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