The living silly putty, episode 2: the spreading!

Douezan et al PNAS 2011

In episode one of this series, I presented a research paper by Stéphane Douezan and his colleagues in which they studied a ball of cells (called a cellular aggregate) sitting on a flat surface. After introducing the concept of cellular aggregate wetting by comparing it to the classical system of a drop of water, today I present the main part of the paper which looks at the dynamics of spreading of the cellular aggregate. I strongly suggest that the reader reads the first post before reading this one.

The living silly putty

Have you ever noticed how drops of water have different shapes on a clean piece of glass and in a frying pan? The frying pan surface is coated with a hydrophobic ("water-repellant") molecule so it does not stick to food, which typically contains a lot of water. As a result, a drop of water will take on a roughly spherical shape to reduce as much as possible its area of contact with the frying pan. If a surface has an even more hydrophobic coating than a frying pan, the drop can even reach a perfectly spherical shape (this is called ultrahydrophobicity).

Dripping, Buckling and Collapsing of a Droplet

The scale bar is 20 micron.

Cell membrane is evolved to be flexible rather than rigid. This fluid 2D sheet plays a key role in cells's survival, be it tailoring the nutrition trafficking or rendering a mechanical toughness. In recent decades, however, artificial membranes have been developed with enhanced mechanical properties. Of such systems are particle-stabilized emulsions and in this post we will look into characterizing mechanical strength of such emulsion.

When espresso evaporates: the physics of coffee rings

I’ve spilled a lot of coffee over the years. Usually not a whole cup, just a drop or two while pouring. And when it’s just a drop, it’s easy to justify waiting to clean it up. When the drop dries on the table, it forms a stain with a ring (Figure 1), giving it the look of a deliberately outlined splotch of brown in a contemporary art piece (In this context, the phrase “coffee ring” refers to this small-scale, spontaneously formed circular stain rather than the imprint left on a table from the bottom of a wet coffee cup). But the appearance of these stains is simply a result of the physics happening inside the drop. Coffee is made of tiny granules of ground up coffee beans suspended in water, so the ring must mean that these granules migrate to the edge of the droplet when it dries. Why do the granules travel as they dry? Today’s paper by Robert D. Deegan, Olgica Bakajin, Todd F. Dupont, Greb Huber, Sidney R. Nagel and Thomas A. Witten provides evidence that coffee rings arise due to capillary flow--  the flow of liquid due to intermolecular forces within the liquid and between the liquid and its surrounding surfaces.

Brick-by-brick to Build Tiny Capsules

In past two decades, several approaches have been developed and optimized to encapsulate a wide variety of materials, from food to cosmetics and the more demanding realm of therapeutic reagents. Inspired by biological cells, the first attempts were to use either natural or synthetic lipid molecules to form encapsulation vessels, i.e., liposomes. Then, with the increasing awareness of controlled release of cargo, especially for therapeutic purposes, advanced materials such as polymers were developed to form carrying vessels. Despite the enormous progress in encapsulation technologies, however, these methods can be limited in their applicability regarding encapsulation efficacy, permeability, mechanical strength, and for biological applications, compatibility.