CONTACT ANGLES ARVIND TOMAR Sr-08471

Contact angle

FACTORS AFFECTING CONTACT ANGLE SURFACE TENSION SURFACE ENERGY OF SOLID SURFACES INTERACTION FORCES BETWEEN LIQUID MOLECULES SURFACE ROUGHNESS TEMERATURE OF LIQUID

SURFACE TENSION

SURFACE TENSION IS A CONTRACTIVE TENDENCY OF SURFACE OF A LIQUID THIS ALLOWS IT TO RESIST AN EXTERNAL FORCE DUE TO SURFACE TENSION A LIQUID ACQUIRE MINIMUM SURFACE AREA DUE TO SURFACE TENSION LIQUID SURFACE BAHAVES AS A STRECHED SKIN SURFACE TENSION IS CAUSED BECAUSE OF MOLECULER ATTRACTION FORCES IN GENERAL DISSOLVED CONTAMINATION IN WATER REDUCES SURFACE TENSION, HENCE ALSO THE CONTACT ANGLE

DUE TO SURFACE TENSION A NEEDLE CAN FLOAT ON A LIQUID

SURFACE TENSION =FORCE/LENGTH =WORK DONE /AREA TO SEPRATE TWO LIQUID SURFACES WE HAVE TO DO WORK THIS PER UNIT AREA WORK IS CALLED SURFACE TENSION THIS WORK INCREASES POTENTIAL ENERGY OF LIQUID

AS SURFACE TENSION REDUCES, DROPLETS TENDS TO SPREADS AND CONTACT ANGLE DECREASES GREATER THE PORTION OF POLAR GROUPS,HIGHER THE ATTRACTIVE FORCES,HIGHER SURFACE TENSION AND HIGH WILL BE THE CONTACT ANGLE EX. WATER HAS HIGHER CONTACT ANGLE AS COMPARED TO OILS

SURFACES BENDS TO BALANCE FORCES

YOUNG-LAPLACE EQUATION ΔP=σ[1/Rx +1/Ry]

HIGHER THE SURFACE TENSION HIGHER WILL BE CONTACT ANGLE

SURFACE ENERGY HIGHER THE SURFACE ENERGY LOWER WILL BE CONTACT ANGLE HIGH SURFACE ENERGY OVERCOMES SURFACE TENSION AND LIQUID DROPLET SPREADS OVER SURFACE HIGHER THE SURFACE ENERGY HIGHER THE ADHESION SURFACE IS ALWAYS AT HIGHER ENERGY AS COMPARED TO BULK

DEPENDENCE OF CONTACT ANGLE ON SURFACE ENERGY AND SURFACE TENSION

SURFACE ENERGY DEPENDS ON CHEMICAL COMPOSITION AT SURFACE POLAR GROUPS CAUSES HIGH SURFACE ENERGY CLEAN METALIC SURFACES HAVE HIGH SURFACE ENERGY BONDING BETWEEN HYDROCARBON MOLECULES IS LESS POLYETHYNES HAVE LESS SURFACE ENERGY AND HIGHER CONTACT ANGLE

FOR UNPOLISHED SURFACE THERE ARE SO MANY POLAR GROUP(EX FOR UNPOLISHED SURFACE THERE ARE SO MANY POLAR GROUP(EX. O-H) SO HAD HIGHER ENERGY SURFACE ENERGY OF SURFACE CAN BE REDUCED BY POLISHING WAX

WATER PROOF FABRICS FLOURINATED FABRICS, WHICH ARTIFICIALLY MAKE A SURFACE LOW ENERGY ONE THUS MORE CONTACT ANGLE AND SURFACE IS NON-WETTING BY FORMING OXYGEN CONTAINING COMPOUNDS AT SURFACE A LOW ENERGY SURFACE CAN BE CONVERTED INTO A HIGH ENERGY ONE THIS CAN BE ACHEIVED BY EXPOSURE TO UV-RADIATION,CORONA/PLASMA DISHCHARGE, ACID TREATMENT etc.

NON-WETTING FABRIC

INTERACTION FORCES BETWEEN LIQUID MOLECULES CONTACT ANGLE (IN DEGREES) DEGREE OF WETTING SOLID-LIQUID INTERACTION LIQUID-LIQUID INTERACTION θ=0 PERFECT WETTING VERY STRONG VERY WEAK 0<θ<90 HIGH WETTING STRONG WEAK 90≤θ<180 LOW WETTING θ=180 PERFECTLY NON-WETTING

SURFACE ROUGHNESS WITH INCREASING SURFACE ROUGHNESS CONTACT ANGLE DECREASES FOR HYDRO-PHILIC SURFACE WITH INCREASING SURFACE ROUGHNESS CONTACT ANGLE INCREASES FOR HYDRO- PHOBIC SURFACE

TEMPERATURE WITH INCREASING OF TEMPERATURE SURFACE TENSION DECREASES AS INTERMOLECULER FORCE DECREASES THUS WITH INCREASING OF TEMPERATURE CONTACT ANGLE DECREASES

BALANCE OF FORCES

γsl +γlg*cosθc =γsg YOUNG'S EQUATION

TWO DIFFRENT LIQUIDS

METHODS FOR MEASURING CONTACT ANGLE THE STATIC SESSILE DROP METHOD THE DYNAMIC SESSILE DROP METHOD DYNAMIC WILHELMY METHOD POWDER CONTACT ANGLE METHOD

Young-dupre equation γ(1+cosθc)= ∆Wsl Here, ∆Wsl=solid-liquid adhesion energy per unit area

CALCULATION FOR CONTACT ANGLE θc=arcCOS[rAcosθA+rRcosθR/rA+rR] Where, CALCULATION FOR CONTACT ANGLE

ΘA= advancing angle ΘR= receding angle ADVANCING ANGLE:- largest contact angle possible without increasing solid-liquid interfacial area by adding volume dynamically RECEDING ANGLE:- if in above case you start removing volume then smaalest possible angle is called receding angle

Calculation of Θa and Θr on a tilted plane

Hysteresis angle H=Θa-Θr Hysteresis angle for an ideal solid surface is zero i.e. Θa=Θr With increasing roughness H increases With increasing roughness Θa increases and Θr decreases Increased liquid penetration leads to increased hysteresis

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