Eleonora Secchi

Last Name

Secchi

First Name

Eleonora

ORCID

0000-0002-0949-9085

Organisational unit

09467 - Stocker, Roman / Stocker, Roman

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Publications 1 - 10 of 29

Transport of Pseudomonas aeruginosa in Polymer Solutions
Savorana, Giovanni; Geisel, Steffen; Cen, Tianyu; et al. (2022)
Frontiers in Physics
Bacteria often live surrounded by polymer solutions, such as in animal respiratory, gastrointestinal, and reproductive tracts. In these systems, polymer solutions are often exposed to fluid flow, and their complex rheology can affect the transport of chemical compounds and microorganisms. Recent studies have focused on the effect of polymer solutions on the motility of bacteria in the absence of fluid flow. However, flow can be a game-changer on bacterial transport, as demonstrated by the depletion of motile bacteria from the low-shear regions and trapping in the high-shear regions in simple fluids, even for flows as simple as the Poiseuille one. Despite the relevance of polymer solutions in many bacterial habitats, the effect of their complex rheology on shear-induced trapping and bacterial transport in flow has remained unexplored. Using microfluidic experiments and numerical modeling, we studied how the shear rate and the rheological behavior of Newtonian and non-Newtonian polymer solutions affect the transport of motile, wild-type Pseudomonas aeruginosa in a Poiseuille flow. Our results show that, in Newtonian solutions, an increase in viscosity reduces bacterial depletion in the low-shear regions at the microchannel center, due to a reduction in the bacterial swimming velocity. Conversely, in the non-Newtonian solution, we observed a depletion comparable to the buffer case, despite its zero-shear viscosity being two orders of magnitude higher. In both cases, bacterial swimming and polymer fluid rheology control the magnitude of bacterial depletion and its shear-rate dependence. Our observations underscore the importance of the rheological behavior of the carrier fluid in controlling bacterial transport, in particular, close to surfaces giving rise to velocity gradients, with potential consequences on surface colonization and biofilm formation in many naturally relevant microbial habitats.
Sequential capillarity-assisted particle assembly in a microfluidic channel
Pioli, Roberto; Fernandez Rodriguez, Miguel Angel; Grillo, Fabio; et al. (2021)
Lab on a Chip
Colloidal patterning enables the placement of a wide range of materials into prescribed spatial arrangements, as required in a variety of applications, including micro- and nano-electronics, sensing, and plasmonics. Directed colloidal assembly methods, which exploit external forces to place particles with high yield and great accuracy, are particularly powerful. However, currently available techniques require specialized equipment, which limits their applicability. Here, we present a microfluidic platform to produce versatile colloidal patterns within a microchannel, based on sequential capillarity-assisted particle assembly (sCAPA). This new microfluidic technology exploits the capillary forces resulting from the controlled motion of an evaporating droplet inside a microfluidic channel to deposit individual particles in an array of traps microfabricated onto a substrate. Sequential depositions allow the generation of a desired spatial layout of colloidal particles of single or multiple types, dictated solely by the geometry of the traps and the filling sequence. We show that the platform can be used to create a variety of patterns and that the microfluidic channel easily allows surface functionalization of trapped particles. By enabling colloidal patterning to be carried out in a controlled environment, exploiting equipment routinely used in microfluidics, we demonstrate an easy-to-build platform that can be implemented in microfluidics labs.
Mapping the local viscosity of non-Newtonian fluids flowing through disordered porous structures
Eberhard, Ursin; Seybold, Hansjörg Florian; Secchi, Eleonora; et al. (2020)
Scientific Reports
Flow of non-Newtonian fluids through topologically complex structures is ubiquitous in most biological, industrial and environmental settings. The interplay between local hydrodynamics and the fluid’s constitutive law determines the distribution of flow paths. Consequently the spatial heterogeneity of the viscous resistance controls mass and solute transport from the micron to the meter scale. Examples range from oil recovery and groundwater engineering to drug delivery, filters and catalysts. Here we present a new methodology to map the spatial variation of the local viscosity of a non-Newtonian fluid flowing through a complex pore geometry. We use high resolution image velocimetry to determine local shear rates. Knowing the local shear rate in combination with a separate measurement of the fluid’s constitutive law allows to quantitatively map the local viscosity at the pore scale. Our experimental results—which closely match with three-dimensional numerical simulations—demonstrate that the exponential decay of the longitudinal velocity distributions, previously observed for Newtonian fluids, is a function of the spatial heterogeneity of the local viscosity. This work sheds light on the relationship between hydraulic properties and the viscosity at the pore scale, which is of fundamental importance for predicting transport properties, mixing, and chemical reactions in many porous systems.
In-flow measurement of molecular diffusion coefficients using differential dynamic microscopy
Eberhard, Ursin; Usuelli, Mattia; Secchi, Eleonora; et al. (2025)
Physical Review E
Understanding diffusive mass transport in fluid flows is fundamental in several fields, ranging from virus particles in chromatographic test kits to nutrients in ocean currents. However, experimentally resolving diffusion processes in flowing fluids is challenging due to local sample deformation caused by advective displacements under varying shear rates. Here we demonstrate that differential dynamic microscopy, combined with image velocimetry, provides a robust method to quantitatively measure molecular diffusion coefficients across a laminar flow profile within a rectangular microfluidic channel. By using subresolution tracer particles homogeneously distributed along the flow profile, we capture the effects of differing shear rates across the flow profile and measure the local diffusion coefficient by scanning through different focal planes with a microscope. While demonstrated with water, this approach can be easily adapted to more complex fluids with shear-dependent viscosity, offering broad applicability for studying diffusion in dynamic environments.
