Autonomous motion and adaptability of microorganisms in fluids are hallmark features of living systems, fueling the emergence of active matter within the realm of soft matter physics. Notably, micron-sized synthetic self-propelled particles (SPPs) have emerged as a distinct class within this domain due to their unique ability to convert internal energy into directed motion, making them ideal models for studying inherently out-of-equilibrium systems. However, a significant challenge persists: the majority of existing synthetic SPPs are ill-suited for probing the governing principles of emergent collective behaviors observed in living systems, such as swarming, active turbulence, and living clusters, particularly in 3D real space. The ultimate aim of SynthAct3D is to pioneer a paradigm shift, transitioning synthetic active matter from the familiar territory of 2D experiments towards the uncharted terrain of 3D materials with advanced functionalities. This fundamentally driven, experiment-centric proposal seeks to unravel the core mechanisms behind emergent phenomena observed in living systems, employing entirely synthetic units. SynthAct3D focuses on two complementary goals, anchored in a novel experimental framework that combines innovative SPP designs, rigorous characterization, and high-speed confocal imaging: I. Phase behavior a. Investigate the influences of dimensionality (2D Vs 3D) and particle shape on the emergence of self-organization, both structurally and dynamically. b. Elucidate the role of particle propulsion and the microscopic nonequilibrium dynamics in dictating the macroscopic behavior of SPPs in 3D. II. Active materials a. Design reconfigurable 3D active materials (e.g. shape-shifting, and glasses) This research is expected to yield unprecedented insights into internally powered systems in 3D, paving the way for a new class of internally driven materials with applications in reconfigurable soft materials.

Lipid-coated microbubbles are fascinating objects rich in nonlinear dynamics. They are used in medicine as ultrasound contrast agents (UCAs) to visualize organ perfusion. The contrast enhancement results from their ultrasound-driven oscillations, which produce a powerful echo. The echo response is sensitive to ambient pressure and the microbubble surroundings so that bubbles have potential sensing capabilities that reach far beyond their current use as contrast agents. However, UCAs contain microbubbles non-uniform in size (1-10 μm diameter) and in shell properties. The resulting ill-defined echo inhibits game-changing applications such as non-invasive pressure sensing and molecular sensing using functionalized bubbles that bind to diseased cells. Microfluidics allows controlled formation of mono-sized bubbles. However, even the echo response of mono-sized bubbles is heterogeneous due to uncontrolled shell properties. I aim to go beyond size-control and enable the microfluidic formation of functional mono-acoustic bubbles with a tuned and predictable acoustic response. The challenge is to bridge the gaps between fluid dynamics, colloid and interface science, interface rheology, and acoustics to unravel the coupled problem of microfluidic bubble-shell formation and ultrasound-driven bubble dynamics in the bulk and near or targeted to a wall. To reach this goal, we will develop highly controlled lab experiments at the sub-microsecond and sub-micrometer level, together with simulations and theory development. The ultimate goal is a physics-based parametrization of the acoustic bubble response as a function of shell formulation, microfluidic control parameters, diffusive gas exchange effects, and targeted molecular binding of the bubble to a boundary.

Melting and dissolution induce temperature and concentration gradients in liquid systems. These gradients induce flows, namely buoyancy driven flows on large scales and phoretic flows on small scales. Such flows locally enhance or delay the melting or dissolution process and thus determine the objects’ shape. On large scales, a relevant example for the climate are glaciers and icebergs melting into the ocean, where cold and fresh meltwater experiences buoyant forces against the surrounding ocean water, leading to flow instabilities, thus shaping the ice and determining its melting rate. Another example is the dissolution of liquid CO2 in brine for CO2 sequestration. Next to buoyant forces also phoretic forces along the interfaces come into play. For dissolving drops at the microscale the phoretic forces become dominant. The resulting Marangoni flow not only affects their dissolution rate, but can also lead to their autochemotactic motion, deformation, or even splitting. In spite of the relevance for these and many other applications, such multicomponent, multiphase systems with melting or dissolution phase transitions are poorly understood, due to their complexity, multiway coupling, feedback mechanisms, memory effects & collective phenomena. The objective of this project is a true scientific breakthrough: We want to come to a quantitative understanding of melting & dissolution processes in multicomponent, multiphase systems, across all scales and on a fundamental level. To achieve this, we perform a number of key controlled experiments & numerical simulations for idealized setups on various length scales, inspired by above sketched problems, but allowing for a one-to-one comparison between experiments and numerics/theory. For the first time, we will perform local measurements of velocity, salt concentration, and temperature and connect them to global transport processes, to arrive at a fundamental understanding of such Stefan problems in multicomponent systems.