Topologically frustrated structures in inkjet printed chiral nematic liquid crystal droplets - experiments and simulations.

Director field alignment in inkjet printed droplets of chiral nematic liquid crystalline materials is investigated using both experiments and numerical simulations. Experimental investigations are performed by depositing droplets of varying sizes and pitches on homeotropic alignment layers. The competition between the bulk behaviour of the chiral nematic liquid crystal and the boundary conditions imposed by the droplet surface leads to the formation of a range of possible internal director configurations.

Drop impact onto immiscible liquid films floating on pools.

The interface dynamics of a droplet impacting onto a liquid pool has been well studied, and the common interfacial velocity quantified for the cases when the pool is both the same and a different fluid to the impacting droplet. In this work we investigate, experimentally and computationally, the scenario of a droplet impacting onto a pool of the same fluid coated by a layer of another fluid with various thicknesses. The effect of the film thickness on the penetration velocity of the upper droplet-film interface is measured for experiments and simulations, and carefully compared to theoretical predictions for early-to-moderate timescales in the limiting cases of: (i) zero film thickness, in which the film has no effect and thus behaves like a fluid on same fluid impact, and (ii) infinite film thickness, in which the underlying pool has no effect.

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).

A new flow-based design for double-lumen needles.

Oocyte retrieval forms a crucial part of in vitro fertilisation treatment and its ultimate outcome. Standard double-lumen needles, which include a sequence of aspiration and flushing steps, are characterised by a similar success rate to single-lumen needles, despite their increased cost. A novel hydrodynamics-based needle called the OxIVF needle is proposed here, which is geared towards the generation of an internal flow field within the full follicular volume via laterally, rather than frontally, oriented flushing, leading to successful retrievals with no additional stress on the oocyte.

Surface tension measurement and calculation of model biomolecular condensates.

The surface tension of liquid-like protein-rich biomolecular condensates is an emerging physical principle governing the mesoscopic interior organisation of biological cells. In this study, we present a method to evaluate the surface tension of model biomolecular condensates, through straighforward sessile drop measurements of capillary lengths and condensate densities. Our approach bypasses the need for characterizing condensate viscosities, which was required in previously reported techniques.

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 ratioμ/μ, 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).

Electrophoretic Molecular Communication With Piecewise Constant Electric Field.

This paper studies a novel electrophoretic molecular communication (EMC) framework utilizing a piecewise constant electric field. EMC is a particular type of molecular communication that exploits electric fields to induce the movement of charged particles to enhance communication performance. Our previous work proposed an EMC framework utilizing a time-varying electric field that exponentially changes; however, the field with such a complicated shape might be challenging to be implemented in practice.

OxVent: Design and evaluation of a rapidly-manufactured Covid-19 ventilator.

Background: The manufacturing of any standard mechanical ventilator cannot rapidly be upscaled to several thousand units per week, largely due to supply chain limitations. The aim of this study was to design, verify and perform a pre-clinical evaluation of a mechanical ventilator based on components not required for standard ventilators, and that met the specifications provided by the Medicines and Healthcare Products Regulatory Agency (MHRA) for rapidly-manufactured ventilator systems (RMVS).

Methods: The design utilises closed-loop negative feedback control, with real-time monitoring and alarms.

Droplet splashing on curved substrates.

Droplets impacting dry solid substrates often splash above a certain threshold impact velocity. We hypothesise that substrate curvature alters splashing thresholds due to a modification to the lift force acting on the lamella at the point of breakup. We have undertaken high-speed imaging experiments of millimetric droplets impacting convex and concave surfaces to establish splashing thresholds and dynamics across a wide range of substrate geometries and impact conditions.

Dipping into a new pool: The interface dynamics of drops impacting onto a different liquid.

When a drop impacts onto a pool of another liquid, the common interface will move down at a well-defined speed for the first few milliseconds. While simple mechanistic models and experiments with the same fluid used for the drop and pool have predicted this speed to be half the impacting drop speed, this is only one small part in a rich and intricate behavior landscape. Factors such as viscosity and density ratios greatly affect the penetration speed.

Creating wounds in cell monolayers using micro-jets.

Many wound-healing assays are used in cell biology and biomedicine; they are often labor intensive and/or require specialized and costly equipment. We describe a contactless method to create wounds with any imaginable 2D pattern in cell monolayers using the micro-jets of either media or an immiscible and biocompatible fluorocarbon (i.e.

Jet-Printing Microfluidic Devices on Demand.

There is an unmet demand for microfluidics in biomedicine. This paper describes contactless fabrication of microfluidic circuits on standard Petri dishes using just a dispensing needle, syringe pump, three-way traverse, cell-culture media, and an immiscible fluorocarbon (FC40). A submerged microjet of FC40 is projected through FC40 and media onto the bottom of a dish, where it washes media away to leave liquid fluorocarbon walls pinned to the substrate by interfacial forces.

