Contact Angle Measurement And Contact Angle Interpretation Pdf
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Contact angle measurements are typically done by using either optical or force tensiometers. Contact angles can be divided into static, dynamic, and roughness corrected contact angles. Contact angle hysteresis and roll-off angles are also measured.
- Surface-wetting characterization using contact-angle measurements
- Surface Innovations
- Surface-wetting characterization using contact-angle measurements
- Techniques of Measuring Contact Angles
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Surface and Colloid Science pp Cite as. The previous chapter was largely theoretical, in that it dealt with the interpretation of contact angle results in terms of solid surface energies. It also delved into the question of how the structure of a solid surface affects the contact angle that a liquid forms on the solid.
Surface-wetting characterization using contact-angle measurements
Contact angle measurements are typically done by using either optical or force tensiometers. Contact angles can be divided into static, dynamic, and roughness corrected contact angles.
Contact angle hysteresis and roll-off angles are also measured. There are three different forces acting on this three-phase contact point between solid, liquid and gas, or in some cases solid, liquid and liquid.
Contact angles can be divided into static, dynamic and roughness corrected contact angle. Static contact angles are by far the most measured wettability values. It is most suitable for relatively smooth and homogenous surfaces.
Static contact angles are also used to define the surface free energy i. Static contact angle offers a quick, easy and quantitative measurement of wettability. When the three-phase boundary is moving, dynamic contact angles can be measured, and are referred to as advancing and receding angles. As the terms imply, advancing contact angle is measured when the droplet front is advancing and receding when the droplet front is receding.
On an ideal surface, these two values are close to each other. However, most often, the measured contact angle depends on the direction on which the contact line is moving. Contact angle hysteresis arises mostly from the chemical and topographical heterogeneity of the surface, solution impurities absorbing on the surface, or swelling, rearrangement or alteration of the surface by the solvent.
The roll-off angle refers to a tilting angle at which the droplet starts to move. A low roll of angle is related to low contact angle hysteresis. Contact angles are affected by roughness. Surfaces with the same chemical composition but different roughness will have different contact angles. Roughness corrected contact angles should especially be used when surface free energies are determined. Optical tensiometers are utilized to measure static, dynamic and roughness corrected contact angles.
The main components of the optical tensiometer are a camera, dispenser to dispense a drop, sample stage and the light source to illuminate the drop on the sample stage.
Optical tensiometers range from completely manual systems to fully automated instruments. Optical tensiometer is sometimes also called optical or contact angle goniometer, contact angle meter or drop shape analyzer. Essentially all of these mean the similar type of instrument which we use the name optical tensiometer. Sessile drop measurement is the most commonly used contact angle measurement method.
In practice, a droplet is placed on the solid surface and an image of the drop is recorded. The static contact angle is then defined by fitting Young-Laplace equation around the droplet, although other fitting methods such as circle and polynomial can also be used.
It is also possible to measure inverted sessile drops called captive bubbles. The captive bubble measurements are especially utilized to study wettability of the contact lens.
It is also suitable for other highly hydrophilic materials where the standard sessile drop method is challenging. Also, it is very much used in applications where the contact angles of immersed objects are being measured. For example, measurement of oil drop on a rock surface immersed in water.
Dynamic contact angle measurements with the optical tensiometer are done with two main methods: needle method and tilting method. In the needle method , a small droplet is first formed and placed on the surface. The needle is then brought close to the surface and the volume of the droplet is gradually increased while recording the contact angle at the same time.
This will give the advancing contact angle. The receding angle is measured the same way but this time, the volume of the droplet is gradually decreased. This method is especially utilized to measure dynamic contact angles on superhydrophobic surfaces.
In the tilting method , the droplet is placed on the substrate which is then gradually tilted. The advancing angle is measured at the front of the droplet just as the droplet starts to move. The receding contact angle is measured at the back of the droplet, at the same time point.
With this method, the roll-off or sliding angle can also be determined. The tilting can be done either by tilting the sample stage or by tilting the whole instrument. Roughness corrected contact angles are measured with the combined surface roughness and contact angle measurements. The optical tensiometer is combined with the topography module to facilitate the roughness measurements.
The roughness is measured at the same spot as the contact angle. The roughness ratio is used to correct the contact angle for surface roughness. Wenzel equation explains the relation between roughness ratio and measured and corrected contact angle. It is important to note that the Wenzel equation assumes that the liquid completely penetrates into the roughness grooves.
