The tendency of a drop to spread out over a flat, solid surface increases as the contact angle decreases . Thus, the contact angle provides an inverse measure of wettability. Wetting of different fluids. A shows a fluid with very little wetting, while C shows a fluid with more wetting. A has a large contact angle, and C has a small contact angle Contact angle (cos θ ) Degree of wetting Strength of interaction between S /L Strength of interaction between L /L θ = 0 Perfect wetting strong weak 0 < θ < 90° High wettability strong strong weak weak 9 0° ≤ θ < 180° Low wettability weak strong θ = 180° Perfectly non-wetting weak strong • A contact angle equal to 0 0 perfect wetting and means that adhesive forces between S/L is more that the cohesive forces between L/L. • A contact angle less than 90° (low contact angle) usually indicates that wetting of the surface is very favorable, and the fluid will spread over a large area of the surface.
• Contact angles greater than 90° (high contact angle) generally means that wetting of the surface is unfavorable so the fluid will minimize contact with the surface and form a compact liquid droplet. To measure the efficiency of wetting agents DR AVE’s TEST is done. Young – Dupre equation: Which relates the surface tensions between the three phases: solid, liquid and gas. Spreading parameter S When S > 0, the liquid wets the surface completely (complete wetting). When S < 0, there is partial wetting
Clarification and filtration Clarification is the term used when sold content donot exceed 1 % and filtrate is primary product. Metafilter: is a pressure filter mainly used for the clarification of syrups, filtration of injections solutions, and clarification of insulin liquors. Filtration is a mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Two main types of filter media are employed in the chemical laboratory — surface filter , a solid sieve which traps the solid particles, with or without the aid of filter paper (e.g. Büchner funnel, Membrane filters, Belt filter, Rotary vacuum-drum filter, Crossflow filters), and a depth filter , a bed of granular material which retains the solid particles as it passes (e.g. sand filter, Sintered and ceramic filter). For the cake filtration rotary drum filter and filtering centrifuge are used. Pressure filter : Plate and frame filters (Filter press) Vacuum filter : Filter leaf Rotatary drum filter: Large scale and continuous Theories of filtration: a. Dracy ’s equation : dV / dt = K A ∆P ηL b. Kozeny- Carmen equation c. Po iseuillie’s law = πr 4 ∆P 8 ηL Membranes filters: have 80% pore of uniform size out of total filter medium volume. They have 400-500 millions pores per square cm of filter membrane. These membrane filters are made up of esters of cellulose or nylon, Teflon, polyamide etc. Cellulosic membrane filter papers can be attacked by bacteria.
- Summer '16
- Physics, Surface tension, Granulations