A new review revisits Reiter and de Gennes' 20-year-old theory of conformational memory in polymer films, and asks what it will take to turn preparation into a tool for designing programmable materials

When polymers are squeezed into extremely thin films, a rich interplay between their geometry, fluctuations and interfaces, can create novel behaviour, named a ‘confinement effect’. Yet some behaviour in these films can't be fully explained by confinement alone. In 2001, physicists Günter Reiter and Pierre-Gilles de Gennes proposed an alternative explanation: that a polymer's preparation stage could imprint the material with a memory of its previous states. But despite promising experimental progress in the time since, several fundamental questions remain about the effect.

Through a new review published in EPJ E, Sivasurender Chandran at the Indian Institute of Technology Kanpur revisits Reiter and de Gennes' original work, tracing key experimental advances since their theory emerged. His insights could help researchers answer long-standing questions about polymer films – including the possibility of programming their mechanical responses.

Wetting describes how liquids spread, recede, or form droplets on various substrates. While classical wetting theory was developed largely for passive liquids, the term “active wetting” has recently been used in diverse nonequilibrium settings, including biomolecular condensates, cell layers and aggregates, and suspensions of self-propelled particles. This growing use makes it timely to ask what, if anything, the term should mean across such different systems.

In a new Colloquium, published in EPJ E, Uwe Thiele (University of Münster, Germany), discusses a tentative classification of wetting phenomena distinguishing equilibrium wetting, where interfacial energies determine a final static state; relaxational wetting, where a system evolves toward such a state; driven wetting, where external forcing maintains motion or deformation; reactive wetting, where chemical or material changes modify the involved interfaces; and active wetting, where internal energy-consuming processes such as motility, growth, or active stresses affect wetting behavior.

Human breathing is governed by fluid mechanics across several regimes that span over a wide range of length and time scales: from turbulent airflow in the upper airways to slow interfacial motion in the smallest bronchioles and alveoli. At the same time, air motion is coupled to deformable tissue, mucus transport, surfactant dynamics, gas exchange, and, in disease, airway narrowing or liquid plugging. This makes the lung a demanding test case for modern multiphysics modelling.

A new editorial in EPJ E reflects on the journal’s role in uniting diverse soft-matter communities, and anticipates the challenges of maintaining interdisciplinary dialogue as the field expands.

Soft matter encompasses a diverse array of structures, including liquid crystals, polymers and biopolymers, and even living cells and tissues. While they are often complex, all of these materials display extremely strong responses to weak perturbations, including mechanical, chemical, and electrical influences. Increasingly, these properties are being explored across a vast array of applications, driving deeper questions about how the fascinating behaviour of soft materials is linked to their constituent molecular parts.

First founded in 2000, EPJ E: Soft Matter and Biological Physics has long been a cornerstone of soft matter research. In one of the first papers of the EPJ E 25th Anniversary Collection: Past Insights, Present Voices, Future Horizons, Jean‑François Joanny at Collège de France, together with Günter Reiter at Albert-Ludwigs-Universität Freiburg, reflect on its prolific past while anticipating the challenges and opportunities it will likely face in the future.

By accounting for long-range interactions between suspended particles, a new model provides a more accurate description of how soft athermal materials transition between reversible and irreversible states.

When soft athermal materials like foams, emulsions, or particle suspensions are gently shaken or sheared back and forth, they can learn to move in perfect rhythm: after each cycle, every particle returns to its original place. But if the driving becomes too strong, that tidy choreography breaks down, and particles wander irreversibly. This reversible–irreversible transition marks the boundary between an ordered and a chaotic state in driven soft matter. So far, however, researchers have struggled to recreate these properties through theoretical models – making it more difficult for them to understand how soft athermal materials behave in real-world applications.

Through new research published in EPJ E, a team led by CNRS researchers Romain Mari and Eric Bertin at Grenoble-Alpes University introduces a new and improved model, which reproduces the reversible–irreversible transition far more accurately. Their approach offers fresh insights into the deeply complex behaviours of soft athermal materials, and could help researchers to develop their application across a diverse range of real-world scenarios.

