Author: Kyle Anthony Baldwin

I am an experimental physicist, focused on fluidic behaviour and soft matter physics. I began my research career at NTU, focusing on the behaviour of evaporating polymer solutions with relevance to the ink-jet printing industry. In particular, solutions that form structures which deviate from the common "coffee-ring" deposit, and instead can grow pillars up to 3x their initial droplet height. Since then, as a post-doctoral researcher, I have stayed within the realm of soft matter physics, exploring: the shapes of levitating, spinning droplets relevant to the geophysical phenomenon of tektite formation; the inhibition of a magnetically induced Rayleigh-Taylor instability by rotational Coriolis forces; vibration induced self-propulsion of a two-sphere dimer; and active emulsions, where droplets swim as the system evolves from an out-of-equilibrium state into a nanoemulsion. The latter has further branched my experience into liquid crystals, which can be used to modify the swimming behaviour via additional symmetry-breaking. Beyond research, I am passionate about disseminating science, through lecturing, blog-writing, and outreach events. I have also forayed into the world of art, by creating beautiful microscopy images of dried droplets (Instagram: @droplarts). My blogging, research highlights, and "art" work can all be found on my website Adropofscience.com. There are bountiful unanswered questions in fluid dynamics and soft matter physics, and pondering these mysteries keep my experimentalist neurons firing and my enthusiasm high. Fluid dynamics also provides a tabletop window into some of the more abstract but connected areas of physics - it is a constant source of amazement to me that, with some pumps, liquids, dyes and magnets, one can probe the physics of tectonic drift, atomic fission, and even supernovae.
Posted on

Soft Matters in Viral Pandemics

Original paper: Soft Matter Science and the COVID-19 Pandemic (arXiv here)

Quintessential soft matter problems, such as the behavior of droplets in ink-jet printing, involve complex interactions between forces and materials. In today’s article, Prof. Wilson Poon points out that coronaviruses are also quintessential soft matter objects, and highlights a range of areas where soft matter science may help better understand, and combat viral pandemics.

To a physicist, a virus is merely an inert particle that drifts around in water (it is a colloidal particle, in the jargon of soft matter physics, see Figure 1), where a single coronavirus particle is roughly spherical and around one tenth of one thousandth of one millimeter in width ($latex 0.1 \mu m$). These inert particles, however, are also covered in tiny specially shaped keys (proteins), and when these keys meet the correctly shaped lock (a receptor at the surface of a cell membrane), they activate. The previously inert particle now instructs the invaded cell’s machinery to make more viruses (imagine a Ford car whose only job is to drive into Rolls Royce factories and rewire the machines to make more Fords). Sickness is then caused by the cells being too busy making viruses to do their jobs, eventual cell death, vast numbers of obstructive virus particles, and the body’s own army of immune response units. 

Although less interesting biologically, while in the inert stage the virus has to “survive” a variety of physical conditions, and ideally this is where we want to destroy it, before it has a chance to invade a cell — before the Fords can find the Rolls Royce factories. In this article, we will focus on a number of open questions about coronaviruses prior to invading a host that soft matter could help answer.

Figure 1. Diagram of a colloidal solution, particles of diameter 0.1 micron. Most are standard surfactant coated solid particles used widely in industry and academia, whereas one is a coronavirus.

Airborne

It is now established that the “dominant route” by which coronaviruses spread is in airborne droplets, through sneezing, coughing and even speaking. In soft matter physics, airborne droplets have been widely studied, both in industry (for example in ink-jet printing) and in academia (for example in finding the scaling laws that govern droplet formation). We may be able to use the lessons we have learned to ask:

  • How are these droplets created?
  • Does the behaviour of these droplets impact virus survival and transmission?
  • Do all droplets contain the same number of active virus particles?
  • Can we modify the air to inhibit viral transmission via the behaviour of these droplets?

When sneezes are observed using high-speed photography, as in this video, an image of a turbulent cloud of droplets is revealed. These droplets break apart, collide, stretch and break apart again in a chaotic cascade that could exert considerable destructive shear forces on the viruses, but it is currently unknown whether the turbulence decreases the active virus concentration or if the virus particles influence the droplet break-up events. Solving this unknown may change the way we understand the role of air flows from ventilation systems in inhibiting active viruses.

