Contact angle determination

Fig. X-8. Use of sessile drops or bubbles for contact angle determination.

Increase Adhesion Tension Maximize Surface Tension Minimize Contact Angle Determine the critical surfactant concentration to maximize adhesion tension. Control impurity levels in particle formation... 

As mentioned in Section 6.1a, surface tension and contact angle determine wetting phenomena we examine this in Section 6.6. We take a closer look at the definition of contact angle and some complications associated with it in Section 6.7. 

M. Mate, J. H. Fendler, J. J. Ramsden, J. Szalma, and Z. Florvolgyi, Eliminating surface pressure gradient effects in contact angle determination of nano- and microparticles using a film balance, Langmuir 14, 6501-6504 (1998). 

F.E. Bartell, J.T. Smith, J. Phys. Chem 57 (1953) 165 vacuum-evacuated, deposited metal film, contact angle determination in evaporated water atmosphere. Surfaces were prepared by evaporating the metal onto a glass surface in an evacuated chamber, contact angle of water drops on the metal surfaces were determined in a water vapour atmosphere at 25°C. 

DouiUard, ].M. and Zajac,]. (2006). Contact Angle Determination from Heat oflmmer-sion and Heat of Wetting. Encyclopedia of Surface and Colloid Science. Marcel Dekker. 

The surface area expansion process in Figure 3.5 must obey the basic thermodynamic reversibility rules so that the movement from equilibrium to both directions should be so slow that the system can be continually relaxed. For most low-viscosity liquids, their surfaces relax very rapidly, and this reversibility criterion is usually met. However, if the viscosity of the liquid is too high, the equilibrium cannot take place and the thermodynamical equilibrium equations cannot be used in these conditions. For solids, it is impossible to expand a solid surface reversibly under normal experimental conditions because it will break or crack rather than flow under pressure. However, this fact should not confuse us surface tension of solids exists but we cannot apply a reversible area expansion method to solids because it cannot happen. Thus, solid surface tension determination can only be made by indirect methods such as liquid drop contact angle determination, or by applying various assumptions to some mechanical tests (see Chapters 8 and 9). 

This equation is equivalent to the static force balance where each of the y terms is considered as the force applied to the vertex. This condition of equilibrium is often called a Neumann triangle. Equation (233) is the basis of Young s equation, which is used in the contact angle determination of liquid drops on flat substrates (see Sections 5.5 and 9.1). 

Dynamic contact angles are the angles which can be measured if the three-phase boundary (liquid/solid/vapor) is in actual motion. A Wilhelmy plate is used in dynamic contact angle measurements, and this method is also called the tensiometric contact angle method. It has been extensively applied to solid-liquid contact angle determinations in recent years. In practice, a solid substrate is cut as a thin rectangular plate, otherwise a solid material is... 

Receding contact angle determination from drop evaporation... 

Contact Angles. Where three phases are in contact with each other, the phase boundaries meet at a given contact angle, determined by the three interfacial tensions (Young s equation). The contact angle determines whether and to what extent the wetting of a surface by a liquid occurs ... 

Figure 4.3 Reversible switching of the wettability of a P4VP brush-modified ETFE surface. The sample was immersed in 0.1 M HCl or H2O prior to contact angle determination, and the process was repeated several times to demonstrate the reversibility of the change in contact angle. The resulting water CAs differed by 36. Source Adapted from Neuhaus et al. [2], with permission from John Wiley Sons Inc.

Testing. In order to characterize the obtained series, the MDF cement specimens were subjected to mechanical tests, contact angle determinations and atomic force microscopy (AFM). 

Surface free energy was calculated from experimental values of dynamic contact angle determined using a tensiometer constructed in the Institute of Chemical Physics of the... 

At a contact angle 3 = 0°, there is complete spreading of the liquid on the surface, whereas at an angle of 9 = 180° no spreading occurs. Real systems have contact angles of between 0° and 180°. Since the contact angle determines the spreadability, its cosine is a direct measure of the wettability of the surface. Equation (13-3) is valid for ideal surfaces in a vacuum. In real systems the equilibrium pressure exerted by the absorbed liquid vapor on the solid must be considered (Young s equation) ... 

