Tag: droplets

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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.

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When you pull on a drop, how does it pull back?

Original paper: How drops start sliding over solid surfaces 

Physicists like to ignore things. In some cases, we may neglect gravity or assume that the temperature is zero degrees Kelvin — colder than any known substance in the universe. And friction is almost comically absent in most models, despite the fact that a world without it would be utterly uninhabitable (this is nicely illustrated in cartoon form here: https://xkcd.com/669/). 

Sometimes, these simplifications are justified. If you’ve ever seen your hair stand on end after taking off a sweater, you know gravity isn’t always the dominant force. And if you’ve ever slipped on ice or slid a puck across an air hockey table, you know that there are situations where friction doesn’t do much.

But there’s another reason we might leave out something like friction: it’s really complicated. Even today, friction between solid objects remains an active topic of research,

with all sorts of interesting topics arising: friction’s role in earthquakes, for example, or how it can encode a kind of memory.

All this talk of friction between solids led Nan Gao and collaborators to ask whether there might be a friction-like effect for liquid-solid interfaces. 

One of the characteristics of friction between two solid objects is that it decreases once the objects start sliding. You may have noticed this when pushing a heavy box: it’s hard to get the box moving, but once it budges it’s easier to continue pushing. This idea is shown in Figure 1 as a plot of force applied to an object versus the amount of time that force is applied. The force increases linearly before the object begins to move, then drops suddenly to a constant value after the object starts sliding. 

Figure 1. Force applied to an object with respect to the amount of time that force is applied for a typical solid object sitting on a solid surface. The force increases linearly before the object begins to move, then drops suddenly to a constant value after the object starts sliding. (Figure adapted from original article)

In this week’s article, Gao and colleagues devise a clever system for measuring the same type of force for a drop of liquid sitting on a solid surface. 

A drop sits on a platform that can be translated in one direction. Inserted in the top of the drop is a thin rod, called a capillary, which remains stationary even as the platform moves. Just as water will stick to your straw as you pull the straw out of your drink, the drop likes to be in contact with the capillary. So rather than simply moving along with the translating platform, the drop pulls on the capillary, causing it to bend. In turn, the capillary pulls back on the drop until the capillary exerts a force on the drop that overcomes the friction between the drop and the platform; the drop stops moving with the platform and remains stuck to the capillary.

As shown in Figure 2, the researchers tracked the position of a laser reflected off the capillary to measure how much the capillary bent. They were then able to back out the so-called “adhesion force,” which is an indicator of the force needed to make the drop move relative to the surface it’s sitting on. 

Figure 2. Cartoon of the setup used. The force due to friction between the drop and the platform, which points in the direction the platform is moving, is represented with a red arrow and labeled “F”. Here, the capillary’s bend is measured by detecting the reflection of a laser (shown in red) off of the capillary. The orange arrows show the length and width of the drop.

Gao and colleagues found that as the platform moves, the adhesion force increases more or less linearly before dropping back to a roughly constant value (see Figure 3). Sound familiar? Just compare with Figure 1 and you’ll see that this behavior strongly mimics friction between two solid surfaces.

Figure 3. Adhesion force of a drop of water on a moving surface of titanium dioxide as a function of time the surface has been moving. Measured values (from the bending of the capillary) are shown as blue circles and calculated values (from an equation for the adhesion force that depends on the shape of the drop) are shown as red squares. In both cases, the force increases roughly linearly for several seconds, reaching a maximum force of 100 uN before dropping to a constant force around 40 uN. (Figure adapted from original article)

This wasn’t just a lucky choice of liquids and solids — they found a similar effect for several fluids on surfaces ranging from titanium dioxide to silicone nanofilaments to goose feathers. The appearance of this effect in such a wide variety of systems suggests that this may be a general phenomenon.

Moreover, their measured values from the bend of the capillary match fairly well with values calculated independently using geometric parameters of their drops like the drop width, drop length, and angles the drop makes with the surface.

Even in systems as simple as a drop of water sitting on a surface, there is an incredible amount of physical richness, which means lots of science still to be explored. And it’s not just how drops stick — how drops land on a surface and how they evaporate both raise unexpectedly subtle and complex questions (see previous posts here and here). What area of drop-related research might physicists slide into next? 

For an even shorter summary of this article, see  https://phys.org/news/2017-11-droplet-friction-similar-solid.html

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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.

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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.

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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.