the solid surface or by tilting the solid and measuring the contact angle on the uphill side. The receding contact angle represents the minimum angle at which the liquid can maintain contact with the surface, indicating the degree of adhesion of the liquid to the surface. The difference between the advancing and receding contact angles is known as the contact angle hysteresis. Contact angle hysteresis occurs due to the presence of surface roughness, chemical heterogeneity, or other surface irregularities that can cause pinning or depinning of the liquid from the solid surface. The contact angle hysteresis is an important value in determining the stability of a liquid droplet on a solid surface, as well as the efficiency of liquid-solid interactions in various applications such as wetting, adhesion, and surface coating. There are two common methods for measuring the advancing and receding contact angles: the tilting base method and the add/remove volume method.
In the case of the tilting base method, as the solid is tilted from 0° to 90°, the uphill angle (or receding angle) decreases while the downhill angle (or advancing angle) increases. As some point the drop may release at the roll-off angle.
The video below shows an example of advancing and receding contact angles using the tilt method. In this experiment a drop of water is dispensed on a PTFE solid. The contact angle is measured twice a second while the tilt is changing at the rate of 1° per second. The drop stays attached all the way to 90° and the final advancing and receding contact angles are used to calculate the contact angle hysteresis. DROPimage Advanced is used in this video on a ramé-hart Model 290.
An alternate method of measuring the advancing and receding angles requires volume of liquid to be added or and removed while dynamically measuring the contact angle as shown below. This method can be automated with the ramé-hart Automated Dispensing System.
The Apparent Contact Angle (θA) is measured using the baseline (shown in red in the graphic below) and the drop tangent. The apparent contact angle is the one captured when measuring with a goniometer. This contrasts with the Young contact angle.
The Baseline occurs at the three-phase line where the drop phase, solid phase, and liquid phase meet. In all editions of DROPimage software, the baseline determines the vertex for the contact angle calculation.
The Captive Bubble method involves producing an air bubble on the underside of a substrate which is immersed in a liquid (typically water). DROPimage software can be used to measure the resulting contact angle. This method is particularly suitable for engineered tissue, breast implants, contact lenses, disposable diapers and other products fabricated from hydrogels and superabsorbent polymers.
Cassie State (or Cassie-Baxter State) is a wetting state of a solid surface that occurs when a liquid droplet rests on a textured or patterned surface, where the liquid partially wets the surface by forming pockets of air between the droplet and the solid surface. This state is named after the scientists who first described it, Bernard Cassie and Sydney Baxter, in 1944.
In the Cassie-Baxter state, the contact angle between the liquid and the surface is greater than the contact angle that would be obtained if the liquid were in direct contact with the surface, i.e., the Wenzel state.
This is because the air pockets reduce the area of the surface that is in contact with the liquid, leading to an apparent increase in the contact angle. The Cassie-Baxter state is favored when the solid surface is textured or patterned in a way that allows air pockets to form between the liquid and the surface. The texture or pattern can be engineered at the micro or nano scale, and can take various forms such as grooves, pillars, or bumps. The Cassie-Baxter state can have important practical applications, such as in self-cleaning surfaces, anti-fouling coatings, and enhanced heat transfer surfaces.
However, it is important to note that the Cassie-Baxter state can be unstable and can transition to the Wenzel state under certain conditions, such as when the surface is subjected to mechanical deformation, or when the liquid penetrates the air pockets due to high pressure or surface tension. Therefore, the stability and reversibility of the Cassie-Baxter state should be carefully considered when designing textured surfaces for practical applications.
Cohesion explains intermolecular attraction between like-molecules. In surface science it's the cohesive force that holds the liquid phase together and contributes to wettability (see graphic under wettability). The Work of Cohesion is defined as the force required to separate the liquid into separate parts and we use this formula:
Contact Angle can be measured by producing a sessile drop of liquid on a solid. The contact angle is measured by viewing the profile of the drop and then measuring the angle whose vertex begins at the three-phase line, i.e., where the drop meets the solid, and then proceeds along the tangency of the semi-spherical drop as shown in the diagram below. A simple contact angle measurement works best on a clean flat homogeneous solid which is horizontally leveled as shown below. Young's equation is used to explain the balance of forces that are present in three-phase system with a solid, a liquid, and an external gas phase. Cohesion explains the force that holds the liquid molecules together while adhesion explains the force that attracts the liquid to the solid.
