Coupled instability modes at a solvent/non-solvent interface to decorate cellulose acetate flowers

Abstract

Dispensing a water drop on the thin film of a solution composed of cellulose acetate (CA) in dimethyl formamide (DMF) forms a thin and porous CA layer at the water–DMF interface. While a denser water drop on a rarer CA–DMF film manifests a Rayleigh–Taylor instability—RTI, the dynamically forming porous layer at the water–DMF interface triggers a Saffman–Taylor instability—STI. The combined effects of RTI and STI enable the formation, growth, coalescence, and branching of an array of periodic finger patterns to finally develop into a flower-like morphology. A general linear stability analysis (GLSA) of a thin bilayer composed of a Newtonian and incompressible water layer resting on a Darcy–Brinkman porous medium could predict the length and the time scales of such a finger formation phenomenon. The GLSA uncovers the crucial roles of pressure gradients originating from the gravitational effects, osmotic forces, the Marangoni effect, and capillary forces on the dynamics of the finger formation. While the density difference between water and CA–DMF layer plays a crucial role in deciding the initial finger spacing, the osmotic pressure dictates the formation, growth, branching, and coalescence of fingers. The length-FL and number-Navg of fingers are found to scale as FL∼We0.33Re−0.25 and Navg∼We0.33Re0.25. Further, an inverse relationship of the concentration of CA (C) with ∼We−0.3 and ∼Re−0.7 highlights its role in the formation and growth of fingers. The loading of CA in DMF, the viscosity and density of the CA–DMF film, and the curvature of the fingers are found to be other parameters that decide morphologies.

Electroosmotic Micromixing in Physicochemically Patterned Microchannels

“…18,19 In particular, the success in the integration of multiplexing of microfluidic devices on the lab-on-a-chip 20,21 platforms has led to the development of portable laboratory prototypes in the diverse areas of biology, 22−24 chemistry, 25−27 medicine, 28,29 and engineering. 30,31 However, several limitations related to microfluidic platforms have emerged over the years, including the diffusion-limited mixing capacity, a relatively lower throughput, and crowding−clogging of transport materials, among others. Of late, such areas have become intense scientific and engineering research.…”

“…It is now well-known that there are multifold advantages of using microfluidic devices over macroscopic ones owing to their portability, ease of use, availability of a higher surface-to-volume ratio for the process intensified engineering processes, control over the reagent parameters owing to their usage of smaller volumes, and capacity to bring in the aspects of very-large-scale integration (VLSI) for a larger throughput and multitasking, among others. Thus, such microfluidic platforms are found to appear in diverse modern-day functionalities that include drug delivery, − point-of-care diagnostics, − tissue engineering, − high-throughput screening, − protein crystallization, − and deoxyribonucleic acid (DNA) analysis. , In particular, the success in the integration of multiplexing of microfluidic devices on the lab-on-a-chip , platforms has led to the development of portable laboratory prototypes in the diverse areas of biology, − chemistry, − medicine, , and engineering. , However, several limitations related to microfluidic platforms have emerged over the years, including the diffusion-limited mixing capacity, a relatively lower throughput, and crowding–clogging of transport materials, among others. Of late, such areas have become intense scientific and engineering research.…”