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 (i.e. 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 0N; 36.7 kDa) was divided into 10 microliter droplets and subjected to extensional strain in a modified tensiometer. Sixteen independent tests were performed where one test on a single droplet comprised 3 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 (~10kHz). The investigation demonstrates an extensional rheological method capable of determining the properties of protein solutions in microliter volumes and that tau protein can aggregate when exposed to a single extensional strain with potentially significant biological implications.

This dissertation consists of a focused study of the low melting point liquid metal known commercially as Galinstan. This metal is notable for being readily liquid at room temperature, being non-toxic, and forming a peculiar oxide layer that is confined to the surface of the liquid. This study consists of four parts. The first is a literature review of the field of flexible and stretchable electronics. The purpose of this review is to assess the state of the field and to make the case for the value of Galinstan as a primary conducting element in the manufacture of these devices. With this case made, the remaining three parts can be sorted into two overarching concepts: experiments designed to produce data relevant to the drop on demand printing of flexible electronics, and to the study of fundamental fluid mechanics phenomenon, often overlapping throughout.

The first study involves the use of tensiometry to measure the effective surface tension, as well as explore the wetting behavior of the liquid metal on glass. For measuring the surface tension, we developed a novel technique that combines principles of both the Wilhelmy plate method, but with a rod modification, with the Du No�y ring method. After measuring the surface tension, we studied the wetting behavior of the metal, namely the dynamic contact angle and found the advancing contact angle (θ??), receding contact angle (θ??), and the contact angle hysteresis θ??−θ??.

Next, we performed droplet impact experiments of liquid metal on glass substrate. Utilizing the wetting information from the tensiometry experiments, we were able to develop a novel model for impact driven spreading of liquid metal on glass for Weber numbers ranging from 10 to 205. This model shows new scaling behavior as compared to other droplet impact models and we compare and contrast these models of impact. This model is also predictive and would serve to modulate voxel size in additive manufacturing operations with Galinstan.

The final set of experiments were of partially coalescing liquid metal droplets in 1 M NaOH. These droplets were observed to bounce on the liquid metal interface and reach different heights as they coalesced into smaller and smaller droplets. We developed a first order model based on the Stokes approximation for droplets in a viscous fluid which we refined with a improved numerical model which was more valid for the range of Reynolds numbers tested.