Illuminating tiny structures: An introduction to small-angle scattering

Bellevue Beach in Klampenborg, Copenhagen, Denmark.

We are surrounded by phenomena caused by the scattering of light. When enjoying a sunny day at the seaside, like in the photo at the top, why is the sky blue? Blue light scatters more than red light. Why is milk opaque? Protein and fat particles scatter light. If you are reading this with blue eyes, your eye color is due to light scattering. Scientists use the same general scattering principle to study the structure of soft materials using the scattering of well-defined radiation. Scattering measurements reveal structures between an ångström and hundreds of nanometers, an important region for studying soft matter. Just as the color of the sky results from light scattered by air molecules, the scattering of X-rays and neutrons tells us about the size and shape of compounds in soft materials along with their interactions, and I will focus on these two types of radiation.

Putting the controversy over atomic-molecular theory to rest

There are many things that we “know” about the world around us. We know that the Earth revolves around the Sun, that gravity makes things fall downward, and that the apparently empty space around us is actually filled with the air that we breathe. We take for granted that these things are true. But how often do we consider whether we have seen evidence that supports these truths instead of trusting our sources of scientific knowledge?

The matter of maternal mucus: permeability and preterm birth

What is the first thing that comes to mind when you hear the word mucus? For most people, it’s probably the last time they had a cold. Mucus is not usually something we think about unless there’s a problem. However, it is always there, working behind the scenes to make sure that our bodies function smoothly. Mucus lines the digestive, respiratory, and reproductive systems, covering a surface area of about 400 square meters- about 200 times more area than is covered by skin. In addition to providing lubrication and keeping the underlying tissue hydrated, mucus also plays a key role the human immune system. It serves as a selectively permeable membrane that protects against unwanted pathogens while also helping to support and control the body’s microbiome.

Self-assembling silk lasers

Rings, spheres, and optical resonators self-assembled out of silk

When I first learned about the coffee ring effect I thought it was super cool, but it seemed like an open-and-shut case. Why do rings form where some liquids, like spilled coffee, are left to dry? Roughness on the table causes the liquid to spread imperfectly across the surface, pinning the edges of the droplet in place with a fixed diameter. Because the diameter of the droplet can’t change during evaporation, new liquid must flow from the droplet’s center to the edges. This flow also pushes dissolved coffee particles to the edges of the droplet, where they are left behind to form a ring as the water evaporates away (Figure 1). More details can be found in our previous post, here. It’s a complicated phenomenon, but after being described in 1997 it doesn’t seem like anything new would be going on here. Right? Well, as it usually happens in science, classic concepts have a way of popping back up in unexpected ways. Last year Itır Bakış Doğru and her colleagues in Prof. Nizamoğlu’s group at Koç University, Turkey published a study using the often troublesome coffee ring effect to their advantage: making self-assembling silk lasers.

Rebuilding hard matter with soft matter

The skeleton is the backbone of the body, both literally and figuratively. Healthy bones protect soft organs from injury and enable the body to move. Starting from childhood, staying active and following a healthy diet helps the body maintain healthy bones. However, as people age, their bones can start to weaken. There are often no early symptoms to weakening bones, and as a result the first indication of a problem may be a painful break once the weakening has already significantly progressed.

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.

Color made from structures inspired by bird feathers

There’s a reason why the word “peacock” has become a verb synonymous with commanding attention. Of course the size of the peacock tail is enough to turn heads, but it wouldn’t be nearly as beautiful without its signature iridescent, or angle-dependent, color. The brilliant colors of the peacock come from the interaction of light with the nanoscale structure of the feathers, which is much different from the origin of color in regular dyes and pigments. In today’s paper, Jason Forster and his colleagues in the Dufresne group developed a simple way to make colors that is inspired by the structures in certain bird feathers. To understand how it works, let’s start from the beginning.