It is generally quite difficult to see through cloudy liquids, but a team of researchers has managed to overcome this challenge.
When driving through a foggy area, car headlights provide limited assistance since the light gets scattered by tiny water droplets in the air. A similar problem occurs when trying to observe the interior of a milk drop in water or the structure of an opal gem using white light. In these scenarios, multiple scattering of light hinders clear examination of what’s inside. Researchers from Johannes Gutenberg University Mainz (JGU) and Heinrich Heine University Düsseldorf (HHU) have developed a new method to explore the inside of a crystalline drop. Their discoveries have been recently published in the journal Soft Matter.
Monochromatic light illuminates the issue
When a drop of ink is placed in water, it’s common knowledge that the ink particles will eventually spread out due to diffusion. However, the behavior of drops made up of particles that repel each other is not as predictable. Though there are some simulations related to unique materials like dusty plasma—which consists of mutually repelling particles similar to those found in the sun—there hasn’t been much research into drops of repelling particles in a liquid. Past experiments aimed at measuring the three-dimensional behavior of such drops have largely failed. Now, scientists have created a simple laboratory method for studying scenarios where white light can’t penetrate, and X-ray usage would be impractical. Their technique takes advantage of the fact that the color of light scattered multiple times depends on the local particle concentration. This effect is especially pronounced in crystalline materials, where areas of different concentrations will show different colors; concentrated regions appear bright blue, while more diluted areas take on a reddish hue. In contrast, when white light—which contains various wavelengths—is used, all colors get scattered at once, making it nearly impossible to pinpoint the origin of each hue within the murky mixture.
“We tackled this challenge by illuminating the drops sequentially with different monochromatic light, meaning light of specific wavelengths,” explained Professor Palberg from JGU. With each wavelength, multiple scattering only occurs in regions with appropriate particle density, allowing other areas of the drop to become transparent to that particular wavelength. “As a result, we can accurately identify where the red or blue light originated deep within the drop,” Palberg added. “With our method, we can now examine the density profiles of crystalline, turbid drops, and even delve into other hazy mediums with high spatial and temporal accuracy.” For example, this technique could help analyze concentration gradients in settling slurries or evaluate the level of homogenization in paint mixed with solvent.
Intricate expansion profile of crystalline drops
In their recent study, the researchers applied their novel approach to investigate drops filled with a suspension of similarly charged, repulsive small polymer spheres mixed in water. Initially, these particles interact so strongly that the undiluted mixture resembles a polycrystalline solid. This appearance is akin to an opal gem and demonstrates significant multiple scattering. However, placing a drop of this mixture in water triggers an expansion. “Through this groundbreaking research, we established that the expansion profile of this crystalline material is quite complex. There isn’t a steady overall density with a clearly defined outer edge, nor is there a simple diffusion pattern as one might anticipate with a drop of non-repelling particles in a liquid,” stated Palberg. Additionally, there is a swift initial expansion of the crystalline sphere due to the repulsion among particles, but as crystals begin to dissolve at the droplet’s edge due to dilution, the drop slowly starts to contract.
While the experiments took place at Mainz University, Professor Hartmut Löwen’s team at HHU worked on theoretical modeling of the density profile using dynamical density functional theory. “We observed a promising correlation between experimental results and theoretical modeling, suggesting the robust predictive capabilities of this theory,” noted Löwen. The calculated density profile indicated a central peak in density and a radial gradient that smoothed over time. Interestingly, even the timing for maximum expansion of the crystalline drop was accurately predicted. Researchers concluded that the size of a drop is influenced by two opposing processes: continuous expansion while simultaneously melting along its edge. “This interaction between these two processes generates an expansion scenario that qualitatively differs from what has been projected based on plasma modeling,” the researchers concluded. They now plan to further investigate by systematically adjusting the level of particle repulsion to understand how it impacts density profiles and expansion dynamics.