Experiment improves our understanding of how compressive stress is linked to changes in how cancer cells grow and move

While a tumour’s development is strongly tied to genetic factors, these are also intertwined with physical and chemical changes in its cells. Many of these changes are linked to compressive stress, which builds up inside a growing tumour as it pushes against surrounding tissues. However, researchers still have much to learn about how these interconnected processes respond to varying levels of stress.

Through a new experiment detailed in EPJ E Morgan Delarue and colleagues at the University of Toulouse have gained deeper insights into the impact of compressive stress on the division and movement of tumour cells. Their results could help improve our understanding of tumour development – potentially opening new routes to effective cancer treatments.

New analysis reveals that the motion of this unicellular amoeba is governed solely by cell–cell collisions, providing a useful model for isolating the effects of density on migration

Cell migration is vital to numerous biological processes. While often guided by long-range biochemical cues, it can also be influenced by direct physical collisions, which trigger biochemical signals within the affected cells. When studying this behaviour, it is important for researchers to consider how individual cell motions are affected by overall cell density. However, experiments have been complicated by the many interacting factors involved, making it difficult to isolate the specific impact of direct cell–cell collisions.

In new research published in EPJ E, a team led by Jean-Paul Rieu at Claude Bernard University Lyon 1 demonstrates how one unicellular amoeba species, Acanthamoeba castellanii (Ac), migrates in ways that are unaffected by long-range biochemical signalling – making it a promising model for studying how density influences cell migration. By using Ac, the researchers aim to gain deeper insights into the mechanisms of cell motion relevant to diverse biological processes, including immune responses, cancer cell invasion, and tissue development.

Guest Editors: Emanuela Bianchi, Jan K.G. Dhont, Gerhard Kahl, Kyongok Kang and Holger Stark.

The topical issue titled “Charged Species in Bulk and at Interfaces: Interaction, Mobility, Transport, and Regulation” is derived from a series of three CECAM workshops held in 2016, 2018, and 2022. The aim of this issue is to advance our understanding of the complex phenomena surrounding charged species in various environments from a fundamental perspective, with some excursions into practical applications. The issue addresses the characterization of the interactions between a broad variety of charged species as well as of the complex macroscopic behaviour arising from these interactions. Modelling pair interactions between highly charged macromolecules and surfaces resulting from (possibly mobile) charges presents significant challenges; equally challenging is understanding how these interactions govern self-assembly, assembly kinetics, transport in dense systems, response to external electric fields, and regulatory mechanisms. In particular, the transport of charged species, both in bulk and at interfaces, has garnered significant attention due to its fundamental and technological implications. The role of electrostatics and dynamics in biological systems – spanning a wide range of length scales has become a rapidly growing area of research.

This paper presents a ‘review of reviews’ of the rapidly growing and diversifying field of active matter research, providing a valuable overview of the field as a whole

Active matter is a term describing systems of particles which harvest the energy in their surrounding environment, usually to propel themselves forward. With examples including flocks of birds, bacteria colonies, and swimming microrobots, active matter can show deeply complex behaviours, which can be exploited for real-world applications.

In recent years, researchers have shown a rapidly growing interest in active matter – with over 1,000 reviews on the topic being published to date. Yet with this growing interest, the field has been divided into an increasingly diverse range of subfields, and it is now becoming increasingly difficult to gain an overview of the field as a whole.

Experiments reveal new insights into how suspended particles rearrange themselves when the flow direction of their suspending fluid is reversed

When particles are suspended in viscous fluids of the same density, their distribution becomes uneven as the fluid is subjected to straining flows. This affects the suspension’s viscosity, especially during flow reversals. Yet although this behaviour is well understood in steady, uniform flows, less is known about how the microstructures of fluid suspensions will evolve in more complex scenarios.

Through new experiments detailed in EPJ E, a team led by Irene Ippolito at the University of Buenos Aires and Georges Gauthier at the University of Paris-Saclay have uncovered new insights into this evolving structure when a suspension’s flow direction is periodically reversed within a narrow gap.