Physicists have created a new model explaining how filaments come together to form active foams.
LMU physicists have created a new model explaining how filaments come together to form active foams.
Many essential life processes, as well as their artificial equivalents in nanotechnology, rely on the self-driven assembly of particles into intricate designs. Professor Erwin Frey from LMU investigates the foundational principles of this self-assembly. Along with his team, he has developed a theoretical model that clarifies how patterns like active foams are formed from a blend of protein filaments and molecular motors. Their discoveries have been published in the journal Physical Review X.
Protein filaments, such as microtubules, and molecular motors are crucial elements of the cytoskeleton found in various cell types. A notable instance of how cellular structures are constructed and reorganized through the interaction between filaments and motors is the mitotic spindle, which is vital for accurate cell division. Research by a team at the University of California, Santa Barbara, using a simplified model, has demonstrated that various structures can arise from the dynamic interactions of microtubules and molecular motors. These structures include aster-like micelles and a new phase called active foam. The fundamental components of this foam consist of microtubule bilayers where the filaments are oriented in opposite directions. These bilayers then merge to create a network that undergoes continuous rearrangements.
“Active foam forms when there is an increase in the number of microtubules,” explains Filippo De Luca, the main author of the study. “Our goal was to uncover the physical mechanism that drives this.” In collaboration with his team, theoretical physicist Frey devised a mathematical model that elucidates the formation of these patterns: “Using numerical simulations, we successfully replicated the patterns we observed in experiments, including the transition from micelles to active foam as influenced by microtubule density,” Frey elaborates.
Ordered foam
The interaction between motors and microtubules plays a crucial role in forming these patterns. Without the motors, microtubules would resemble a chaotic pile of pick-up sticks, lacking the organized structure required for complex cellular arrangements. The motors connect microtubules in pairs and travel along the filaments, arranging them in parallel. “They effectively zip the filaments together as they move along,” Frey remarks. Through this process, the two filaments can slide against one another and be repeatedly reorganized, which is essential for foam formation.
The transition from micelles to foams is dependent on the quantities of motors and microtubules involved. When the component numbers are low, the particles enjoy significant freedom, allowing individual micelles to develop. “However, as the number of components rises, band-like layers form, leading to more intricate structures like foams,” Frey explains. “These foams exhibit an ordered formation with a combination of pentagons, hexagons, and heptagons, resembling honeycombs.” But unlike honeycombs, active foams continually change their arrangement.
This theoretical model generally applies to all forms of filaments and motors, providing fresh insights into active matter. According to the researchers, it may also pave the way for advancements in bionanotechnology in the future.
