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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
ΔP=σ[1/Rx +1/Ry]
YOUNG-LAPLACE EQUATION
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. 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 STRONG
WEAK WEAK
90≤θ<180 LOW WETTING WEAK STRONG
θ=180 PERFECTLY NON-WETTING
VERY WEAK VERY STRONG
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
YOUNG'S EQUATION
γsl +γlg*cosθc =γsg
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,
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
THANK YOU