Force tensiometry consists of a highly sensitive balance where the sample is attached through a hook. The test liquid is placed on a measurement cup on the sample stage that can be moved either manually or automatically. Force tensiometers range from completely manual stand-alone systems to fully automated computer-controlled instruments.
For contact angle measurements the fully automated instruments are used. With the force tensiometer, the measured contact angles are always advancing and receding angles and contact angle hysteresis. Due to dynamic nature of the method it is not possible to measure static contact angles. The contact angle measurement with the force tensiometer is based on so-called Wilhelmy plate method. Force tensiometer measures the mass affecting to the balance when a solid sample is brought in contact with the test liquid.
The contact angle can then be calculated when the surface tension of the liquid and the perimeter of the sample are known. The advancing angle is measured as the sample is immersed into the liquid and receding angle when the sample is withdrawn out from the liquid.
As the perimeter of the sample must be known, the sample has to be rectangular or rod shape. The contact angle is averaged across the whole area dipped into the test liquid. This means that the surface has to be the same on both sides and the edges of the sample.
Wilhelmy plate is then not suitable for the contact angle measurements of surface treatments done just one side of the sample. Fibers and other thin objects can be measured by using optical tensiometer equipped with picoliter dispenser, with the meniscus method or with force tensiometer using Wilhelmy method.
With special optics and a high-speed camera, it is possible to take an image of this small droplet and the contact angle can be determined in a similar fashion than by using microliter size droplets.
Meniscus measurement can also be used for fibers. In this method, the sample is first immersed into liquid. When the sample is pulled from the liquid a meniscus is formed and the contact angle is optically measured. Force tensiometer, on the other hand, can be used by utilizing a special holder for the fiber. It should be noted that the measurement with picoliter droplets leads to static contact angles whereas the Wilhelmy method is dynamic, giving both advancing and receding angles.
The contact angle measured with the meniscus method is somewhere in between and cannot be directly compared with contact angles measured with the other two methods. Contact angles on powders can be measured by using sessile drop measurement on the compressed powder tablet or by using the force tensiometer with the Washburn method. When compressed tablets are measured, it can behave like the absorbing substrate if the powder is hydrophilic or the droplet can stay on the surface if the powder is hydrophobic.
The main issue with the tablet compression is that the surface properties of the tablets can vary a lot, which can lead to misinterpretation of the results. Roughness on the tablet surface could be considered by measuring the roughness corrected contact angle. To study loose powders, the Washburn method must be utilized. In this method, the powder is packed in a cylinder with holes at the bottom.
The time it takes for the liquid to penetrate is related to contact angle through Washburn equation. In some cases, the contact angle measurements need to be done on small areas.
This could be anything from a printed circuit board to a catheter surface. For these measurements, picoliter-sized drops are needed.
The measurements can be done with the optical tensiometer equipped with a picoliter dispenser that produces small enough drops to fit on the location measured. High magnification optic is required for drop visualization. Apart from that, the measurement is similar to the standard static contact angle measurement.
Contact angle measurements are most commonly done at ambient conditions but in some cases, temperatures and even pressures need to be controlled. For temperature control, there are cooling and heating options for both optical and force tensiometer.
For measurements at high pressures, a high-pressure tensiometer is required. In addition to the standard camera and light source needed for contact angle measurements, the high-pressure tensiometer includes a high-pressure chamber.
The high pressure tensiometer is especially utilized to study wettability at reservoir conditions for enhanced oil recovery optimization.
There are several different standards for contact angle measurements with optical tensiometers. Standards are mainly related to very practical applications such as testing of surface cleanliness and evaluation of surface after plasma or corona treatment. There are also standards for surface free energy determination through contact angle measurements and more general guidelines for how to perform dynamic contact angle measurements with the needle or tilt method. We have put all the knowledge of this page in a guide for you to read when you have the time.
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Download PDF. Wettability refers to how easily a liquid spreads over the surface of a contact lens. Clinically, this can be observed by viewing the interaction between the tears and the lens surface. The contact angle is the angle formed between a drop of liquid and the surface of the lens. Small contact angles are associated with an increased ability of the tears to spread out over the surface of a contact lens and lead to a more stable tear film. The three main laboratory techniques used to measure the contact angle of contact lenses are the sessile drop , captive bubble , and Wilhelmy plate methods, which are described in detail below.
