Exploring the Lotus Effect Using Candle Soot

Hydrophobic and super-hydrophobic surfaces are ubiquitous in the natural world. You do not need to search much to find good examples: just walk out in your garden after a light rain and look at the plenty of weed leaves pearly decorated by water droplets. If you have an ornamental pond, you may have even the chance to see floating better examples of plants having a super-hydrophobic surface. Notably, wettability in Nature is present in a different form that subtle differences in the function and effect on the water droplets. Plant leaves need to keep their surfaces clean for light-harvesting efficiency. A water repellent leaves let water drops roll over its surface and mechanically removes dust particles. This effect was first noted on leaves of the Lotus plant, and for that reason, it is also called the Lotus effect.

Several novel technological materials exploit the properties of super-hydrophobic. For example, in your kitchen, Teflon pans are used to avoid sticking food residuals and therefore easily cleaned. Your car windows are teated to let the water easily roll over the surface.

Candle soot is an artificial material that is easy to produce and can be used to demonstrate some of the properties of the (super)-hydrophobic surface existing in nature.

This Instructable shows how to prepare these simple surfaces and make some interesting observations with them.

Supplies

• A medium-sized candle.
• Microscope slides.
• A curved glass surface such as a lab hourglass or a Petri dish.
• A smartphone camera.
• A white background screen.
• A pipette with a small tip or a syringe with the needle to drop small droplets of water.
• OPTIONAL: A USB microscope.

Step 1: PREPARATION OF THE SOOTED SURFACE

The procedure to make a super-hydrophobic surface is very simple. You need a wax candle and a clean microscope objective slide.

Hold the slide on one side, and carefully pass one surface several times on the candle flame, until a black and uniform layer of soot is deposited on the slide surface.

For experiments on rolling the droplets on the soothed surface, you can use the bottom of a Petri disk or an hourglass as shown in the video of the Instructable.

HEALTH AND SAFETY NOTE
Non-Pyrex glass can easily shatter when undergoes non-homogeneous heating. Therefore, protect your eyes with protection google and your hand with hard duties gloves when you prepare the soot-coated glasses.

Step 2: THE CONTACT ANGLE OF LIQUID DROPLETS

If you deposit a small drop of water on the microscope slide and look at it on the side, you will note that the droplet forms a spherical cap on the surface. The contact of the droplet with the surface is characterized by an angle between the water surface and the glass. This is called the Young angle, from Thomas Young (1773–1829) who first studied systematically the wetting of surfaces. This angle is the result of the surface energy of the different materials involved in the contact: air, water, and glass. When the force acting between the different materials is balanced then the contact angle takes an equilibrium value. The word hydrophobicity derives from the combination of two old Greek words: hydros (water) and phobos (afraid): afraid of the water. A hydrophobic surface is characterized by a contact angle or Young angle. That angle is measured as shown in the figure and it is determined by the equilibrium of forces called surface tension forces (indicated with the greek letter gamma in the figure) that pull the drop to spread on the solid surfaces. For not hydrophobic surfaces as the common glass, water droplets spread reducing the contact angles to less than 90 degrees. In the case of hydrophobic surfaces, the droplet will not spread but will tend to retain a more spherical shape with contact angles larger than 90 degrees. The wetting properties of surfaces are therefore classified according to the value of the Young angle of a droplet of solvent on the solvent of different nature (polar or apolar). Polar solvents have contact angles ≥90 on hydrophobic surfaces and < 90 on hydrophilic surfaces.

If you could give a closer look with an electron microscope to the lotus’ leaf, you will be surprised to see that the surface is far being flat. There are myriads of small pegs or hairs protruding from it. This patterning renders the surface rough and changes dramatically the surface tension properties. The petal and lotus leaf effects are based on the different microscopic morphology of the vegetal surfaces. This peculiar biological surface patterning with microscopic protrusion has been used to design bio-inspired super-hydrophobic material using the silica nano-pillars showed in the following graphics representation.