Contaminated surfaces inhibit wetting and thus produce higher contact angles than are produced on clean surfaces. In addition to contamination, many other environmental conditions such as temperature, relative humidity, surface roughness, material homogeneity, and static electricity can affect contact angle. See also, ideal surface.
Dynamic measurements (where the contact angle is measured periodically over time) are useful for quantifying the efficacy of surfactants and other surface treatments. Other factors that can affect dynamic measurements include evaporation, molecular relaxation, absorption of the liquid into a porous solid, and adsorption.
Contact Angle Hysteresis is the difference between the advancing and receding contact angles and is used to characterize surface roughness and surface heterogeneity. It is affected by cleanliness, the presence of surfactants, and other surface treatments. In general, when the contact angle hysteresis is small, this would indicate that the solid surface is homogeneous in terms of structure (roughness) and chemical composition so long as the system is in a Wenzel state.
Dewetting can be the result of evaporation or absorption. If a sessile drop loses volume, the three-phase line may retract as shown below on the left. The dewetting phenomenon contrasts with pinning as shown on the right. Dewetting is also used to describe when a thin liquid film on a solid or on another liquid phase ruptures and produces drops. Dewetting is the opposite of wetting.
A Dynamic Contact Angle is measured on a drop that is in motion -- for example, while volume is being added to or removed from the drop -- or while tilt is occurring.
Goniometer or Contact Angle Goniometer is an instrument that is used to precisely measure static and dynamic contact angles of liquids on solids. The modern contact angle goniometer was invented by Dr. William Albert Zisman at the Naval Research Laboratory in Washington DC and built by ramé-hart in New Jersey. The original Zisman design, aka the NRL Goniometer Model 100-00, was in production for nearly four decades. The current generation ramé-hart goniometer replaces the microscope with a digital camera and imaging software to collect and measure contact angle. Additionally, the new generation of instruments can calculate surface energy, surface tension as well as perform advancing and receding measurements and other more advanced tasks.
Hydrophilicity occurs when a water drop forms with a small contact angle and wetting is nearly complete; surface energy is very high. If the water contact angle approaches 0°, the material is said to be superhydrophilic. Strictly speaking a hydrophilic surface is one that attracts water (hydro = water, philic = likes). When referring to other liquids, other descriptors (such as omniphobic may be more suitable.
Hydrophobicity occurs when a water drop forms with a large contact angle, say over 90°. In this condition wetting is considered poor and surface energy is low. If the contact angle exceeds 150°, such as water on a lotus leaf, then the material is considered superhydrophobic. If the contact angle exceeds 160°, then the term ultrahydrophobic is often used.
Hyperhydrophobicity and its converse, hyperhydrophilicity, are both terms used by Dr. Carel van Oss in an attempt to describe what most surface scientist refer to as superhydrophobicity and superhydrophilicity respectively. The argument is that Latin and Greek roots cannot be combined to form a new word. Since hydro is a Greek root, the correct prefix to connote an extreme condition should be hyper a Greek root, not super a Latin root.
An Ideal Surface is one that is perfectly smooth, flat, rigid, and chemically homogenous. An ideal surface will have no contact angle hysteresis. Most surfaces are not ideal. Virtually all surfaces are considered real surfaces.
Interfacial Tension is the surface free energy between two immiscible liquid phases (e.g. oil and water). It is similar to surface tension except that the external phase is a liquid rather than a gas. An emulsifier may be used to lower the surface free energy thus allowing the disparate liquids to mix.
Lipophilicity refers to an affinity for fats, lipids, and nonpolar solvents such as hexane. Such a surface is lipophilic.
Lipophobicity is the converse to lipophilicity.
Lotus Effect refers to the superhydrophobic condition produced by the leaf of a lotus plant which typically exceeds 170°. The lotus effect exhibits a low roll-off angle and is characterized by a Cassie regime in which the adhesive forces between the water drop and the surface are extremely weak thus providing a self-cleaning property. The Lotus Effect contrasts with the Petal Effect.
