Probing Flow-Induced Biomolecular Interactions With Micro-Extensional Rheology: Tau Protein Aggregation.

Biomolecules in solutions subjected to extensional strain can form aggregates, which may be important for our understanding of pathologies involving insoluble protein structures where mechanical forces are thought to be causative (e.g., tau fibers in chronic traumatic encephalopathy (CTE)). To examine the behavior of biomolecules in solution under mechanical strains requires applying rheological methods, often to very small sample volumes. There were two primary objectives in this investigation: (1) To probe flow-induced aggregation of proteins in microliter-sized samples and (2) To test the hypothesis that tau protein aggregates under extensional flow. Tau protein (isoform:3R 0 N; 36.7 kDa) was divided into 10 µl droplets and subjected to extensional strain in a modified tensiometer. Sixteen independent tests were performed where one test on a single droplet comprised three extensional events. To assess the rheological performance of the fluid/tau mixture, the diameter of the filament that formed during extension was tracked as function of time and analyzed for signs of aggregation (i.e., increased relaxation time). The results were compared to two molecules of similar and greater size (Polyethylene Oxide: PEO35, 35 kDa and PEO100, 100 kDa). Analysis showed that the tau protein solution and PEO35 are likely to have formed aggregates, albeit at relatively high extensional strain rates (∼10 kHz). The investigation demonstrates an extensional rheological method capable of determining the properties of protein solutions in µl volumes and that tau protein can aggregate when exposed to a single extensional strain with potentially significant biological implications.

Forced versus Spontaneous Spreading of Liquids.

Two sets of experiments are performed, one for the free spreading of a liquid drop on a glass substrate and the other for the forced motion of a glass plate through a gas-liquid interface. The measured macroscopic advancing contact angle, θA, versus the contact line speed, U, differ markedly between the two configurations. The hydrodynamic theory (HDT) and the molecular kinetic theory (MKT) are shown to apply separately to the two systems. This distinction has not been previously noted. Rules of thumb are given that for an experimentalist involve a priori knowledge of the expected behavior.

Spreading and arrest of a molten liquid on cold substrates.

Understanding the spreading and solidification of liquids on cold solid surfaces is a problem of fundamental importance and general utility. The physics of nonisothermal spreading followed by phase change is still a mystery. The present work focuses on the dynamics and thermal characteristics of liquid drop spreading and their subsequent arrest due to freezing. The spreading of liquid is recorded, and the evolution of the liquid spreading diameter and liquid-solid contact angle is measured from the recordings of a high-speed digital camera. After the initiation of solidification, the liquid drops are pinned to the substrate, showing fixed footprints and contact angles. A physical hypothesis using scaling is provided to explain the relationship between the arrested base diameter (D*) and arrested contact angle (θ*) with respect to the Stefan number (Ste). The experimental observations of solidified drops on cold substrates corroborate the derived physical theory.

Effect of surface mobility on the uniformity of a thin film under a bubble.

The shape of a soap bubble placed on a solid surface is familiar to everyone-a thin hemispherical dome that thickens near the solid surface. This structure is stabilized by the balance between the film's elasticity, provided by surfactant molecules, and the pressure inside the bubble. However, there is also a soap film on the flat solid surface that has been mostly ignored in previous studies; its thickness is typically assumed to be constant or varying monotonically. In this letter, for the first time, we show that the thickness of this film is not always monotonic. Depending on the surfactant type, it can exhibit a significant dip, similar to marginal pinching. This finding has a significant influence in numerous applications in which solid/foam interactions are important, such as oil extraction or foam-based drug delivery.

Dynamics of nanoscale precursor film near a moving contact line of spreading drops.

The spreading of liquids on solids is a commonplace phenomenon, discernible in various instances of everyday life. Despite the apparent simplicity of spreading, the underlying mechanisms are still not fully understood on a microscopic level, particularly at the moving edge between liquid and solid known as the contact line region. Here we show the time-dependent evolution of nanoscale films on a clean solid surface near the moving contact line. Our work contributes to the body of experimental evidence required to assemble a comprehensive understanding of microstructures at the vicinity of the contact line, bridging the gap between computational methods and theory. Moreover, this research will provide insight into the fundamental behavior of fluid spreading and other surface phenomena.

Hydrodynamic instabilities of viscous coalescing droplets.

Droplets coalescing at a planar fluid-fluid interface are studied in detail in the Stokes regime with high speed photography. Attention is paid to the expansion of the interfacial bridge, formed between the droplet and interface, as it expands during the process. We report a hydrodynamic instability at the rim of the interfacial bridge. As the rim becomes unstable, it forms a series of tendrils which themselves become unstable and produce micron sized droplets. We show that rim stability depends on drop and medium viscosities as well as the rim geometry.

Astonishing life of a coalescing drop on a free surface.

We report an interesting feature in the consecutive steps of coalescing of a drop, which is called a cascade of partial coalescence. It is observed that as the secondary drop gets smaller, it bounces higher. We show that the capillary force is the main driving force for this phenomenon. By using ultra-high-speed video, it is revealed that the capillary force at the pinch off pulls the drop to the planar interface. The drop bounces off the interface and moves upward until it reaches the maximum height. A theory is developed that includes the capillary and gravitational forces and predicts this process.