Droplet impacts onto soft solids entrap more air.

We investigate the effects of surface stiffness on the air cushioning at the bottom of a liquid drop impacting onto a soft solid and the resulting entrapment of a central bubble. This was achieved using ultra-high-speed interferometry at 5 million frames per second and spatial resolution of 1.05 μm per pixel.

Using Fluid Walls for Single-Cell Cloning Provides Assurance in Monoclonality.

Single-cell isolation and cloning are essential steps in many applications, ranging from the production of biotherapeutics to stem cell therapy. Having confidence in monoclonality in such applications is essential from both research and commercial perspectives, for example, to ensure that data are of high quality and regulatory requirements are met. Consequently, several approaches have been developed to improve confidence in monoclonality.

Dynamics of droplets on cones: self-propulsion due to curvature gradients.

We study the dynamics of droplets driven by a gradient of curvature, as may be achieved by placing a drop on the surface of a cone. The curvature gradient induces a pressure gradient within the drop, which in turn leads to spontaneous propulsion of the droplet. To investigate the resulting driving force we perform a series of experiments in which we track a droplet's displacement, s, from the apex of a cone whose surface is treated to exhibit near-zero pinning effects.

The Effect of Surface Roughness on the Contact Line and Splashing Dynamics of Impacting Droplets.

Whether a droplet splashes upon impact onto a solid is known to depend not only on the fluid properties and its speed, but also on the substrate characteristics. Past research has shown that splashing is heavily influenced by the substrate roughness. Indeed, in this manuscript, we demonstrate that splashing is ruled by the surface roughness, the splashing ratio, and the dynamic contact angle.

Dynamics of wrinkling in ultrathin elastic sheets.

The wrinkling of thin elastic objects provides a means of generating regular patterning at small scales in applications ranging from photovoltaics to microfluidic devices. Static wrinkle patterns are known to be governed by an energetic balance between the object's bending stiffness and an effective substrate stiffness, which may originate from a true substrate stiffness or from tension and curvature along the wrinkles. Here, we investigate dynamic wrinkling induced by the impact of a solid sphere onto an ultrathin polymer sheet floating on water.

Role of the Dynamic Contact Angle on Splashing.

In this Letter, we study the splashing behavior of droplets upon impact onto a variety of substrates with different wetting properties, ranging from hydrophilic to superhydrophobic surfaces. In particular, we study the effects of the dynamic contact angle on splashing. The experimental approach uses high-speed imaging and image analysis to recover the apparent contact angle as a function of the spreading speed.

Raising fluid walls around living cells.

An effective transformation of the cell culture dishes that biologists use every day into microfluidic devices would open many avenues for miniaturizing cell-based workflows. In this article, we report a simple method for creating microfluidic arrangements around cells already growing on the surface of standard petri dishes, using the interface between immiscible fluids as a "building material." Conventional dishes are repurposed into sophisticated microfluidic devices by reshaping, on demand, the fluid structures around living cells.

Controlled cavity collapse: scaling laws of drop formation.

The formation of transient cavities at liquid interfaces occurs in an immense variety of natural processes, among which the bursting of surface bubbles and the impact of a drop on a liquid pool are salient. The collapse of a surface liquid cavity is a well documented natural process that leads to the ejection of a thin and fast jet. Droplets generated through this process can be one order of magnitude smaller than the cavity's aperture, and they are consequently of interest in drop on demand inkjet applications.

Microfluidic chambers using fluid walls for cell biology.

Many proofs of concept have demonstrated the potential of microfluidics in cell biology. However, the technology remains inaccessible to many biologists, as it often requires complex manufacturing facilities (such as soft lithography) and uses materials foreign to cell biology (such as polydimethylsiloxane). Here, we present a method for creating microfluidic environments by simply reshaping fluids on a substrate.

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.

Plethora of transitions during breakup of liquid filaments.

Thinning and breakup of liquid filaments are central to dripping of leaky faucets, inkjet drop formation, and raindrop fragmentation. As the filament radius decreases, curvature and capillary pressure, both inversely proportional to radius, increase and fluid is expelled with increasing velocity from the neck. As the neck radius vanishes, the governing equations become singular and the filament breaks.

Breakup of liquid filaments.

Whether a thin filament of liquid separates into two or more droplets or eventually condenses lengthwise to form a single larger drop depends on the liquid's density, viscosity, and surface tension and on the initial dimensions of the filament. Surface tension drives two competing processes, pinching-off and shortening, and the relative time scales of these, controlled by the balance between capillary and viscous forces, determine the final outcome. Here we provide experimental evidence for the conditions under which a liquid filament will break up into drops, in terms of a wide range of two dimensionless quantities: the aspect ratio of the filament and the Ohnesorge number.