材料分析作业第二单元

Materials Characterization techniques

Chapter 2 Contact Angle

1. Interpretation of the following terms

Surface energy :the work required to increase the surface area of a substance by unit area.

surface tension :The free energy per unit area is termed the surface energy for solids,and the surface tension in liquids when one of the bulk phases is a gas. Young's equation :γγθγsl

sv Y lv -=cos contact angle :The contact angle is the angle at which a liquid-vapor interface meets the solid surface.

advancing contact angle :Liquid - solid interface to replace the gas - solid interface formation after the contact angle is called the advancing angle.

receding contact angle :The gas - solid interface to replace the formation of liquid - solid interface contact angle is called receding angle.

contact angle hysteresis :The hysteresis can result from sample prehydration,surface roughness,chemical heterogneity,evaporation,or molecular movement.The change between the two angle is relatively stable, and this phenomenon is called contact angle hysteresis, limit for the advancing contact angle A θlower limit for the receding contact angle R θ,the difference between the twoΔθ=-A θ-R θis defined as the contact angle hysteresis.

Wettability :It is a solid interface transition by the gas - solid interface for the solid-liquid interfacial phenomena.

Hydrophobic :Hydrophobic refers to a molecule ( hydrophobe ) physical properties are mutually exclusive with water.

Hydrophilic :Hydrophilic molecules can form refers to physical properties short bonding through hydrogen bonds with water.

2. Blank filling

Advancing angles are normally measured to determine the ( definitive ) of a solid. However the receding angle is not suitable for calculating ( surface energies ).

Dynamic Wilhelmy Method can only be used for measuring average advancing and receding contact angles on solid of (a uniform geometry).

Another method for measuring contact angle under a ( temperature-controlled ) condition is the captive air bubble method.

The capillary rise method is the only method of contact angle measurement available

for the measurement of ( tubular ) materials and coatings.

Tilted-drop measurement can measure both the advancing and receding contact angles ( until the droplet reaches a point where it almost moves).

3.Short answers

What’s the reasons cause to the contact angle hysteresis?

Answer:The hysteresis can result from sample prehydration,surface roughness, chemical heterogneity,evaporation,or molecular movement.

What’s the measurement range of contact angle?

Answer:0.3~2nm

How many ways that the contact angle can be measured?

Answer:5ways.Sessile drop method,Wilhelmy plate method,Captive Air Bubble Method,Capillary Rise Method and Tilted-Drop Measurement.

How many interactions which could cause the contact angle to change with time? Answer: 1.Evaporation of the liquid;

2.Migration of surfactants from the solid surface to the liquid surface;

3.Substance dissolved in the drop migrating to the surface(or in the opposite direction);

4.Chemical reactions between the solid and liquid;

5.The solid being dissolved or swollen by the liquid.