Lyophobicity is generally used to describe colloids. A lyophobic colloid, for example, is one that repels liquids (solvents) due to the limited interaction between the dispersed phase and the continuous phase. A solid is lyophobic if it repels a solvent.
Lyophilicity occurs when a liquid (typically a solvent) wets well on a surface and thus produces low contact angles.
NRL refers to the Naval Research Lab and also the first generation ramé-hart goniometer (e.g., ramé-hart Model 100-00 NRL Goniometer) which was designed by Dr. William Zisman of the Naval Research Lab in the 1950s and 60s.
Oleophilicity is characterized by a strong affinity for oils.
Oleophobic surfaces, by contrast, repel oils. Cell phones, for example, feature a touch screen with an oleophobic polymer film designed to resist fingerprints.
An omniphilic surface exhibits excellent wetting and produces low contact angles for both polar and apolar liquids. The condition is omniphilicity.
A Pendant Drop hangs, typically from a needle, and can be used to measure surface tension. An inverted pendant drop is used measure interfacial tension when the drop phase is less dense than the external phase.
The Petal Effect is the result of a Cassie impregnated wetting state and can be found on rose petals. The surface is hydrophobic like the Lotus leaf but the roll-off angle is typically high due to a greater adhesion between the petal and the water drop.
A Phase in surface science refers to a liquid (drop), solid (substrate), and gas (external phase). Commonly the liquid phase is water and the gas phase is air. Surface Tension is when a liquid pendant drop is suspended in a gaseous external phase. Interfacial Tension is measured when the external phase is another liquid. The temperature and relative humidity of the external phase as well as the temperature of the solid and liquid phases can affect contact angle and wettability.
Pinning (also referred to as contact line pinning) refers to the propensity for the three-phase line of a sessile drop to remain static even while the volume of the drop is decreasing - e.g., due to evaporation or absorption - as shown below on right - or increasing. The inverse is dewetting, see graphic on left below. Pinning can also occur when volume is added to a drop. If enough volume is added, eventually, the three-phase line will slip and wetting will occur. Pinning is typically caused by surface roughness.
Roll-off Angle is the maximum tilt angle that a solid can be tilted before a drop releases. This is most often measured using a tilting base option. See Advancing and Receding Contact Angles. Drops on hydrophobic solids tend to have lower roll-off angles than drops on hydrophilic surfaces which often don't even roll off. A higher roll-off angle indicates stronger adhesion forces. While generally liquids with higher contact angles exhibit lower roll-off angles, there are examples of superhydrophobic surfaces with high roll-off angles. This phenomenon is explained by high adhesive forces combined with even higher cohesive forces within the liquid molecules.
A Real Surface is one that is less than ideal. That is, it exhibits some degree of heterogeneity in terms of structure and chemical composition. Most surfaces are considered real surfaces. See also ideal surface.
cos(θA) = r cos(θY)
where θA is the apparent contact angle (or measured contact angle, θM), r is the roughness ratio and θY is the Young contact angle. As long as a drop is in the Wenzel state, roughness will amplify wetting. That is hydrophobic surfaces become more hydrophobic with increased roughness while hydrophilic surfaces become more hydrophilic. Roughness is typical with real surfaces.
A Static Contact Angle is measured on a sessile liquid drop on a solid in a gas phase or a captive bubble gas drop in a liquid phase on (the bottom of) a solid when the drop is not changing. The static contact angle will be less than the advancing contact angle but greater than the receding contact angle.
Superhydrophobicity occurs when the measured water contact angle greater than 150°. Super is often added to other descriptors such as superomniphobic, superlyophobic, etc.
Superhydrophilicity is used to describe complete or nearly complete wetting when the contact angle is at or close to 0°. The term ultrahydrophobicity is often used interchangeably.
Surface Energy or Surface Free Energy is the excess energy at the surface of a material compared with the material as a whole. There are various methods for measuring surface energy. Young's equation is one method which can be used to quantify surface energy. Other methods for measuring Surface Energy include: the Acid-Base Tool, the Work of Adhesion Tool, Zisman's Plot, Solid-Liquid-Liquid Surface Energy Tool, and the One-Drop Tool. Note that Surface Energy is measured in milli-Newtons per meter, or mN/m which is the same as mJ/m2. For more on surface energy tools, see this page.
