The dynamics of a non-equilibrium bubble near bio-materials

Abstract

In many medical treatments oscillating (non-equilibrium) bubbles appear. They can be the result of high-intensity-focused ultrasound, laser treatments or shock wave lithotripsy for example. The physics of such oscillating bubbles is often not very well understood. This is especially so if the bubbles are oscillating near (soft) bio-materials. It is well known that bubbles oscillating near (hard) materials have a tendency to form a high speed jet directed towards the material during the collapse phase of the bubble. It is equally well studied that bubbles near a free interface (air) tend to collapse with a jet directed away from this interface. If the interface is neither 'free' nor 'hard', such as often occurs in bio-materials, the resulting flow physics can be very complex. Yet, in many bio-applications, it is crucial to know in which direction the jet will go (if there is a jet at all). Some applications require a jet towards the tissue, for example to destroy it. For other applications, damage due to impacting jets is to be prevented at all cost. This paper tries to address some of the physics involved in these treatments by using a numerical method, the boundary element method (BEM), to study the dynamics of such bubbles near several bio-materials. In the present work, the behaviour of a bubble placed in a water-like medium near various bio-materials (modelled as elastic fluids) is investigated. It is found that its behaviour depends on the material properties (Young's modulus, Poisson ratio and density) of the bio-material. For soft bio-materials (fat, skin, brain and muscle), the bubble tends to split into smaller bubbles. In certain cases, the resulting bubbles develop opposing jets. For hard bio-materials (cornea, cartilage and bone), the bubble collapses towards the interface with high speed jets (between 100 and about 250 m s(-1)). A summary graph is provided identifying the combined effects of the dimensionless elasticity (kappa) and density ratio (alpha) of the elastic materials which will result in a nearby oscillating bubble jetting towards, splitting or jetting away from the elastic material interface. Since the phenomenon of a bubble jetting away from an elastic material as it collapses has not been reported before in the literature, experiments were performed to validate the numerical observation. A bubble is created in a heavy fluid (hydrofluoroether (HFE)) using a laser pulse. The bubble collapses near the elastic material polydimethylsiloxane (PDMS). The experimental results obtained are compared with the corresponding simulation. The simulation provides spatial and temporal details about the bubble dynamics beyond experimental limits and can therefore be considered as a very useful tool to get a better understanding of the physics involved.

Observations of Translation and Jetting of Ultrasound-Activated Microbubbles in Mesenteric Microvessels

“…A bubble oscillating near a pressure-release boundary ( e.g ., an air-water interface) moves away from the boundary during collapse and can form a liquid jet directed away from the boundary (Chahine 1977; Robinson et al 2001). Near a compliant flat boundary, such as viscoelastic gel, the direction of bubble translation and jetting can be toward or away from the boundary, depending on the mechanical properties of the boundary, bubble size and distance from the boundary (Brujan et al 2001a; Brujan et al 2001b; Fong et al 2006; Gibson and Blake 1982; Kodama and Tomita 2000; Ohl et al 2009; Shima et al 1989). However, many of these previous studies focused on freely collapsing, millimeter-sized bubbles that are much larger than the microbubbles used in diagnostic and therapeutic ultrasound.…”

Ultrasonic bubbles in microfluidics for red blood cells, bacteria and yeast lysis

“…During growth bubble pushes the cell to 0 μm in frame 4. The rapid collapse of the bubble cau gated after the bubble has completely collapsed for so yborg (1962) To understand the mechanisms b is based on potential flow and the n refer to previous publications of the and Khoo (2004), Ohl et al (2009)) For the simulations, the pressure bubble to expand, accelerating the f with surface tension) to deform. Ini collapses, causing the lower portion …”

Modeling Microbubble Dynamics in Biomedical Applications

“…The interaction of a collapsing bubble with a nearby tissue is complex and has received a lot of attention by the research community because of the potential for tissue or vessel damage [19,24,25,33,56,57] . The tissue significantly deforms during the bubble dynamics and observations indicate potential occurrence of both poration [53] and invagination [19] depending on the experimental configuration.…”

“…The elastic modulus of bio-materials varies in a very wide range (see Ref. [57] ). Since the elasticity is a crucial parameter for this problem, we show the results for three elastic modulii: E = 10 kPa, 100 kPa and 1 000 kPa.…”