Despite the lack of information on virus viability in the turbulent sneeze cloud, Poon and colleagues point out that studies suggest that the viral load of virus-carrying droplets is small – around 1 viable virus particle per droplet. However, sneezes typically generate up to 40,000 droplets, whereas a single cough (or talking for 5 minutes) will generate up to 3000. One also needs to know how many viruses a person must absorb before their dose becomes infectious – the minimum infective dose (MID). For the flu, the MID is reported to be around a few thousand, but the equivalent figure is not known for coronaviruses. All of these numbers play a role in infection but collectively point to the importance of limiting the amount of direct droplet transmission, and the importance of ventilation.

Finally, an airborne droplet containing a virus evaporates, at a rate depending on the local humidity, and will eventually completely dry. Interestingly, studies suggest that bacteriophages (viruses that invade bacteria) are more likely to survive in very dry or very humid air, but not in between. The reason for this seems to lie in how dissolved salt destructively interacts with the virus at steady (not slow or fast) evaporation rates. Other saliva components (like the gelation of the mucin proteins that make mucus slimy) also impact evaporation, further complicating the problem of viral transport and survival, but these clues indicate that controlling air humidity could be a viable option for hampering the virus.

On Surfaces

In the previous section, we’ve seen that how droplets behave in the air seems to be important, and this has been well-studied in a variety of commercial and academic contexts. As in ink-jet printing, we should also consider how droplets impact surfaces, as contact from surface to skin to mucus-membrane is another route through which viruses spread.

Here we will summarize the five main questions highlighted by Poon and colleagues in how droplets interact with surfaces, which could be important for understanding viral survival and transmission:

  • Splashdown. While ink-jet printed droplets need to avoid bouncing off surfaces, in a pandemic we ideally want the opposite. Polymers are known to give an anti-bounce property to droplets, but do mucins in saliva also have this effect? (see Figure 2 a)
  • Coffee rings. Much research has been done on the coffee-ring effect (When Espresso Evaporates), where suspended material is dragged to a droplet’s edges as it dries. Do viruses cluster at the edges in this way, and do these dragging forces impact their viability? This is further complicated by the salts and mucins present in these droplets, which will also accumulate at the edges (see Figure 2 b).
  • Material. Surfaces can have complex structures (smooth, ridged, or fibrous) and variable chemistry (metal, glass, or oily skin cells). Interestingly, virus viability tends to follow an exponential decay once on a surface, but the decay rate is higher on some surfaces (e.g. copper) than others (e.g. plastics). A question remains: is this due to a chemical catalytic property, or are some surfaces retaining moisture better than others?
  • Capillary Bridges. Watery bridges can form when two wet surfaces come into close contact, but do microscopic bridges form when your skin comes into contact with surfaces, and do they enhance viral transport? Interestingly, bacteriophages are known to better transfer from surface to surface in humid environments, which suggests that such liquid bridges may be important (see Figure 2 c).
  • Fluidic Forces. It is well documented that bubbling gas through a solution or filtering it through a bed of glass beads deactivates viruses, but why? Poon argues that there could be significant capillary forces (which are known to deform latex particles in drying paint) pulling on virus particles if they get stuck at the interface between air and water, which will likely happen often in a vigorously bubbled or filtered solution. The behaviour of colloidal particles at interfaces is a rich area of ongoing study, so answers here are anticipated soon.
Figure 2. Schematic of processes to consider with viral loaded droplets on surfaces. (a). Droplets impacting surfaces. Bottom: droplet impact is absorbed, and it comes to rest in a single spherical cap. Middle: Droplet splashes into multiple droplets. Top: Droplet bounces away. (b) Droplet evaporation. Particles, salts, mucins and viruses accumulate at droplet edges via the coffee-ring effect. (c) Capillary bridges. Schematic shows a pathway viruses may take via capillary bridges between the resting surface and skin.

Final Note

The possibility of a global viral pandemic had been predicted by scientists for decades, and yet was not prevented, and the current one is seemingly unlikely to fully disappear in the near future. While the world’s hopes currently rest with developing and deploying effective vaccines, we should in parallel use every single weapon at our disposal to understand and treat the causes and symptoms of viral pandemics. The lessons we learn may also allow us to improve on some of the problems that the current measures are creating, such as the unfortunate and unsustainable use of single-use plastics in disposable protective gear that contribute to the “plastic pandemic”. Soft matter science may not be central to solving the problem of COVID-19, but is important, and could have a significant role to play in preventing the next global pandemic.

