A team of scientists from America, China, and Japan has developed a mechanism that can explain the stability of nanodroplets composed of biomolecules. It was found that a weak positive charge forms on the surface of the smallest droplets, which causes electrostatic repulsion and prevents them from merging.
When water is mixed with oil and vigorously shaken, a large number of small oil droplets initially form. However, this system is unstable: to minimize surface energy, small droplets tend to disappear, and large ones tend to increase in size. There are two main processes of enlargement: droplets can merge upon contact due to Brownian motion, or Ostwald ripening occurs, where small droplets dissolve, and the released substance transfers to larger ones.
It is known that nanodroplets containing proteins, nucleic acids, and other biomolecules (with a diameter of about tens of nanometers) in living cells are capable of maintaining their stability for several hours or even days. Previously, scientists hypothesized that this stability was due to biochemical reactions, as well as the influence of the cytoskeleton and surfactants, but a complete answer to this question was lacking.
The team of American, Chinese, and Japanese physicists, led by Chen Feipeng from the University of Hong Kong, identified an additional stabilization mechanism for nanodroplets. The researchers studied aqueous solutions of two polyelectrolytes with opposite charges—positively charged PDDA and negatively charged PMA. These systems are often used as a simplified model to simulate biomolecular condensates formed in solutions of proteins and nucleic acids.
The physicists prepared solutions with different initial concentrations, while the ratio of PMA to PDDA in each solution was maintained at 1:1. As a result, droplets of polyelectrolytes formed in the aqueous medium, and in more concentrated samples, the droplets were initially larger. Over twelve hours, the growth of these droplets was observed using dynamic light scattering. It was found that the growth rate depended on the initial size: the largest droplets (with a diameter exceeding 500 nanometers) grew quickly. Medium droplets initially showed very slow growth, but after reaching a diameter of 250–300 nanometers, they began to grow at the same rate as the large ones. The smallest droplets (less than 200 nanometers) grew the least actively and ultimately hardly changed their volume during the entire observation period.
The authors of the calculations concluded that Ostwald ripening for such large molecules occurs extremely slowly; therefore, the dominant process should be coalescence upon collision. However, this coalescence is blocked by an electrostatic barrier. Since the positively charged PDDA chain is significantly longer than the negatively charged PMA, it is more advantageous for the short PMA chains to remain in the surrounding liquid, where they have greater freedom of movement and bending, than inside a dense droplet. This leads to some negative charges being outside the droplet, causing a slight excess of positive charges on its surface. Data obtained from zeta potential measurements and computer modeling confirmed that the charge density on the surface is maximal precisely for smaller droplets. Consequently, in a system where small droplets predominate, the electrostatic barrier prevents their merging. As the droplet size increases, this barrier weakens, allowing some droplets to merge, increase in size, and their subsequent growth becomes practically unlimited.
In 2024, a group of scientists from the USA, the UK, the Netherlands, and Germany studied the freezing process of oil with droplets. It was discovered that upon rapid cooling, oil droplets deform the ice differently than expected: instead of being pushed outward, they are pressed into the ice layer. The authors explained this phenomenon using Marangoni effect: when the surface tension of silicone oil changes sharply, the front part of the droplet experiences high surface tension, causing fluid to move from warmer areas to colder ones.
The physicists also modeled the movement of a walrus whisker in water and found that due to the curvature of the vibrissae, Karman vortex streets form. These streets lead to an increase in self-induced noise and a decrease in potential sensitivity. The authors of the article published in Physics of Fluids noted that the results obtained may be useful in developing hydrodynamic sensors for underwater vehicles.