Step 3: THE DEPOSITION WATER DROPLETS ON THE COATED SURFACE

Prepare horizontal tall support (for example a tall glass) to lay the coated against a uniform white background. Align the smartphone camera to have a lateral view of the slide. Adjust the distance and zoom on the droplet and focus on it. If the image is too small you can put a reading magnifier glass between the phone and the slide. Try to use daylight illumination or you can play with a spotlight to find the illumination that gives the best contrast between the background and the droplet shape.

Now using a pipette with a fine tip or a syringe with a needle, deposit a small water drop of water on the freshly fabricated sooty surface and observe what happened. Take care not to touch the surface with the tip of the needle or the pipette as this will remove the soot coating. For this experiment, keep the pipette near the surface to avoid the drop bouncing off of the slide. Deposit a water drop also on the not coated surface of the same slide to see the difference. For example, the picture shows large water drops on the soothed (right) and clean glass surface (left).

Step 4: MEASURING THE CONTACT ANGLE

The contact angle can be approximately determined by taking a picture of the droplet on the surface and by measuring the contact angle using software for image analysis.

We are going to use the public domain software ImageJ (https://imagej.nih.gov/ij/). This is a powerful program that can be used to perform sophisticated image analysis. One popular distribution is called Fiji and it contains a customized version of ImageJ with a collection of selected plugins (https://fiji.sc). Once the image is load by the program, it is possible to use the angle tool in the toolbar as shown in the figure. To take the angles, select a point on the water/slide interface inside the drop, drag a line to the triple contact point, and then drag another line tangent to the drop. To measure the selected angle measure in the Analyze menu item of the program. Repeat the measure for the other side of the drop and calculate the mean of the two measurements. The figure shows the results for the drop on the soot surface of the slide that gives and mean angle of 130.5 degrees while for the water on the clean glass 47.5 degrees.

Step 5: ROLLING WATER DROPLETS ON THE SOOTED SURFACE

Another interesting experiment is to observe what happens when we roll the droplets on the water surface. As in nature, the dusty soot tends to sick on the surface droplet and removed by the glass surface.

For this experiment, you can the microscope slide or better a curved coated glass surface obtained by a Petri dish or an hourglass. Adding drops on the top of the curved part of the glass, you can see rolling down the droplet and darkening of the droplet due to the accumulation of soot particles adsorbed on the water surface (as shown in the drop on the left picture compared to a clean one on the right).

Using a video recording, it is possible to analyze the detail of this process. The video of this instructable show shows some examples of this experiment.

Step 6: BOUNCING WATER DROPLETS ON THE COATED SURFACE

To perform the bouncing of the water droplet experiment, you can drop the water from different highs and observe the bouncing when to land on the soot.

The collision of the droplet causes a dent to the sooted surface how it can be observed with a USB microscope.

We have used our Mighty Roto-microscope (https://www.instructables.com/The-Mighty-Roto-Microscope/). For this purpose, a small cover glass for microscopy was coated with soot to fit on the microscope table.

The video of this instructable shows some examples of this experiment.

Step 7: CONCLUSION

In this instructable, we have presented a simple way to obtain hydrophobic surfaces to study their interaction with water. It is a simple and useful learning activity for students of different levels although for children the supervision of adults in the preparation of the surface is required.

I will let conclude this instructable to personal best student, my son:

Hydrophobic surfaces have caught many scientists’ attention over the years, not only because of their starkly beautiful effect on liquids but their helpful property on understanding fluid dynamics. From leaves of water-based plants to sooted glass surfaces, hydrophobic surfaces are widespread, and the function varies. For example, most leaves (some more prominently like a lotus leave) use their hydrophobic properties to clean themselves for an efficient day of photosynthesis. How the liquids react to these peculiar surfaces and what is their use is the question that resulted in our fascination and studies of the surfaces. As a result, we wanted to share these collaborations with the wider public in the hope that it might spark new interest in this fascinating and complex world of chemical or physical interactions.