Determination of Contact Angle of Colloid Particles
The determination of contact angles in colloid particle uses three major processes. These methods include wilhelmy plate, pendant drop, and sessile. However, there is a difficulty experienced when measuring the colloids particle contact angle. The small size of these particles makes it a challenge with the existing measurement methods for contact angles. Contact angle denotes the angle conventionally calculated via the liquid, where a vapor or liquid interface comes into contact with a solid surface. Colloidal particles are system of two phases that consist of a dispersed and continuous phase. The size of these particles is very miniature and range from 1 nanometer to 1 micrometer. The various examples of dispersions of colloidal particles are emulsions or solid or liquid, suspensions or liquid or liquids, and foams or gas or liquids. If the size of the particle decreases, there occurs the increase of surface area because of the functionality of the total volume. This paper aims at detailing the methods necessary for determination of contact angles between particles. This paper details all these processes via steps including what contact angles are and how they are calculated.
There are various methods, which are used to measure contact angles. Contact angles are either dynamic or static. Dynamic contact angles are the processes at the boundary of the solid or liquid during the decrease or increase in volume of the drop. The formation of a boundary is not that instantaneous, but a dynamic equilibrium takes time before it is established. A high flow rate is discouraged for the measurement of retreating and advancing of an angle as the contact angle will be calculated at a boundary that has not formed completely. Measurement of the static contact angle is not altered by the drop size. The time effect type determines the constant angle as it can decrease or increase with time (Okatan, 45).
Contact angle denotes the angle conventionally calculated via the liquid, where a vapor or liquid interface comes into contact with a solid surface. This is classified using a diagram as:
There are three main methods which are used in the calculation of contact angles. These methods include wilhelmy plate, pendant drop, and sessile. The sessile method is used to measure dynamic and static contact angles. The sessile method requires a surface that is flat, where the liquid drop is placed. Colloids are mainly deposited onto slides of a microscope as a film. Particles, which are spherical monodisperse like those of microspheres, are mainly associated with the sessile method. Soil colloids are also calculated using the sessile method. Smectites, which are soil colloids, are very suitable with the sessile method, because they are said to form a water surface that is impermeable. Penetration of the liquid is dependent on the porous nature of the colloids, and its consequence is the change of the contact angle with time. The sessile method is very important in the measurement of contact angles in soil particles (Mittal, 57).
The use of the static sessile method to calculate contact angle applies the use of a goniometer. This machine uses subsystem, which uses optical as a way of capturing a pure liquid profile on a solid substrate. The angle formed between the solid or liquid interface and the vapor or liquid interface is denoted as the contact angle. Older generation applied the use of a backlight in a microscope optical system. The use of softwares and cameras with high resolution as a way of analyzing the contact angle is used today (Okatan, 76). The following is a goniometer in use.
The following picture shows how a goniometer views the contact angles. The first picture indicates when the volume of the drop is being increased and the other when it is being decreased.
The measurement of a contact angle uses the dynamic sessile method. This method has similarities with the static method, but the modification of the drop is required. The study involving dynamic sessile drop, which is very common defines the largest contact angle available without the increment of the interfacial area of the liquid or solid by dynamically adding volume. The maximum angle attained is also the advancing angle. The removal of volume makes it possible to obtain the smallest possible angle, or the receding angle. Hysteresis is the difference between the receding and the advancing angle (Mittal, 83). The following denotes the dynamic sessile method.
The use of the sessile method has various advantages. One of the advantages is that it is very straight forward. The other advantage is that, with a solid surface which is large enough, multiple droplets placed on the sample in various locations are used for the determination of heterogeneity. The reflection of heterogeneity is achieved as a result of reproducibility of particular values of the properties of the surface’s energy. However, the disadvantage is heterogeneity may be difficult if that the sample is large enough to accommodate one droplet, thus may be difficult for the assumption of homogeneity (Okatan, 89).
The other alternative method used for calculation of the contact angle is the wilhelmy. This method applies meter of sensitive force to calculate the force translated to the contact angle value. A sample of the solid which is a small plate shaped is attached to the force meter arm and dipped to a probe liquid pool vertically, and the force exerted by the liquid to the sample is calculated by the use of a force meter (Mittal, 99). The following equation denotes the relationship between the force and the contact angle:
Where the total force calculated by the force meter is denoted by F, the force of buoyancy which occurs as a result of liquid displacement by solid sample is denoted by Fb, the wetted length is denoted by I, and the liquid surface tension is denoted by sigma. The following picture explains this:
The plate used by this method is mainly extracted from filter paper, platinum, or glass and to ensure complete wetting, it is roughened. If the materials used in this method are wetted, then the experiment results are deemed irrelevant in terms of the materials used. The plate used is thoroughly cleaned and attached to a balance or scale using a metal wire that is thin. A microbalance or tension meter measures the force exerted on the plate as a result of wetting and is used in the calculation of surface tension using the following equation: Where the wetted perimeter of the plate is denoted by l and the constant angle between the plate and the liquid phase is denoted by. The use of this method is advantageous as the measurement produces data which is essentially averaged and fairly objective over the wetted length. This does not clearly give the heterogeneity, but gives a value that is more accurate. This method has a disadvantage that it is more complicated in regards to a goniometer. The other disadvantage is that the appropriate sample size needs to be produced with a cross section that is uniform in the wetted direction and the submersion direction needs to be measured with precision (Okatan, 103).
Contact and angles in planar services are said to be substrates that lie on rough chemically homogenous. Contact angles in spherical states are said to be in smooth surface. The difference that exists between these two aspects of measuring the contact angle is the difference of the receding and advancing angle. In rough surfaces, there exists the aspect of lack of correlations between the angles of receding and advancing. According to the equation of young, contact angles existing on rough surfaces are meaningless. There lack general guidelines, which regard how smooth a surface should be for surface roughness to lack the obvious impact towards the contact angle. Planar surfaces have a poor constant angle in comparison to spherical surfaces.
Rough surfaces need to be smoothened to ensure that the contact angle is not obsolete. Wetter surfaces tend to present the difference between the receding and advancing angle efficiently. The contact angle that is attained as a result of spherical surfaces is very perfect in comparison to the one obtained from the planar surface. The use of spherical particles makes it possible to calculate the contact angle as a result of the distance curve force. The use of these particles also eliminates the availability of equilibrium deflection. This goes on until a cantilever is achieved. The cantilever can bend and equate to the equilibrium deflection an aspect that can be used to calculate the contact angle (Mittal, 107).
The calculation of contact angle is dependent on various issues. This calculation makes it very important to industries that have to clearly outline the necessary surface tension or force available between two interfaces. The calculation of contact angle is, however, hampered by planar or rough surfaces, which make it possible for the achievement of an obsolete angle. The calculation of a contact angle should be extensively done, and the roughness of the surface is eliminated. The measurement of contact angle is well measured using the dynamic drop sessile method, which has lesser assumptions made. However, the use of spherical surfaces needs implementation as they cause a lesser assumption to the difference between the receding and advancing angle.
Mittal, K L. Contact Angle, Wettability and Adhesion: Vol. 6. Leiden: VSP, 2009. Print.
Okatan, Leyla. A Study of Drop Size Dependence of Contact Angles for Small Drops. Ottawa: National Library of Canada = Bibliothèque nationale du Canada, 2003. Print.
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