Impact elasto-hydrodynamics of non-newtonian fluid droplets

Abstract

The present thesis investigates the influence of impact elasto-hydrodynamics of non-Newtonian fluid droplets on the different wettability, temperature and curvature targets. Research into droplet deposition following impact on superhydrophobic surfaces has recently become increasingly significant because of its potential application in reducing pesticide consumption. It has been demonstrated that small amounts of long-chain polymers applied to water can prevent the droplet bouncing effect on superhydrophobic substrates from occurring. Past investigations have attributed several explanations, such as extensional viscosity, the predominance of elastic stresses, or the delaying movement of the contact line during the recoiling phase as a result of the lengthening of polymer chains. The current investigation determines whether or not there is any importance for PAAM concentration and collision velocity that influence the suppression of drop rebound. The collision velocity will indirectly affect the shear rate throughout the recoiling phase, and the PAAM concentration determines the timescale for the elastic fluids to relax once they have been compressed. In the end, we demonstrate that the Weissenberg number (at the time of recoiling), which captures both the elastic characteristics of polymer linkages and the post-impact dynamics, is the vital factor in defining the system. The initiation of rebound inhibition occurs, and further that there exists an optimal value that governs the initiation of jump arrest. Weissenberg number is a factor that can be used to compare the three reasons listed above that are expressions of elastic phenomena in non-Newtonian fluids. This text emphasises the contribution of Bogor or elastic fluids (non-newtonian) results on the drop impact events at temperatures notably above the boiling reason or on top of the Leidenfrost point. For a fixed elasticity (polymer polyacrylamide concentrations), with an increase in the impact velocity (based on the impact height of the droplet), Leidenfrost point (LFP) goes on decreasing. Water droplets (Newtonian) disintegrated at lower impact velocity (manifested as Weber number), while the elastic droplets (non-Newtonian) resisted disintegration up to higher impact velocity. We conjointly varied the fluid elasticity (PAAM concentration) and discovered that, up to 1000 ppm, the Leidenfrost point was more than the water droplet. This reveals that the utilisation of Bogor fluids delays the result. We have established the potential feature of elastic results (manifested by way of the formation of long-lasting filaments) in the course of retraction during the increase of the Leidenfrost point. Although the LFP for 1500 ppm was less than that of water, it showed an identical dwell time to water in the early impact period. Additionally, we investigated the influence of the Weber number and elastic factors on the bouncing characteristic of the droplet at 405 degrees Celsius. We discovered that the critical Weber number up to the threshold from where the drop could withstand breakup at 405°C started rising in direct proportion to the concentration of PAAM used. For a given Weber number, the drop bouncing height and hovering period improved up to 500 ppm, after which they started to decline again. The observed conversion of bounceback kinetic energy into rotational energy during the floating period was linked to the non-monotonic nature of bounce heights. Last but not least, a correlation between the dimensionless Leidenfrost temperature and the corresponding Weber and Weissenberg numbers is established, and a scaling relationship is provided. Non-Newtonian elastic or Boger fluid drops (polyacrylamide (PAAM) solution in water) collide with convex and concave substrates of various sizes, and the elasto-hydrodynamics of the droplets are investigated. Investigations on both hydrophilic and superhydrophobic (SH) substrates are used to establish the importance of wettability. To better understand the diverse post-impact hydrodynamics of a drop, numerous determining parameters such as target size, Weber number, and fluid elasticity (different polymer concentrations) were adjusted systematic way. The development of long-lasting, narrow fluid filaments was seen in PAAM droplets (non-Newtonian) on hydrophilic substrates, in contrast to the capillary breakdown in the presence of water drops (Newtonian). These filaments displayed satellite beads when stretched, which were created by blistering or pearling instability (known as a beads-on-a-string (BOAS) effect) in some cases. PAAM drops bounce with greater cylindrical diameters and a higher Weber number than water drops on SH substrates. Fleeting filaments connected to the curved surface, which are thin and transient, gradually restrict droplet bounce. Water drops (Newtonian) on cylindrical SH surfaces usually observed bounce and disintegration, whereas non-Newtonian PAAM droplets on curved SHS never exhibited these characteristics (Rebound inhibition initiated). Finally, we show section maps in which the distinct regimes of post-impact elasto-hydrodynamics of the drop are associated with one another. The suggested elastic Weber number is described as the combination of two non-dimensional numbers (which incorporates the effects of both the Weber (inertia) and the Weissenberg numbers (elastic property)) and the non-dimensional diameter D∗(diameter ratio). We demonstrate that multiple scaling regimes emerge in the elasto-hydrodynamic behaviour of post-impact droplets of elastic fluids.