2-20232024bwd11802chapter4surfaceproperties (1).pdf

Semester II Session 2023/2024 Physical Properties of Food (BWD11802) Chapter 4 SURFACE PROPERTIES Assoc. Prof. Ts. Dr. Norazlin binti Abdullah Surface Tension tendency of a surface of a liquid to behave like a stretched elastic membrane. Liquids that have high surface tension values al...

Semester II Session 2023/2024 Physical Properties of Food (BWD11802) Chapter 4 SURFACE PROPERTIES Assoc. Prof. Ts. Dr. Norazlin binti Abdullah Surface Tension tendency of a surface of a liquid to behave like a stretched elastic membrane. Liquids that have high surface tension values also have high latent heat values. The surface tensions of most liquids decrease as the temperature increases. The surface tension value becomes very low in the region of critical temperature as the intermolecular cohesive forces approach zero. The small droplet adapts its shape to an almost perfect sphere since it has the least surface area per unit volume. The shape becomes flatter as the size increases because of gravity. Surface Tension Surface tension (σ) is expressed as free energy per unit surface area or work required to extend a surface under isothermal conditions. It is also defined as the force per unit length on the surface that opposes the expansion of the surface. where d is the distance between wires A and B, L is the distance that the bar is advanced to the left, and the factor 2 is introduced since there are two liquid surfaces (one at the front and one at the back). Schematic A bar is pulled with diagram for force F to expand a determination of liquid film that is the surface stretched like a bubble tension of a film on a wire frame. liquid. The surface tension has dimensions of force per unit length. In the SI system, the unit for surface tension is N/m.  Surface tension appears in situations involving either free surfaces (liquid–gas or liquid–solid boundaries) or interfaces (liquid–liquid boundaries). If it takes place in interfaces, it is called interfacial tension. Interfacial tension arises at the boundary of two immiscible liquid due to the imbalance of intermolecular forces. Emulsifiers and detergents function by lowering the interfacial tension. Generally, the higher the interfacial tension, the lower is the solubility of the solvents in each other. Emulsion a colloidal system in which a liquid is dispersed as droplets in another liquid with which it is immiscible. can be classified as oil-in- water (o/w) and water-in- oil emulsions (w/o). In oil-in-water emulsions, oil is dispersed in water as droplets. The typical example for o/w emulsions is mayonnaise. In water-in-oil emulsions, such as butter, droplets of water are dispersed in oil. Emulsion Viscosities of the emulsions are very high as compared to the viscosities of either of the liquids. For example, the viscosity of mayonnaise is higher than that of the vinegar and oil used in preparing mayonnaise. The stability of emulsions can be determined by the viscosity of the continuous phase, the presence and concentration of emulsifier, the size of the droplets, and the ratio of dispersed phase to the continuous phase.  Foam a colloidal dispersion in which gas is the dispersed phase and liquid is the continuous phase. The dispersion medium is usually a liquid that is sometimes modified into a solid by heating or strengthened by a solid. Ice cream, whipped cream, or cake batter are some examples of foams. Foams are important because of their contribution to volume and texture of food products. Foam The inclusion of air reduces the density of the product. The term overrun is used in relation to foams to describe the amount of air incorporated. where Vf is the volume of the foam and Vl is the volume of the original liquid  Foam Liquids should have low surface tension to form foams. As a result of low surface tension, the liquid can be stretched or spread easily and does not coalescence. If the surface tension is high, formation of the foam will be difficult because of its resistance to spreading and strong tendency to expose the least surface area. If the liquid has high surface tension, the foam collapses quickly as a result of the tendency to coalescence. Liquids should have low vapor pressure to prevent evaporation. This will help retain the gaseous phase within the liquid and minimize its tendency to break out into the surrounding medium. Stability can be provided to foams if some solid matter can be incorporated into the films to increase the rigidity of the walls surrounding the gas. In protein foams, the denaturation of the protein provides stability to cell walls.  CONTACT ANGLE MEASUREMENT METHODS can be measured with a contact angle goniometer. This instrument consists of a light source, an illuminating stage on which the liquid drop–solid material three-phase system (liquid, vapor, solid) rests, and a telescope. By viewing the drop through the telescope, the contact angle is measured. An inexpensive instrument of a video-based contact angle meters is also available to measure contact angle.  SURFACE TENSION MEASUREMENT METHODS DROP BUBBLE WEIGHT PRESSURE CAPILLARY RISE TENSIOMETER DYNAMIC METHODS REFERENCES Thank you