Experiment 2 Effect of surfactants on surface tension

Goal

1
Understanding and visualising the effect of surface tension
2
Practising hands-on techniques for measuring surface tension.
3
Experimenting with and comprehending the effects of surfactants.

Background

We apply scientific methods and concepts to comprehend and manipulate physical properties in the natural world. One crucial property that impacts various systems is the surface tension of liquids.
In simplified terms, surface tension arises from molecular attractions observed at the interface between two bulk phases. The force at this interface results from intermolecular attractions within the liquid being stronger than those between liquid and air molecules. As a result, molecules at the liquid's surface are more attracted to those within the liquid than to those in the air (see figure below). This phenomenon causes the liquid's surface to behave like an elastic membrane, akin to a trampoline, striving to minimize its surface area and resist external forces.
Effect of surfactants on surface tension

Apparatus required

1 Syringe
2 Two 100mL Beaker
3 Detergent
4 Glass rod

Procedure

Step 1 - Fill the two beakers

Fill the two beakers with 75mL of water. Add 2mL of detergent into one of the beakers using a Syringe. Stir the detergent solution using a glass rod.

Step 2 - Open App

Open the Droplet Lab app on the Computer.

Step 3 - Set the density and needle diameter

Set the density and needle diameter as 0.998g/ml (Water Density) and 1.8mm (Needle used), respectively.

Step 4 - Fill the syringe with water

Fill the syringe with water. Lock the syringe in the syringe holder once it is filled. Tip: Make sure the position of the tip of the needle is in the middle of the live window of the screen as shown below.

 

Experiment 2: Effect of surfactants on surface tension

Step 5 - Slowly generate a drop with the syringe

Slowly generate a drop with the syringe. The ideal situation for drop measurement is to aim for when the drop is just about to detach from the needle tip.

Step 6 - Adjust the image quality

Adjust the image quality (including light condition and focus distance) using the two filters in the right-hand top corner. Once we have a suitable image click a picture by pressing the orange button in the center of the bottom edge.
i. Tip: If you cannot focus on the droplet or its blurry please move the syringe holder further from the phone.

 

Experiment 2: Effect of surfactants on surface tension

Step 7 - Calibrate the image

Calibrate the image of the drop.

Step 8 - Go into the measurement interface

Go into the measurement interface by clicking on the drop image. Drag the horizontal orange line into the middle of the needle.

Step 9 - Drag the 2 vertical red bars to the needle

Drag the 2 vertical red bars to the needle. And then click on the blue calibrate button besides the calculate button. Once the calibration is done, drag the horizontal line towards the contact point between the needle and the droplet.

Questions & Discovery

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