Surface properties of dental polymers: measurements of contact angles, roughness and fluoride release. In the present study, five dental polymers were investigated to ascertain their contact angles, wettability, roughness, and fluoride release in dry or wet conditions. Fluoride did not modify the contact angles, but increased the roughness of the finished material. Keywords: dental polymers, surface roughness, surface hydrophobicity, release of fluoride. Aesthetic restorative materials of composite resins regularly used in dentistry are composed of polymers to which inorganic particles are usually associated
Contact angle is one of the common ways to measure the wettability of a surface or material. Wetting refers to the study of how a liquid deposited on a solid or liquid substrate spreads out or the ability of liquids to form boundary surfaces with solid states. The wetting, as mentioned before is determined by measuring the contact angle, which the liquid forms in contact with the solids or liquids. The wetting tendency is larger, the smaller the contact angle or the surface tension is. The contact angle is an angle that a liquid creates with a solid surface or capillary walls of a porous material when both materials come in contact together. This angle is determined by both properties of the solid and the liquid and the interaction and repulsion forces between liquid and solid and by the three phase interface properties gas, liquid and solid.
Interpretation and meaning of the measured contact angles can be made simpler if the basic concepts are clearly understood and theory-based.
Surface-wetting characterization using contact-angle measurements
The contact angle is one of the most sensitive experimental values describing a junction between three phases, being influenced by the composition and properties of contacting media as well as the structure and composition of interfaces involved. Since then, the contact angle has remained one of the most important values measured experimentally during characterization of solids and their wetting characteristics. As a result, the attention of scientists and researchers in the past two centuries has been on development of methods for accurate contact angle measurements, interpretation of experimental values and understanding of the causes of contact angle value variation and contact angle hysteresis. This paper reviews advancements made in interpretation of experimental contact angles and their use in characterization of solid surfaces. This is a relatively broad window of time for a scientific community to study the contact angle and understand the physics behind it.
E-mail: marmur technion. The measurement and interpretation of contact angles deceptively appear to be simple. This paper attempts to summarize the pitfalls in the field, and how to avoid them. First, the fundamental underlying theory that is necessary in order to properly measure and interpret contact angles is discussed, emphasizing recent developments. Then, the practical implications of these theoretical aspects are presented.
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Techniques of Measuring Contact Angles
Protocol DOI: Wetting, the process of water interacting with a surface, is critical in our everyday lives and in many biological and technological systems. The contact angle is the angle at the interface where water, air and solid meet, and its value is a measure. The contact angle is the angle at the interface where water, air and solid meet, and its value is a measure of how likely the surface is to be wetted by the water. The most common method for surface-wetting characterization is sessile-drop goniometry, due to its simplicity. The method determines the contact angle from the shape of the droplet and can be applied to a wide variety of materials, from biological surfaces to polymers, metals, ceramics, minerals and so on.
The contact angle, as a vital measured parameter of wettability of material surface, has long been in dispute whether it is affected by gravity. Herein, we measured the advancing and receding contact angles on extremely low contact angle hysteresis surfaces under different gravities G and found that both of them decrease with the increase of the gravity. The underlying mechanism is revealed to be the contact angle hysteresis and the deformation of the liquid-vapor interface away from the solid surface caused by gradient distribution of the hydrostatic pressure. The real contact angle is not affected by gravity and cannot measured by an optical method. The measured apparent contact angles are angles of inclination of the liquid-vapor interface away from the solid surface. Furthermore, a new equation is proposed based on the balance of forces acting on the three-phase contact region, which quantitatively reveals the relation of the apparent contact angle with the interfacial tensions and gravity. This finding can provide new horizons for solving the debate on whether gravity affects the contact angle and may be useful for the accurate measurement of the contact angle and the development of a new contact angle measurement system.
The contact angle is the angle , conventionally measured through the liquid, where a liquid — vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liquid, and vapor at a given temperature and pressure has a unique equilibrium contact angle. However, in practice a dynamic phenomenon of contact angle hysteresis is often observed, ranging from the advancing maximal contact angle to the receding minimal contact angle. The equilibrium contact angle reflects the relative strength of the liquid, solid, and vapour molecular interaction. The contact angle depends upon the medium above the free surface of the liquid, and the nature of the liquid and solid in contact. It is independent of the inclination of solid to the liquid surface.
Request PDF | Contact angle interpretation in terms of solid surface droplet onto the surface of samples and measuring the contact angle (Â.
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