Birds do it, bees do it, even West Ham supporters do it



Starling murmuration – an example of metachronal wave (Image by , via wikimedia Commons, CC-BY)

The world’s most renowned TV naturalist, Sir David Attenborough,  stands in a tropical forest. It’s dark. Suddenly a pinpoint of green light flashes underneath him, in the grass. Another flash, and another until they become too numerous to count. And then a pattern emerges – instead of random light flashes, which would create a steady background, like individual drops of rain create steady rain noise, the flashing fireflies synchronise. They create a rhythm, not unlike flashing traffic lights – or a lighthouse.

The synchronising of rhythms of individual insects is not limited to the fireflies. Perhaps less surprisingly, bees, the notorious collective, do it. Not the torch-like flashing but they shimmer in response to hornet approach.  So do starlings and fish that create mesmerising collective movements.

This type of movement is called metachronal rhythm or metachronal wave.  It’s produced by the sequential action (as opposed to synchronized) of structures such as cilia, segments of worms or legs. These movements produce the appearance of a travelling wave.


A Mexican wave in Brazil. (Image by Danilo Borges via Wikimedia Commons, CC-BY 3.0)

It’s made by reacting and repeating the movement of your neighbours be it cilia  in a single cell organism or a human.  West Ham football supporters (and all the rest of them) succumb to a metachronal rhythm during a Mexican wave.


Restless Creatures: The Story of Life in Ten Movements by Matt Wilkinson, 
  • ASIN: B01B39IRJ2

On origin of life: Packman goes forth

The line between life and non-life is becoming increasingly blurred.  We use biological molecules as molecular machines and even as a base of computing. On the other side of the spectrum, inorganic materials are constructed to display a cell-like behaviour.

But there’s a gap between these organisation levels and the most primitive living cells capable of matter and energy exchange with the environment (metabolism) and reproduction. Experiments that bridge the gap between complex inorganic and a living cell brings us closer to understanding how life came to be.


Two consequent of optical microscopy images showing spontaneous transfer of silica colloidosome (red object, dotted line) into a magnetic droplet through a fatty acid stabilized aperture. Scale bar = 100 µm. (Image by University of Bristol).

Magnetite + organic solvent =

The authors of a recent article in Nature materials (Rodrigues-Arco et al. (2017), DOI: 10.1038/NMAT 4916) tried to bridge the gap between inorganic and organic. They mixed magnetic particles of iron oxide (magnetite) with droplets of an organic solvent and water. The particles with diameter of 500 ± 250 µm self-assembled on the surface of the solvent and were stable for several weeks.

Applying a magnetic field to the magnetic droplets opened the spheres along the surface, but they didn’t lose structural integrity and returned to the spherical shape. 


Magnetite droplets open and close on   magnetic field application (Image by  Fr4zer  via Wikimedia Commons)

Increasing pH of the water phase to 10.5 and oleate concentration led to the creation of Matryoshka structures: parts of droplets remained covered by magnetic nanoparticles, and the rest of the structure was bordered by a monolayer of organic molecules.  At an optimal concentration, 3/4 magnetite with 1/4 water surface droplets resembled the hero of the classic game, Packman in appearance and behaviour.


Adding silica 

Under the same conditions, silica particles form smaller spheres, 50 ± 20 µm in diameter.  Silica colloids mixed with magnetic spheres do not interact in the absence of oleic acid. However, applying a magnetic field to the mix opened apertures in the magnetic spheres led to their self-propelled movement and random engulfment of colloidosomes. Only spheres with apertures – Packmen –  were able to move. The authors call this the engulfment ‘phagocytic-like behavior’ after ameba-like white blood cells that eat bacteria.

Particles movement explained by Marangoni effect – a movement due to surface tension gradient because of the uneven distribution of oleate on the surface of magnetic particles.  As the oleate gradient dissipate, the droplets moved only for several seconds. 

The authors proposed a model of ‘phagocytosis’. Non-magnetite covered surface Packman aperture covered by oleic acid molecules acts as a single layer proto-membrane. The silica colloidosomes have this layer as well. Fusion of molecular layers on the surface of Packmen aquatic opening and release of colloidosomes into the inner space creates semi-double membrane particles.

See the pictures and models on Nature web-site.   If by some miracle you have access to the original paper, do look into the supplemental videos of Packmen moving, Packmen ‘eating’, it’s mesmerising.

What is it good for

The authors propose using composite droplets mixtures for development of new material and nanoscale engineering approach, for example in microfluidics and delivering reagents for spatially controlled reactions. This sounds plausible and the author’s intention to mimic predation and chemical communication even more interesting.

However,  the scientists also call droplets ‘protocells’ and talk about ‘populations’ and their ‘collective behaviour’. In my opinion, this is a bridge too far. Life is characterised by sustained metabolism and ability of self-propagation. Magnetite and silica droplets display neither.  The reports about creating synthetic life is overhyping, the chronic disease of modern science.

Laser in a droplet



      A typical laser – Terminator rules. (Image by US Air Force, via Wikimedia Commons)

When you hear about a laser, you imagine a medium-size apparatus with a light beam coming out of it. You don’t imagine bacteria in a drop of liquid. Well, Turkish and British scientists went beyond ordinary imagination and published their findings in Lab on Chip


If you don’t own a laser in a form of a laser pointer, you certainly use one when you scan your purchases. To construct a laser, you need three things – a source of energy to get it going, a material capable of amplifying it and a feedback mechanism allowing to amplify the initial energy even more.

In a run of the mill lasers, the initial electromagnetic waves are trapped by mirrors, which bounce and enhance them. One of the mirrors is semi-transparent, allowing some of the amplified energy to escape. If the escape is a narrow slit and the energy in the visible light part of the spectrum, it creates a laser beam, the weapon of choice of SF battles.

(A nice  animation of laser)

In a typical laser, you have a gas-filled cylinder or a glass rod with ions for the energy amplification. However, the internal paraphernalia can be replaced by an illuminated liquid droplet suspended in mid-air via a standing sonic wave (Whovians, rejoice!) or optical tweezers.


Water droplet

A suspended water droplet – unlikely laser. (Image by Thomas Bresson [CC BY 2.0 (, via Wikimedia Commons

A  suspended droplet is a perfect sphere, which allows the initial light to go inside and bounce from the internal edges of the sphere. A photon from an excitation laser can make tens of thousands of bounces.  The liquid inside the drop serves as the amplification medium and the droplet edge as mirrors.


Some of the light leak from the droplet-cavity in all directions. Because the leakage is omnidirectional, there is no beam, but the droplets size – from nano to micrometers – and variable droplet composition allow using the droplet lasers for various applications.



Bacteria expressing differently coloured fluorescent proteins glow in the dark.  (Image  by Nathan Shaner [GFDL  or CC-BY-SA-3.0 ] via Wikimedia Commons)

Once you’ve done your initial setup of an external light source and supply of droplets, you can vary the droplet composition. You can put in fluorescent dyes for a signal amplification.  Or you can use a fluorescent protein, which emits light.


The Turkish scientists went further and instead of a purified protein, used a live E.coli cell, which synthesized a fluorescent protein. One bacterium containing lots of fluorescent molecules was enough to create a tiny laser.



Bacteria E.coli seen via tunneling electron microscope – even unlikelier laser.  (Image by    NIAID [CC BY 2.0] via Wikimedia Commons)

The droplet-based lasers, containing biomolecules and even live cells can be used in biosensors, environmental analysis and lab-on-chip.