Giorgia Sinibaldi

Aeroacoustics and aerodynamics of impinging supersonic jets: Analysis of the screech tones

The interaction between acoustics and aerodynamics of a supersonic jet is an actual fundamental topic which has been a matter of discussion in the last decades. The present paper is devoted to the experimental analysis of free and impinging jets with particular attention on the effect of an impinging surface on screech tones. The acoustics is studied using free-field microphones, while Particle Image Velocimetry is used to investigate the velocity field. The analysis of acquired data allowed to verify and explain the coupling between acoustic discrete tones and mean and fluctuating flow velocities.

Swimming and rafting of E.coli microcolonies at air–liquid interfaces

The dynamics of swimming microorganisms is strongly affected by solid‐liquid and air‐liquid interfaces. In this paper, we characterize the motion of both single bacteria and microcolonies at an air‐liquid interface. Both of them follow circular trajectories. Single bacteria preferentially show a counter‐clockwise motion, in agreement with previous experimental and theoretical findings. Instead, no preferential rotation direction is observed for microcolonies suggesting that their motion is due to a different physical mechanism. We propose a simple mechanical model where the microcolonies move like rafts constrained to the air‐liquid interface. Finally, we observed that the microcolony growth is due to the aggregation of colliding single‐swimmers, suggesting that the microcolony formation resembles a condensation process where the first nucleus originates by the collision between two single‐swimmers. Implications of microcolony splitting and aggregation on biofilm growth and dispersion at air‐liquid interface are discussed.

Towards cavitation-enhanced permeability in blood vessel on a chip

The development of targeted delivery systems releasing pharmaceutical agents directly at the desired site of action may improve their therapeutic efficiency while minimizing damage to healthy tissues, toxicity to the patient and drug waste. In this context, we have developed a bio-inspired microdevice mimicking the tumour microvasculature which represents a valuable tool for assessing the enhancement of blood vessel permeability due to cavitation. This novel system allows us to investigate the effects of ultrasound-driven microbubbles that temporarily open the endothelial intercellular junctions allowing drug to extravasate blood vessels into tumour tissues. The blood vessel on a chip consists of a tissue chamber and two independent vascular channels (width 200 µm, height 100 µm, length 2762 µm) cultured with endothelial cells placed side-by-side and separated by a series of 3 µm pores. Its geometry and dimensions mimic the three-dimensional morphology, size and flow characteristics of microvessels in vivo. The early stage of this project had a twofold objective: 1. To define the protocol for culturing of Human Umbilical Vein Endothelial Cells (HUVECs) within the vascular channel; 2. To develop a fluorescence based microscopy technique for measuring permeability. We have developed a reliable and reproducible protocol to culture endothelial cells within the artificial vessels in a realistic manner: HUVECs show the typical elongated shape in the direction of flow, exhibit tight junction formation and form a continuous layer with a central lumen that completely covers the channels wall. As expected, the permeability of cell-free device is higher than the one cultured with HUVECs in the vascular channels. The proposed blood vessel on a chip and the permeability measurement protocol have a significant potential to allow for the study of cavitation-enhanced permeability of the endothelium and improve efficiency in screening drug delivery systems.The development of targeted delivery systems releasing pharmaceutical agents directly at the desired site of action may improve their therapeutic efficiency while minimizing damage to healthy tissues, toxicity to the patient and drug waste. In this context, we have developed a bio-inspired microdevice mimicking the tumour microvasculature which represents a valuable tool for assessing the enhancement of blood vessel permeability due to cavitation. This novel system allows us to investigate the effects of ultrasound-driven microbubbles that temporarily open the endothelial intercellular junctions allowing drug to extravasate blood vessels into tumour tissues. The blood vessel on a chip consists of a tissue chamber and two independent vascular channels (width 200 µm, height 100 µm, length 2762 µm) cultured with endothelial cells placed side-by-side and separated by a series of 3 µm pores. Its geometry and dimensions mimic the three-dimensional morphology, size and flow characteristics of microvessels in viv...

Analysis of the characteristic acoustic tones of an impinging jet

The acoustic emission of a supersonic jet is an interesting and fundamental issue due to the several technological applications and the diverse physical aspects to be still clarified. An experiment ...

A T-junction device allowing for two simultaneous orthogonal views: application to bubble formation and break-up

A novel design for the classical microfluidic device known as T-junction is proposed with the purpose of obtaining a simultaneous measurement of the in-plane velocity components in two orthogonal planes. A crucial feature of the proposed configuration is that all three velocity components are available along the intersection of the two planes. A dedicated optical set-up is developed to convey the two simultaneous views from the orthogonal planes into the sensor of a single camera, where a compound image is formed showing on either half one of the two views. A commercial micro-particle image velocimetry system is used to measure the velocity in the two planes. Feeding the T-junction with a liquid continuous phase and a dispersed gas phase, the velocity is measured by phase averaging along the bubble formation and break-up process showing the potentialities of the new design. The accuracy analysis shows that the error is dominated by a systematic component due to the thickness of the measurement slice. The error can be reduced by applying confocal microscopy to the present system with no further modifications so as to reduce the thickness of the measurement slab thereby reducing the error. Moreover, by sweeping the planes across the region of interest, a full three-dimensional reconstruction of the velocity field can be readily obtained. Finally, the simultaneous views offer the possibility to extract the principal curvatures of the bubble meniscus thereby providing access to the Laplace pressure.

Perspectives on cavitation enhanced endothelial layer permeability

Traditional drug delivery systems, where pharmaceutical agents are conveyed to the target tissue through the blood circulation, suffer of poor therapeutic efficiency and limited selectivity largely due to the low permeability of the highly specialised biological interface represented by the endothelial layer. Examples concern cancer therapeutics or degenerative disorders where drug delivery is inhibited by the blood-brain barrier (BBB). Microbubbles injected into the bloodstream undergo volume oscillations under localised ultrasound irradiation and possibly collapse near the site of interest, with no effect on the rest of the endothelium. The resulting mechanical action induces a transient increase of the inter-cellular spaces and facilitates drug extravasation. This approach, already pursed in in vivo animal models, is extremely expensive and time-consuming. On the other hand in vitro studies using different kinds of microfluidic networks are firmly established in the pharmaceutical industry for drug delivery testing. The combination of the in vitro approach with ultrasound used to control microbubbles oscillations is expected to provide crucial information for developing cavitation enhanced drug delivery protocols and for screening the properties of the biological interface in presence of healthy or diseased tissues. Purpose of the present review is providing the state of the art in this rapidly growing field where cavitation is exploited as a viable technology to transiently modify the permeability of the biological interface. After describing current in vivo studies, particular emphasis will be placed on illustrating characteristics of micro-devices, biological functionalisation, properties of the artificial endothelium and ultrasound irradiation techniques.

Load identification by coherence analysis of structural response

Aim of this paper is the identification of uncorrelated forces acting on a structure based on a coherence analysis of the structure response, performed entirely in operative condition. In order to identify the position and the amplitude of the applied load only the responses of the structure and the experimental FRF are required. The proposed procedure consists of three steps. First, the number of acting loads is established by the analysis of the responses coherence, second the position of the acting forces is identified using an index obtained by the knowledge of experimental FRF and of the responses of the structure, then, the amplitude of the acting forces is computed in correspondence of the excited points. The procedure is tested by two experiments. First experiment consists in the excitation of a complex structure in several places with an instrumented hammer. The identification is performed by the accelerations measured on a set of points of the structure itself. The second experiment is carried o...