This wark was done at NJIT's microfluidics lab by NM and Prof. Lou Kondic throughout 2007.

We report on instabilities which occur during spreading of volatile liquid drops. The motivation for this experimental work stems from curious phenomena recorded in the semiconductor industry, where octopus-shaped instabilities appear close to the contact line of evaporating isopropyl alcohol (IPA) drops (2006 PRL paper).

The experiments are carried out in the Capstone Laboratory of the Department of Mathematical Sciences at NJIT. The recording equipment consists of TechnolPhotron 1024PCI high-speed camera mounted on top of Nikon Eclipse LV100P polarizing microscope, so that a view from the top was established and microscope's mobile stage allows for accurate tracking of the drop's contact line. Two different magnifications were used: 20x which provides a 2.5mm x 1.75mm viewing window, and 40x which provides a 0.6mm x 0.4mm view. All experiments were performed in room conditions (1 atm, 50% relative humidity, 25 deg. C). Volatile drops of the volume of approximately 5 microliters were deposited onto a solid substrate manually, using a Hamilton gas-tight syringe. The set-up is shown on the picture below.

Series of experiments were performed in short time span of only a few hours, with each experiment repeated multiple times. Various liquid/solid combinations were examined. The liquids used were either aqueous solution of IPA, with varying IPA content (70% IPA or 91% IPA), or pure semiconductor grade IPA (100% IPA). The substrates were either semiconductor grade silicon wafers (thickness 0.5mm, surface roughness 0.5nm) or plain microscope glass slides (thickness 1.1mm, pre-cleaned). The substrates were not heated.

The movie reveals rich nature of instabilities which appear ahead of the contact line and is composed of five segments, corresponding to different liquid/solid configurations. The segments are arranged in such a way so as to show how increase of IPA content (and overall volatility of liquid) influences changes in size and shape of the developing patterns. Note that the field-of-view is not fixed; the camera follows the contact line as it spreads/retracts. The movies are shown in real time.

The first segment presents beautiful mushroom-like features which appear for a 70% IPA drop spreading on a plain glass slide in a 2.5mm x 1.75mm view. The mushroom-like features appear uniformly (both in size and shape) along drop's perimeter. As the drop spreads, neighboring mushroom-like features coarsen and form larger and larger mushroom heads. Eventually, as spreading subdues, remaining large mushroom heads collide to form a perimeter ring. A still image of the fully developed mushrooms is shown below.

The second movie segment also features 70% IPA drop in a 2.5mm x 1.75mm view, but now on a silicone wafer. The spreading of the drop is less rapid, and the early stage of instability formation is captured. This fascinating beading process occurs in isolated zones ahead of the expanding contact line. The beads appear simultaneously in quartets, quintets or sextets and eventually grow into mushroom-like features, resembling those from the first movie. These mushrooms are not uniform and grow in a variety of shapes and sizes. Neighboring mushrooms interact and coarsen to form larger mushrooms, making the late stages of this process similar to what is shown in the first movie segment. The very early stage of beading process is shown on the picture below.

The third segment shows 91% IPA drop on a plain glass slide in a 0.6mm x 0.4mm view. The spreading of the drop is faster than in the first 2 movies. Bumps of various sizes appear on (and not ahead of) the contact line, as can be seen on the picture below.

Soon, as the spreading continues, the film between neighboring bumps rupture. These bumps, that break away ahead of the contact line, become either mushroom-like or finger-like objects of wide range of sizes, all smaller than the objects seen in the first 2 movies. Neighboring objects do not coarsen as before, but instead a ridge forms along the perimeter of the drop, some distance behind the objects. As the spreading of the main body of drop stops, the objects continue their forward motion for a short while. Eventually, as this forward motion subdues, rupture occurs in the thin valley behind the ridge, leaving the objects stranded as the main body of the drop recedes in a dramatic fashion. Mushroom-like features can be seen on the image below.

The fourth movie shows spreading of pure 100% IPA drop on a plain glass slide in a 0.6mm x 0.4mm view. Small comet-like circular objects appear ahead of the spreading contact line. These are mostly uniform in shape and size along the perimeter, and, from the very start, each appears to be connected to the main body of the drop only through a very narrow tail, which becomes less visible as spreading motion proceeds. Eventually, as the spreading of the main drop stops, the comet-like objects are pushed some distance ahead of the contact line, loose their tails completely, and remain stranded as the main body of the drop shrinks to complete dry-out. Comet-like structures can be cleary seen on the image included below.

Finally, the fifth movie segment shows spreading of pure 100% IPA drop on a silicon wafer in a 0.6mm x 0.4mm view. Elliptical objects appear ahead of the contact line of the spreading drop. These object are smaller than the ones seen in previous movies, and appear to be perfectly uniform in both size and shape. A steady stream of smaller satellite drops (several tentacle arms) connects each elliptical head to the main body of the drop, and hence we nickname these objects octopi. The neighboring octopi do not coarsen, and, as spreading continues, they maintain uniform distance ahead of the contact line.