Surface Tension

In this experiment you will explore the concepts of surface free energy, adhesion and wetting

Experimental procedure

  1. Contact angles of different substrates:
    1. Measure the contact angle of water on 5 samples: aluminum, copper, PVC, Teflon, glass. Use Table 1.

    Table 1: Contact angles of water on different surfaces

    Sample

    Contact angle [degrees]

    Aluminum

     

    Cu

     

    PVC

     

    Teflon

     

    Glass

     
  2. Determine the effect of surface modification of copper on the contact angle of water:
    1. Immerse a Cu disk in 30% H2O2 solution for 30 minutes in a covered plastic cup.
    2. Rinse with de-ionized water and dry carefully.
    3. Measure the contact angle of water on the surface.
    4. Immerse a Cu disk in 20mM sodium dodecyl sulfate (SDS), which is a monosulfate salt, in ethanol for 30 minutes in a covered plastic cup.
    5. Rinse with ethanol and dry carefully.
    6. Measure the contact angle of water on the surface.
  3. Create a Zisman plot for n-propanol/water solutions on Teflon:
    1. Make sure you are familiar with the Zisman plot paper.
    2. Prepare n-propanol/water solutions of concentrations calculated during the lab preparations.
    3. Measure the contact angle on a Teflon substrate.
      In order to use a single disk for all measurements, the mixtures must be measured in ascending order of concentration.
    4. Calculate γLG using the following equation:

      Remember that this equation is true only for 0.001 ≤ X ≤ 1, where X is the molar fraction of alcohol in water. Plot γLG as a function of cos(θ) and extract γSG.
  4. Determine the effect of surfactant (Triton X-100) on water surface tension:
    1. Make sure you are familiar with the CMC paper.
    2. Measure the contact angle of Triton X-100/water solutions of different concentrations on a Teflon substrate. The initial concentration of the Triton X-100 solution is 5mM. Record the contact angles for each solution in Table 2.

      3. Table 2: Contact angle dependence on surfactant concentration

      C[M]

      Contact angle [degree]

       

       

       

       

       

       

       

       

       

       

       

       

       

       
    3. Calculate γLG (γWA) using the following equation:
    4. Calculate γSL using Young's equation: γSG = γSL + γLG cos(θ) and the value of γSG you calculated in procedure 3. A typical plot of Teflon contact angle dependence on surfactant concentration is shown in Figure 1.
Change in contact angle with the concentration of TX-100 in aqueous solution on Teflon at 25°C. Reference