On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part II.

Particle separation using surface acoustic waves (SAWs) has been a focus of ongoing research for several years, leading to promising technologies based on Lab-on-a-Chip devices. In many of them, scattering effects of acoustic waves on suspended particles are utilized to manipulate their motion by means of the acoustic radiation force (FARF). Due to viscous damping of radiated waves within a fluid, known as the acoustic streaming effect, a superimposed fluid flow is generated, which additionally affects the trajectories of the particles by drag forces. To evaluate the influence of this acoustically induced flow on the fractionation of suspended particles, the present study gives a deep insight into the pattern and scaling of the resulting vortex structures by quantitative three-dimensional, three component (3D3C) velocity measurements. Following the analysis of translationally invariant structures at the center of a pseudo-standing surface acoustic wave (sSAW) in Part I, the focus in Part II turns to the outer regions of acoustic actuation. The impact of key parameters on the formation of the outer vortices, such as the wavelength of the SAW λSAW, the channel height H and electrical power Pel, is investigated with respect to the design of corresponding separation systems. As a result of large gradients in the acoustic fields, broadly extended vortices are formed, which can cause a lateral displacement of particles and are thus essential for a holistic analysis of the flow phenomena. The interaction with an externally imposed main flow reveals local recirculation regions, while the extent of the vortices is quantified based on the displacement of the main flow.

On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part I.

By integrating surface acoustic waves (SAW) into microfluidic devices, microparticle systems can be fractionated precisely in flexible and easily scalable Lab-on-a-Chip platforms. The widely adopted driving mechanism behind this principle is the acoustic radiation force, which depends on the size and acoustic properties of the suspended particles. Superimposed fluid motion caused by the acoustic streaming effect can further manipulate particle trajectories and might have a negative influence on the fractionation result. A characterization of the crucial parameters that affect the pattern and scaling of the acoustically induced flow is thus essential for the design of acoustofluidic separation systems. For the first time, the fluid flow induced by pseudo-standing acoustic wave fields with a wavelength much smaller than the width of the confined microchannel is experimentally revealed in detail, using quantitative three-dimensional measurements of all three velocity components (3D3C). In Part I of this study, we focus on the fluid flow close to the center of the surface acoustic wave field, while in Part II the outer regions with strong acoustic gradients are investigated. By systematic variations of the SAW-wavelength λSAW and channel height H, a transition from vortex pairs extending over the entire channel width W to periodic flows resembling the pseudo-standing wave field is revealed. An adaptation of the electrical power, however, only affects the velocity scaling. Based on the experimental data, a validated numerical model was developed in which critical material parameters and boundary conditions were systematically adjusted. Considering a Navier slip length at the substrate-fluid interface, the simulations provide a strong agreement with the measured velocity data over a large frequency range and enable an energetic consideration of the first and second-order fields. Based on the results of this study, critical parameters were identified for the particle size as well as for channel height and width. Progress for the research on SAW-based separation systems is obtained not only by these findings but also by providing all experimental velocity data to allow for further developments on other sites.

[Learning with digital media in emergency medicine-a path through the jungle of possibilities]. / Lernen mit digitalen Medien in der Notfallmedizin ­ ein Pfad durch den Dschungel der Möglichkeiten.

Background: Digital media, such as podcasts, wikis, ePortfolios, and extended reality applications, provide excellent learning opportunities with a high degree of connectivity and flexibility for learners, as well as for learning facilitators. This not only enables location-independent and pandemic-resilient learning, but also a high degree of autonomy for the learners. The megatrend of digitalization opens up many possibilities, but there are also stumbling blocks and limitations. Objectives: This article is intended to provide readers in the emergency medicine/rescue field with an overview, various aspects to consider, and awareness of stumbling blocks. However, a balancing act between didactics and medicine as well as the heterogeneous group of addressees is necessary. Methods: By means of a narrative review, an assessment of digital media is made and subjected to an evaluation from the perspective of educational practice. Conclusion: Learning is not only changing due to new learning technologies, but also due to the growing importance of informal learning, the increasing significance of the ability to quickly access high-quality knowledge, faster-changing professional biographies, and the use of digital universal tools. Thus, in the jungle of possibilities, an estimation of scientific quality criteria is often difficult and a differentiated consideration is necessary. Basically, the question of appropriate methods must be asked and it must be critically questioned whether the learning/competency objectives can be achieved with the planned digitalized media. Digital media cannot and should not replace practical training in the workplace.