Happy New Year for everyone, and my best wishes for you all in 2008! 

[DELFT - 1 JAN 2008].

On 14 January 2008, at 12:30 in the Aula of TU Delft, I will have the public defense of my doctorate dissertation. Prior to that, a very short 'layman talk' will be given starting at 12:00. After the defence, there will be a small reception. You are all invited to come! And to warm you up for this special occasion, I have prepared a mini quiz below. Enjoy...

[UPDATE: see also the post-defense news article published by TU Delta.]

Above you can see a picture of a chip floating on a water surface (the same picture is also used on the cover of my dissertation). A light-and-dark pattern can be seen on the water surface around the chip. The questions are:

1. What makes the chip stay floating on the water surface?
a) The density of the chip material is lower than the density of water.
b) Some air bubbles are trapped inside the chip, helping the chip floating on the water.
c) Neither a) or b).

2. What is the origin of the observed pattern on the water surface around the chip?
a) Ripples of water waves around the chip produce a periodic shadow.
b) The pattern is induced by the interference between [1] the light reflected by the water surface around the chip and [2] the light reflected by the bottom of the water basin.
c) Neither a) or b).

To see the correct answers, please scroll down.

1. The answer is c). Why?

a) The chip is mainly made of glass, which has a mass density (of not less than 2370 kg/m^3) that is significantly larger than the mass density of liquid water (around 998 kg/m^3 at 20 degree Celcius).

b) As soon as the openings of the channels touch a liquid water, the water is spontaneously sucked into the channels due to capillary forces. Therefore it is unlikely that the whole channels are filled with air. Even if there are some air bubbles trapped within the chip, the volume of the bubbles would not be significant enough to induce a buoyant force to help the chip floating.

c) It has been widely observed that objects denser than water can float on a water surface (for an example, click here). This phenomenon can be explain by surface tension, in which a layer of liquid behaves like an elastic sheet due to the (subtle) attraction among water molecules. To take this picture (yes, I took this picture myself), I had to put the chip very slowly on the water surface while ensuring the chip surface is always (approximately) parallel to the water surface; otherwise, the attraction among water molecules becomes disturbed and the chip will sink straight to the bottom of the water basin. For other 'strange' daily phenomena that can be explained by surface tension, you can go to this gallery.

2. The answer is c). Why?

a) The presence of the relatively small chip (area = 2 cm x 2 cm) on the liquid water would not sufficiently disturb the water surface to induce water ripples. Moreover, we can see that the surface of the water is slightly curved along the edges of the chip (this is explained by the 'surface tension' effect discussed above), but the light-and-dark pattern is not consistent with (and hence not caused by) the shape of the curved surface.

b) A curved thin layer of water may cause an interference pattern called the Newton's rings. However, such rings only occur as concentric rings, while the light-and-dark pattern in our picture is clearly not concentric (in fact it looks more like a linear pattern).

c) A closer look at the top side of the chip reveals that the same light-and-dark pattern is also observed, albeit forming straight lines rather than curved lines as seen on the water surface. This is a strong clue that we have a spatially-modulated light source nearby, where the reflection on the chip is undistorted by the flat chip surface (hence the straight lines) while the reflection on the water is distorted by the curved water surface. This spatially-modulated light source is actually a  simple venetian blind, which is a kind of window covering composed of long strips of solid material. I took the picture in my old office room in the Quantitative Imaging group, where I had a venetian blind at my window.