Several methods are available to determine the physical parameters of polymer surfaces. Biomaterials penetrate liquids like blood or water present in soft tissue. It is known that the free surface energy at the biomaterial/water interface is the driving force for the reorientation processes of the polar groups of the uppermost molecular layers of the polymer surface towards the aqueous phase. The chemical composition of the surface of the biomaterial is different depending on its contact with an aqueous medium or with air. Hydrophilic domains of polymer systems like those found in block copolymers, for example, are mostly located at the aqueous interface, while the hydrophobic ones tend to remain at the air interface. The investigation of surface wettability by means of contact angle determination and the measurement of the streaming potential ( -potential) is of special interest in the characterization of the polymer surface. 

The contact angles of water and suitable solvents at the solid/liquid/gas interface allow the determination of the surface tension of solids as well as the dispersive (y ) and polar (yP) components. A semiquantitative prediction of the hy-drophilicity and hydrophobicity of polymer surfaces has already been achieved by contact angle determination with water [74,75]. 

The contact angle determination at surfaces of biomaterials is carried out by several methods which are schematically presented in Fig. 7,i.e. 

Fig. 7. Schematic illustration of dynamic and static contact angle determinations using die Wihehny plate method, the sessile drop method and the captive huhhle method...

A distinction can be made among the available methods between static and dynamic contact angle determination methods. In the case of a static determination the contact angle of a drop with an immobile solid/liquid/gas interface is determined microscopically (sessile drop). In the captive bubble method the contact angle of an air bubble, which is located under the solid surface in contact with the liquid, is determined. In contrast to the sessile drop method, in the captive bubble method the contact angle is measured at a completely wet surface. 

Special efforts are being carried out to improve the blood compatibility of biomaterials which are already in use for medical devices. In order to hydrophilize the surface of the commonly used cycloaliphatic poly(ether urethane), Tecoflex and argon (Ar) and sulfur dioxide (SO2) plasma treatment has been appHed. The surfaces of treated Tecoflex foils were characterized as a function of plasma treatment time (10,60 and 300 s). XPS shows SO groups in the Tecoflex smface after SO2 plasma treatment. The results of the dynamic contact angle determination of Ar or SO2 plasma treated Tecoflex surfaces are presented in Fig. 13. 

The IR-ATR spectra of the untreated and modified PES as well as that of the PHEMA reference surface are shown in Fig. 15. The spectra of the modified PES surface shows the superposition of the bands of the pure PES and PHEMA. The surface tension of the PES surface modified with the hydrogel was investigated by means of the captive bubble technique by measurement of the air/water and octane/water contact angle (Fig. 16). The polar and dispersive components of the surface tension, yP and were calculated from the contact angles determined [114],... 

The contact angle determines the degree to which a solid surface is being wetted by a liquid coating. The liquid spreads freely over the surface when the contact angle is zero and is said to wet it completely. Complete wetting takes place when the molecular attraction between the sofid molecules and the fiquid is greater than that between similar liquid molecules. 

Drops Deposited on Dirty Surfaces. We have implicitly assumed an ideal surface. On a real surface, the contact angle of a drop is often slightly dependent on the preparation conditions. Its value lies between two limits (larger) and 6 (smaller). The hysteresis of the contact angle, determined via the force 5 = 7 cos Or cos a), will be discussed in chapter 3. The difference 0a. is a measure of the state of cleanliness and roughness of a surface. It is used as a test in the automobile industry to ensure that surfaces are perfectly clean before applying paint. Oa Or must be sufficiently small for good adhesion. 

Figure 17 The influence of contact angle on nuclei size formed in fluid-bed granulation of lactose/salicylic acid mixtures. Powder contact angle determined by goniometry and percent lactose of each formulation are given in parentheses. (From Ref. 31.)...

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