Guido Bolognesi

Evidence of slippage breakdown for a superhydrophobic microchannel

A full characterization of the water flow past a silicon superhydrophobic surface with longitudinal micro-grooves enclosed in a microfluidic device is presented. Fluorescence microscopy images of the flow seeded with fluorescent passive tracers were digitally processed to measure both the velocity field and the position and shape of the liquid-air interfaces at the superhydrophobic surface. The simultaneous access to the meniscus and velocity profiles allows us to put under a strict test the no-shear boundary condition at the liquid-air interface. Surprisingly, our measurements show that air pockets in the surface cavities can sustain non-zero interfacial shear stresses, thereby hampering the friction reduction capabilities of the surface. The effects of the meniscus position and shape as well as of the liquid-air interfacial friction on the surface performances are separately assessed and quantified.

Microfluidic generation of monodisperse ultra-low interfacial tension oil droplets in water

We present a novel microfluidic approach for the generation of monodisperse oil droplets in water with interfacial tensions of the order of 1 μN m−1. Using an oil-in-water emulsion containing the surfactant aerosol OT, heptane, water and sodium chloride under conditions close to the microemulsion phase transition, we actively controlled the surface tension at the liquid–liquid interface within the microfluidic device in order to produce monodisperse droplets. These droplets exhibited high levels of stability with respect to rupture and coalescence rates. Confirmation that the resultant emulsions were in the ultra-low tension regime was determined using real space detection of thermally-induced capillary waves at the droplet interface.

A novel technique for simultaneous velocity and interface profile measurements on micro-structured surfaces

We present a novel approach which allows simultaneous measurement of the velocity field and the interface profile close to a composite liquid–gas and solid–gas interface. The proposed scheme is the method of choice for the characterization of those flows where the velocity field is highly dependent on the actual shape and position assumed by liquid–gas and liquid–solid interfaces. The new method is based on the digital processing of microscopy images of a flow seeded with fluorescent passive tracers. The relative position and the shape of both liquid–gas and liquid–solid interfaces can be determined with a resolution of few tens of nanometers. The results for the liquid–solid interfaces are also compared to an additional detection method we devised to accurately determine the absolute position of the solid walls.

Digital holographic tracking of microprobes for multipoint viscosity measurements

Digital holographic microscopy provides an ideal tool for 3D tracking of microspheres while simultaneously allowing a full and accurate characterization of their main physical properties such as: radius and refractive index. We demonstrate that the combination of high resolution multipoint tracking and accurate optical sizing of tracers provides an ideal tool for precise multipoint viscosity measurements. We also report a detailed evaluation of the techniques accuracy and precision in relation to the primary sources of error.

Multiplexed droplet Interface bilayer formation

We present a simple method for the multiplexed formation of droplet interface bilayers (DIBs) using a mechanically operated linear acrylic chamber array. To demonstrate the functionality of the chip design, a lipid membrane permeability assay is performed. We show that multiple, symmetric DIBs can be created and separated using this robust low-cost approach.

Optical characterization of an individual polymer-shelled microbubble structure via digital holography

Microbubbles are often used as ultrasound contrast agents for sonography. However, polymer-shelled microbubbles have the potential to support also additional functionalities such as, for example, targeted drug delivery. Using digital holographic microscopy we demonstrate that structural information on individual bubbles can be retrieved with high precision combined with low costs and short acquisition time. We have characterized the distributions of size, shell thickness and refractive index for hundreds of polymeric microbubbles showing that shell thickness displays a large variation that is strongly correlated with its refractive index. A large thickness corresponds to a smaller refractive index indicating a possible mechanism of progressive shell swelling with water.

Real-time digital holographic microscopy of multiple and arbitrarily oriented planes

Digital holographic microscopy is used to numerically refocus a recorded hologram at an arbitrary axial distance. However, as a straightforward property of coherent light fields, image reconstruction on an arbitrary tilted plane could be directly obtained by a rotation in k-space. We demonstrate that this property allows the real-time microscopic inspection of particle distribution over three mutually orthogonal planes at the same time. As a straightforward application we use the proposed technique for real-time monitoring of fluid flow over the three cross sections of a microfluidic channel.

Sculpting and fusing biomimetic vesicle networks using optical tweezers

Constructing higher-order vesicle assemblies has discipline-spanning potential from responsive soft-matter materials to artificial cell networks in synthetic biology. This potential is ultimately derived from the ability to compartmentalise and order chemical species in space. To unlock such applications, spatial organisation of vesicles in relation to one another must be controlled, and techniques to deliver cargo to compartments developed. Herein, we use optical tweezers to assemble, reconfigure and dismantle networks of cell-sized vesicles that, in different experimental scenarios, we engineer to exhibit several interesting properties. Vesicles are connected through double-bilayer junctions formed via electrostatically controlled adhesion. Chemically distinct vesicles are linked across length scales, from several nanometres to hundreds of micrometres, by axon-like tethers. In the former regime, patterning membranes with proteins and nanoparticles facilitates material exchange between compartments and enables laser-triggered vesicle merging. This allows us to mix and dilute content, and to initiate protein expression by delivering biomolecular reaction components.Assembly of higher-order artificial vesicles can unlock new applications. Here, the authors use optical tweezers to construct user-defined 2D and 3D architectures of chemically distinct vesicles and demonstrate inter-vesicle communication and light-enabled compartment merging.

Engineering thermoresponsive phase separated vesicles formed: Via emulsion phase transfer as a content-release platform

Elucidation of cholesterol insertion efficiency into phase-transfer vesicles enables the rational design of phase-separated membranes as thermally-responsive platforms for artificial cell construction.

Rheological droplet interface bilayers (rheo-DIBs): Probing the unstirred water layer effect on membrane permeability via spinning disk induced shear stress

A new rheological droplet interface bilayer (rheo-DIB) device is presented as a tool to apply shear stress on biological lipid membranes. Despite their exciting potential for affecting high-throughput membrane translocation studies, permeability assays conducted using DIBs have neglected the effect of the unstirred water layer (UWL). However as demonstrated in this study, neglecting this phenomenon can cause significant underestimates in membrane permeability measurements which in turn limits their ability to predict key processes such as drug translocation rates across lipid membranes. With the use of the rheo-DIB chip, the effective bilayer permeability can be modulated by applying shear stress to the droplet interfaces, inducing flow parallel to the DIB membranes. By analysing the relation between the effective membrane permeability and the applied stress, both the intrinsic membrane permeability and UWL thickness can be determined for the first time using this model membrane approach, thereby unlocking the potential of DIBs for undertaking diffusion assays. The results are also validated with numerical simulations.