Featured image for the article is an edited version of a figure from the original article.

Posted on

Spider silk: Sticky when wet

Original paper: Hygroscopic Compounds in Spider Aggregate Glue Remove Interfacial Water to Maintain Adhesion in Humid Conditions 

If you were Spider-Man, how would you catch your criminals? You could tangle them up in different types of threads, but to really keep them from escaping you would probably want your web to be sticky (not to mention the utility of sticky silk for swinging between buildings). Like Spider-Man, the furrow spider spins a web with sticky capture silk to trap its prey. This silk gets its stickiness from a layer of glue that coats the thread. What makes this capture silk really interesting is that, unlike commercial glues, these spider glues don’t fail when wet.

The tendency for water to interfere with glues should come as no surprise. For example, sticky bandages become unstuck when they’re wet, whether it’s because of swimming, taking a shower, or going for a run on a humid day. This interference occurs on the microscopic scale, where water prevents the components of a glue from forming adhesive chemical bonds. Even just high humidity provides enough water vapor in the air for it to condense on nearby surfaces and interfere with adhesion. One would naturally expect this very general and simple mechanism to cause problems for spiders that lay traps near water, as our furrow spider does. As you may have guessed, our furrow spider is a bit more clever than that: their glues are highly effective regardless of the water content of the air, and this humidity-resilience has caught the attention of Saranshu Singla and colleagues at the University of Akron, Ohio.

spider-516653_960_720
Figure 1. A spider unperturbed by the water droplets formed on its sticky web.

The furrow spider glue being studied by Singla and co-workers is essentially a cocktail of 3 main components: specialized “glycoproteins” that act as the primary adhesive molecule, a group of smaller low molecular mass compounds (LMMCs), and water. The LMMCs group covers a wide range of chemicals (both organic and inorganic), but the main distinguishing feature of this group is that they are hygroscopic, which means they are water absorbing. The exact recipe of this glue is specific to each spider species, and previous research has shown that individual species’ glues stick best in the climate that spiders evolved in—rather than humidity causing them problems, tropical spider webs are in fact most effective in humid conditions.

To understand how spiders achieve this, the researchers used a combination of spectroscopy [1] techniques to observe the arrangement of molecules during adhesion. They took a densely packed layer of web threads collected from the furrow spider and stuck them to one side of a sapphire prism, an ideal surface for its smoothness and transparency to the light rays used for spectroscopy (See Figure 1 for experimental schematic). They then measured the chemical bonds at the point of contact between the glue droplets and the sticking surface over a range of humidity conditions. These measurements allowed them to figure out what happens when these sticky glues get coated in water.

41467_2018_4263_Fig1_HTML
Figure 2. Experimental setup schematic from the manuscript. The white scale bar in c is 0.1 mm. Here “flagelliform” refers to the silk material prior to the glue layer being added, and “BOAS” refers to the classic beads-on-a-string structure that droplets form on threads. SFG stands for “sum frequency generation” spectroscopy, the noninvasive technique used in this research for analyzing the molecular arrangement at the sticking interface between the glue droplets and the sapphire surface.

Singla and her colleagues find that there is very little liquid water at the sticking interface, despite water being one of the three main glue elements. They concluded that the hygroscopic LMMCs are drawing water away from the droplet surface and storing it near the center. The LMMCs make it possible for the sticky glycoproteins to fulfill their role: in high humidity the glue droplet first absorbs nearby water, and then draws that water away from the droplet surface, preventing it from interfering with the sticky molecules’ adhesive chemical bonds. The researchers also conclude that the glue’s efficiency at drawing water to the center of the droplet is controlled by the local humidity and the ratio of the three components. Tweaking this ratio would then make the glue better adapted to different humidities. This suggests that the addition of hygroscopic compounds provides a simple method to tune adhesives to suit specific environments.

This continues to be an exciting time for materials science as scientists unlock the secrets of nature, but perhaps more importantly, Peter Parker can now rest easy with the knowledge that Humidity-Man will be a highly ineffective foe.