Surface Tension is described as a measurement of energy on the surface of a liquid surrounded by a gas which allows it to behave like an elastic sheet and can be measured using the pendant drop or sessile drop methods on a ramé-hart tensiometer. Surface tension is measured in milli-Newtons per meter, or mN/M. One mN/m is the same as 1 dyne/cm. If the external phase is another liquid, it's called interfacial tension.
Surface Dilatational Elasticity and Viscosity are measured using an Oscillator which provides periodic oscillatory deformation of sessile and pendant drops and bubbles. The axisymmetric drop and bubble shape methods are used to measure these properties.
Surface Treatments consist of surfactants, which minimize surface tension of liquids, surface coatings and surface altering treatments (such as hardening, diffusion, and glazing) which are designed to alter the surface energy of solids.
Surfactant, derived from "surface acting agent", is a wetting agent used to lower the surface tension of a liquid allowing improved wetting. They can also reduce the interfacial tension between two liquids (such as oil and water). Soap is a common surfactant.
Tensiometer is any instrument that can measure surface tension. The ramé-hart Model 250, 290, 500, and 590 instruments with DROPimage Advanced, for example, measure surface tension and interfacial tension via the pendant and sessile drop method based on the Young-Laplace equation.
Three-phase Line is defined at the line (typically circular) that occurs at the intersection of the liquid drop phase, external gas phase, and solid phase. The three-phase line can increase or decrease in size due to wetting and dewetting.
Tilting Base Method. See Advancing and Receding Contact Angles.
Ultrahydrophobicity occurs when the measured contact angle of a water drop on a surface is greater than 160°. This is also often referred to as the Lotus Effect due to the super water repelling properties of the Lotus leaf. The ultra root can be added to other descriptors, such as in ultralyophobicity.
Ultrahydrophilicity is often used interchangeably with superhydrophilicity to indicate a condition where complete wetting or very low water contact angles are measured.
Wenzel State exists when the liquid of a drop fills the voids in the solid on which it sits. See diagram above under Cassie State. According to Wenzel's formula, a roughness variable is used to account for the increased surface area which results in higher contact angles on superhydrophobic solids.
Wettability defines the degree to which a solid will wet. If a drop spreads out indefinitely and the contact angle approaches 0°, then total wetting is occurring. In most cases, however, the drop will bead up and only partial wetting (or non-wetting) will occur. The extent to which a solid will wet can be quantified by measuring the contact angle.
Wettability determined by the cohesive forces of the liquid molecules among themselves and the adhesive forces that result from the molecular interactions between the liquid and the solid as illustrated in the diagram below. (In real life, the molecules are not so neatly organized.)
Wettability can be explained by the relative strength of the cohesive (Liquid/Liquid) and adhesive (Solid/Liquid) forces as shown above and below. Strong adhesion with weak cohesion produces very low contact angles with nearly complete wetting. As the solid/liquid interactions weaken and the liquid/liquid interactions strengthen, wetting diminishes and contact angle increases.
Wetting is the action of a liquid moving onto a solid. For example, when the volume of a sessile drop is increased, wetting can occur when the three-phase line increases in diameter. Wetting also occurs as a drop rolls along a surface: while wetting is occurring on the side in the direction the drop is moving, dewetting is occurring on the opposite side.
Work of Adhesion determines an index of wetting for a liquid on a given solid. The Adsorption Theory explains that van der Waals interactions should be sufficient for good adhesion.
Young-Laplace Equation in surface science is used to describe the capillary pressure between the interfaces of two phases (such as water and air) which results from surface tension. The equation is represented as follows:
∆p is the difference in pressure between liquids at the interface.
γ is the surface tension.
ň is a unit normal to the surface.
H is the mean curvature.
R1 and R2 correspond to the principal radii of curvature.
Young's Equation is used to explain the balance of forces of a liquid drop on a solid surface.
The Young Contact Angle is measured between the drop tangent and the surface tangent at the microscopic level.
Zisman, Dr. William is the inventor of the modern contact angle goniometer which he designed at the Naval Research Lab in Washington DC in the 1960's. For more information, see this article and this newsletter. Also available is the article An Example of Why Research Cannot Be Planned, written in 1973 and reprinted with permission.