Dynamic nozzles for drop generators

received: 2015-06-03 accepted: 2015-10-16 published: 2015-11-03received: 2015-06-03 accepted: 2015-10-16 published: 2015-11-03This work was funded by the UK Engineering and Physical Sciences Research Council (Grant No. EP/H018913/1), the John Fell Oxford University Press Research Fund, and the Royal Society

Nasca Lines: A Mystery wrapped in an Enigma

We analyze the geometrical structure of the astonishing Nasca geoglyphs in terms of their fractal dimension with the idea of dating these manifestations of human cultural engagements in relation to one another. Our findings suggest that the first delineated images consist of straight, parallel lines and that having sophisticated their abilities, Nasca artist moved on to the design of more complex structures.Comment: 6 pages, 1 color figure and 2 graphs. To appear in Chao

Effect of surfactants on the splashing dynamics of drops impacting smooth substrates

We present the results of a systematic study elucidating the role that dynamic surface tension has on the spreading and splashing dynamics of surfactant-laden droplets during the impact on hydrophobic substrates. Using four different surfactants at various concentrations, we generated a range of solutions whose dynamic surface tension were characterized to submillisecond timescales using maximum bubble-pressure tensiometry. Impact dynamics of these solutions were observed by high-speed imaging with subsequent quantitative image processing to determine the impact parameters (droplet size and speed) and dynamic wetting properties (dynamic contact angle). Droplets were slowly formed by dripping to allow the surfactants to achieve equilibrium at the free surface prior to impact. Our results indicate that while only the fastest surfactants appreciably affect the maximum spreading diameter, the droplet morphology during the initial stages of spreading is different to water for all surfactant solutions studied. Moreover, we show that surfactant-laden droplets splash more easily than pure liquid (water). Based on the association of the splashing ratio to our tensiometry measurements, we are able to predict the effective surface tension acting during splashing. These results suggest that droplet splashing characteristics are primarily defined by the stretching of the equilibrated droplet free surface

A fate-alternating transitional regime in contracting liquid filaments

The fate of a contracting liquid filament depends on the Ohnesorge number, the initial aspect ratio and surface perturbation. Generally, it is believed that there exists a critical aspect ratio such that longer filaments break up and shorter ones recoil into a single drop. Through computational and experimental studies, we report a transitional regime for filaments with a broad range of intermediate aspect ratios, where there exist multiple thresholds at which a novel breakup mode alternates with a no-break mode. We develop a simple model considering the superposition of capillary waves, which can predict the complicated new phase diagram. In this model, the breakup results from constructive interference between the capillary waves that originate from the ends of the filament

It's Harder to Splash on Soft Solids

Droplets splash when they impact dry, flat substrates above a critical velocity that depends on parameters such as droplet size, viscosity and air pressure. By imaging ethanol drops impacting silicone gels of different stiffnesses we show that substrate stiffness also affects the splashing threshold. Splashing is reduced or even eliminated: droplets on the softest substrates need over 70\% more kinetic energy to splash than they do on rigid substrates. We show that this is due to energy losses caused by deformations of soft substrates during the first few microseconds of impact. We find that solids with Young's moduli $\lesssim 100$kPa reduce splashing, in agreement with simple scaling arguments. Thus materials like soft gels and elastomers can be used as simple coatings for effective splash prevention. Soft substrates also serve as a useful system for testing splash-formation theories and sheet-ejection mechanisms, as they allow the characteristics of ejection sheets to be controlled independently of the bulk impact dynamics of droplets.Comment: 5 pages, 4 figure

A self-assembly based supramolecular bioink with hierarchical control As a new bioprinting tool