1. Broadly, spectroscopy is a study of the interaction between matter and light. There are many different types of spectroscopy, as there are many different ways that light and matter interact, but typically, a beam of light covering a range of the electromagnetic spectrum (hence the “spectro” prefix) is shone onto a substance, and then regathered by a light detector. The brightness of the detected light at each wavelength can then be used to carefully analyze the properties of the substance. Here, the researchers combined infrared spectroscopy and SFG, a non-invasive technique that is specifically tailored to probe molecular arrangements at interfaces, and so is perfectly suited for probing interfacial adhesion.

Posted on

Are squid the key to invisibility?

Original paper: Adaptive infrared-reflecting systems inspired by cephalopods

While many today would associate a “cloak of invisibility” with Harry Potter, the idea of a magical item that renders the wearer invisible is not a new one. In Ancient Greek, Hades was gifted a cap of invisibility in order to overthrow the Titans, whereas, in Japanese folklore, Momotar? loots a straw-cloak of invisibility from an ogre, a story which is strangely similar to the English fairytale Jack the Giant-Slayer. Looking to the future in Star Trek, Gene Roddenberry imagined a terrible foe known as the Klingons, a war-driven race that could appear at any moment from behind their cloaking devices – indeed, any modern military would bite your arm off to get hold of this kind of device. Clearly, invisibility is a concept that has captured minds across many cultures, genres, and eras, so it should be no wonder that scientists are working on making it a reality.

As is often the case in materials science, a good starting point for inspiration is to look to biology, after all, life has had billions of years of competition-driven evolution to craft its tools. To that end, Gorodetsky and his team at of the University of California, Irvine, have been attempting to replicate the master of disguise: the cephalopod. From this class of mollusk, squid and octopuses in particular excel at adaptively altering the color, texture, and patterning of their skin to camouflage against a wide variety of oceanic backdrops (see Figure 1). They accomplish this primarily by stretching and contracting pigment-containing skin cells. More importantly to this research, some species of squid have additional skin cells called ‘iridocytes’, which are structured reflective cells that resemble a microscopic comb or folded pleats. These folds reflect light at specific wavelengths that correspond to the fold size. By actively stretching and contracting these cells on-demand, the squid effectively becomes a self-modifying bioelectronic display.

SquidCamouflage (1)
Figure 1: Squid before (left) and after (right) deploying camouflage. Images reproduced from
a video by H. Steenfeldt under the YouTube Creative Commons Attribution license.

Following this technique of stretch-induced camouflage, the authors devise a technique for replicating some squid-like properties in an artificial material. The procedure consists of using electron-beam evaporation[1] to deposit an aluminum layer onto a stretched polymer film held under strain. When they release the strain, the metallically coated material shrinks and buckles to form microscopic wrinkles that are analogous to the structures in squid iridocyte cells. The polymer film of choice is an excellent proton conductor, so when mounted to electrodes, it can be stretched back to the flat state simply by applying a voltage.

When stretched, the aluminum coated film will reflect infrared light like a perfect mirror, whereas when wrinkled, the incoming light is reflected diffusively in multiple directions, like sunlight hitting the moon. To see this effect, the scientists shine infrared light – a heat source – at the material and position an infrared camera at a specific angle so that when flat, it reflects all the incoming radiation toward the camera and appears hot; when relaxed and wrinkled, much less of the light is scattered towards the camera, making the material appears to take on the thermal properties of the background and disappear from view.

Perhaps as a head-nod to their biological inspiration, the team then recreate this material in the likeness of a squid and watch as its infrared silhouette disappears from the camera’s view (Figure 2).

InfraInvisibleSquidShape_Fixed (1)
Figure 2: The flexible squid-shaped material, viewed under an infrared camera. When relaxed (left), very little infrared is reflected towards the camera, and it appears cold. When stretched (right), it reflects most incoming infrared light towards the camera and appears hot. Image edited from Fig. 5 in the manuscript.

The authors conclude that this ready-for-manufacturing material will have immediate applications in heat-regulating technology, and while it is currently limited to the infrared part of the spectrum, they also note that there is no reason why this technique couldn’t be adapted to the optical range.

Squid haven’t solved our desires for a cloak of invisibility just yet, but these mysterious creatures may hold more secrets than we realize. We would be wise to keep an eye on them … if we can.

[1]  Electron beam evaporation is the technique of bombarding a solid metal with energetic electrons, causing it to evaporate. This metal vapor then cools and condenses uniformly on all nearby surfaces, forming a uniform metallic coating.