Characterization of liquids with modulated density profiles

In many industrial settings, such as inside machines, liquids can be confined to very small spaces. For instance, liquid lubricants work in the small spaces between solid components to reduce friction and wear. Other examples of confined liquids include when they are inside a porous solid, and in biological systems. In these confined geometries, the properties of the liquid are fundamentally different from the properties of the same liquid in a larger, macroscopic, volume. The viscosity of the liquid is significantly enhanced and the rheological properties differ drastically. Such liquids are extremely common, and can affect the behavior of the structures around them. Therefore, an in-depth theoretical understanding is essential for progress in many practical applications. However, it can be difficult to study these liquids directly because very small length scales, and correspondingly very small timescales, must be resolved. Since the walls are generally rough, the interaction between the liquid and the walls is non-trivial and must also be considered. A key feature of confined liquids is that their density is not uniform, but it varies close to the walls, i.e it is higher in some regions and lower in others. This feature can be mimicked in macroscopic systems. In experiments, this can be done with lasers that exert a force on all particles, in such a way that the strength and direction of the force depends on the location of the particle. Thus, the density profile can be controlled precisely. In this project, we will consider similar forces applied in theoretical and simulated models to mimic the varying densities seen in confined fluids. In the future, it should be possible to compare our results directly with experiments. Through this project we expect to gain valuable insight into liquids with varying density, thereby expanding the understanding of confined liquids.

In many industrial settings liquids are confined to very small spaces. For example, liquid lubricants work in the small spaces between solid components in machines. Other examples of confined liquids include when they are inside a porous solid, like crude oil during the extraction process, and in biological systems, like synovial fluid in knee joints or the intracellular fluid inside cells. In these confined geometries, the properties of the liquid are fundamentally different from the properties of the same liquid in a larger, macroscopic, volume. For example, the viscosity of the liquid is significantly increased and the rheological properties change drastically. For liquids confined between two parallel walls, their structure has two particularly apparent features. The first is a very complicated relationship between how ordered the particles are, and how confined the liquid is. The second noteworthy feature is that the density is not uniform, but it varies close to the walls, i.e it is higher in some regions and lower in others; the particles tend to layer against the walls. A critical question in understanding confined liquids is determining if these two features are intrinsically linked. To disentangle these features, we devised a way to study liquids which are confined, but do not have layering. We did this by confining the liquid to the surface of a 4D cylinder. Despite the fact that this quasi-confined liquid cannot exist experimentally, we can still gain valuable insight by studying it with computer simulations and theory. We find that the amount of order in this quasi-confined fluid exhibits a similar non-monotonic behavior to a liquid confined between two walls, even though the density is homogeneous. However, the size of this effect is much smaller. Approaching this question from the opposite side, we artificially controlled the density profile in a macroscopic system. We find that the degree of ordering in this system shows a strong dependence on modulation wavelength. This means that it's both the confinement itself, as well as the density modulation, that is the cause of the complicated relationship between particle ordering and confinement length scale in real confined liquids. The degree of ordering of the particles directly influences how readily the liquid transitions to a glass, and goes from acting like a fluid to acting like a solid. The improved understanding of particle ordering and density modulation we have gained in this project could be exploited in the future to design confined liquids with desired properties. For example, the surface of the confining walls could be tuned to create a particular density profile, which could be selected to minimize the degree of ordering in the fluid. This has the potential to improve the effectiveness of lubricants.