Tissue engineering aims to capture details of the extracellular matrix (ECM) that stimulate cell growth and tissue regeneration. Molecularly complex materials or advanced additive fabrication techniques are often used to capture aspects of the ECM. Promising biofabrication techniques often lack nano and molecular scale control, as well as materials that can recreate the natural ECM or selectively guide cell behaviour. On the other hand, complex biomaterials based on molecular self-assembly tend to lack reproducibility and order beyond the nanoscale. We propose a new material fabrication platform that integrates the benefits of bioprinting and molecular self-assembly to overcome the current major limitations. Our approach relies on the co-assembly of peptide amphiphiles (PAs) with biomolecules and/or proteins found in the ECM, whilst exploiting the droplet-on-demand (DoD) printing process. Taking advantage of the interfacial fluid forces during printing, it is possible to guide the self-assembly into aligned or disordered nanofibers, hydrogel structures of different geometries and sizes, surface topographies and higher-ordered structures made from multiple hydrogels. The co-assembly process can be performed during printing and in cell-friendly conditions, whilst exhibiting high cell viability (\u3e 88 %). Moreover, multiple cell types can be spatially distributed on the outside or embedded within the tuneable biomimetic scaffolds. The combination of self-assembly with 3D-bioprinting, provides a basis for a new biofabrication platform to create hydrogels of complex geometry, structural hierarchy and tuneable chemical composition. Please click Additional Files below to see the full abstract

Drop splashing after impact onto immiscible pools of different viscosities

Droplet impact onto liquid pools is a canonical scenario relevant to numerous natural phenomena and industrial processes. However, despite their ubiquity, multi-fluid systems with the drop and pool consisting of different liquids are far less well understood. Our hypothesis is that the post-impact dynamics greatly depends on the pool-to-droplet viscosity ratio , which we explore over a range of six orders of magnitude using a combination of experiments and theoretical approaches (mathematical modelling and direct numerical simulation). Our findings indicate that in this scenario the splashing threshold and the composition of the ejecta sheet are controlled by the viscosity ratio. We uncover that increasing the pool viscosity decreases the splashing threshold for high viscosity pools () when the splash comes from the droplet. By contrast, for low viscosity pools, the splash sheet comes from the pool and increasing the pool viscosity increases the splashing threshold. Surprisingly, there are conditions for which no splashing is observed under the conditions attainable in our laboratory. Furthermore, considering the interface velocity together with asymptotic arguments underlying the generation of the ejecta has allowed us to understand meaningful variations in the pressure during impact and rationalise the observed changes in the splashing threshold

Phase synchronization of baroclinic waves in a differentially heated rotating annulus experiment subject to periodic forcing with a variable duty cycle

A series of laboratory experiments in a thermally driven, rotating fluid annulus are presented that investigate the onset and characteristics of phase synchronization and frequency entrainment between the intrinsic, chaotic, oscillatory amplitude modulation of travelling baroclinic waves and a periodic modulation of the (axisymmetric) thermal boundary conditions, subject to time-dependent coupling. The time-dependence is in the form of a prescribed duty cycle in which the periodic forcing of the boundary conditions is applied for only a fraction ߜ of each oscillation. For the rest of the oscillation, the boundary conditions are held fixed. Two profiles of forcing were investigated that capture different parts of the sinusoidal variation and ߜ was varied over the range 0.1 ൑ ߜ ൑ 1. Reducing ߜ was found to act in a similar way to a reduction in a constant coupling coefficient in reducing the width of the interval in forcing frequency or period over which complete synchronization was observed (the “Arnol’d tongue”) with respect to the detuning, though for the strongest pulselike forcing profile some degree of synchronization was discernible even at ߜ ൌ 0.1. Complete phase synchronization was obtained within the Arnol’d tongue itself, though the strength of the amplitude modulation of the baroclinic wave was not significantly affected. These experiments demonstrate a possible mechanism for intraseasonal and/or interannual “teleconnections” within the climate system of the Earth and other planets that does not rely upon Rossby wave propagation across the planet along great circles

The dynamics of the impact and coalescence of droplets on a solid surface.

A simple experimental setup to study the impact and coalescence of deposited droplets is described. Droplet impact and coalescence have been investigated by high-speed particle image velocimetry. Velocity fields near the liquid-substrate interface have been observed for the impact and coalescence of 2.4 mm diameter droplets of glycerol∕water striking a flat transparent substrate in air. The experimental arrangement images the internal flow in the droplets from below the substrate with a high-speed camera and continuous laser illumination. Experimental results are in the form of digital images that are processed by particle image velocimetry and image processing algorithms to obtain velocity fields, droplet geometries, and contact line positions. Experimental results are compared with numerical simulations by the lattice Boltzmann method