The effect of flow on swimming bacteria controls the initial colonization of curved surfaces
Secchi, Eleonora; Vitale, Alessandra; Miño, Gastón L.; et al. (2020)
Nature Communications
The colonization of surfaces by bacteria is a widespread phenomenon with consequences on environmental processes and human health. While much is known about the molecular mechanisms of surface colonization, the influence of the physical environment remains poorly understood. Here we show that the colonization of non-planar surfaces by motile bacteria is largely controlled by flow. Using microfluidic experiments with Pseudomonas aeruginosa and Escherichia coli, we demonstrate that the velocity gradients created by a curved surface drive preferential attachment to specific regions of the collecting surface, namely the leeward side of cylinders and immediately downstream of apexes on corrugated surfaces, in stark contrast to where nonmotile cells attach. Attachment location and rate depend on the local hydrodynamics and, as revealed by a mathematical model benchmarked on the observations, on cell morphology and swimming traits. These results highlight the importance of flow on the magnitude and location of bacterial colonization of surfaces.
The role of surface adhesion on the macroscopic wrinkling of biofilms
Geisel, Steffen; Secchi, Eleonora; Vermant, Jan (2022)
eLife
Biofilms, bacterial communities of cells encased by a self-produced matrix, exhibit a variety of three-dimensional structures. Specifically, channel networks formed within the bulk of the biofilm have been identified to play an important role in the colonies' viability by promoting the transport of nutrients and chemicals. Here, we study channel formation and focus on the role of the adhesion of the biofilm matrix to the substrate in Pseudomonas aeruginosa biofilms grown under constant flow in microfluidic channels. We perform phase contrast and confocal laser scanning microscopy to examine the development of the biofilm structure as a function of the substrates' surface energy. The formation of the wrinkles and folds is triggered by a mechanical buckling instability, controlled by biofilm growth rate and the film's adhesion to the substrate. The three-dimensional folding gives rise to hollow channels that rapidly increase the effective volume occupied by the biofilm and facilitate bacterial movement inside them. The experiments and analysis on mechanical instabilities for the relevant case of a bacterial biofilm grown during flow enable us to predict and control the biofilm morphology.
Localization in Flow of Non-Newtonian Fluids Through Disordered Porous Media
Seybold, Hansjörg Florian; Eberhard, U.; Secchi, Eleonora; et al. (2021)
Frontiers in Physics
We combine results of high-resolution microfluidic experiments with extensive numerical simulations to show how the flow patterns inside a “swiss-cheese” type of pore geometry can be systematically controlled through the intrinsic rheological properties of the fluid. Precisely, our analysis reveals that the velocity field in the interstitial pore space tends to display enhanced channeling under certain flow conditions. This observed flow “localization”, quantified by the spatial distribution of kinetic energy, can then be explained in terms of the strong interplay between the disordered geometry of the pore space and the nonlinear rheology of the fluid. Our results disclose the possibility that the constitutive properties of the fluid can enhance the performance of chemical reactors and chromatographic devices through control of the channeling patterns inside disordered porous media.
The influence of bodily fluid rheology on biofilm formation: Known facts and open questions
Savorana, Giovanni; Di Claudio, Alessia; Rusconi, Roberto; et al. (2025)
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Biofilms - microbial communities encased in a self-produced extracellular matrix - pose a significant challenge in clinical settings due to their association with chronic infections and antibiotic resistance. Their formation in the human body is governed by a complex interplay of biological and environmental factors, including the biochemical composition of bodily fluids, fluid dynamics, and cell-cell and cell-surface interactions. Improving therapeutic strategies requires a deeper understanding of how host-specific conditions shape biofilm development. Despite efforts to replicate in vivo conditions, in vitro models are often insufficient for capturing the complex dynamics of biofilm formation within the host. This limitation hinders the translation of experimental findings into clinical applications and slows the development of targeted therapies. In this review, we examine the role of fluid dynamics within the human body, with a particular focus on how the biochemical composition, rheological properties of bodily fluids and local flow conditions influence surface colonization and biofilm formation. We survey the current state of the field and outline key open challenges, intending to inform and inspire the development of next-generation experimental models that more closely reflect physiological reality.
Stress-hardening behaviour of biofilm streamers
Savorana, Giovanni; Redaelli, Tommaso; Truzzolillo, Domenico; et al. (2025)
Nature Communications
Bacteria’s ability to withstand mechanical challenges is enhanced in their biofilm lifestyle, where they are encased in a viscoelastic polymer matrix. Under fluid flow, biofilms can form as streamers – slender filaments tethered to solid surfaces and suspended in the flowing fluid. Streamers thrive in environments subjected to intense hydrodynamic stresses, such as medical devices and water filters, often resulting in catastrophic clogging. Their colonisation success may depend on a highly adaptable mechanical response to varying stress conditions, though the evidence and underlying mechanisms of this adaptation remain elusive. Here, we demonstrate that biofilm streamers exhibit a stress-hardening behaviour, with both differential elastic modulus and effective viscosity increasing linearly with external stress. This stress-hardening is consistent across biofilms with different matrix compositions, formed by various bacterial species, and under diverse growth conditions. We further demonstrate that this mechanical response originates from the properties of extracellular DNA (eDNA) molecules, which constitute the structural backbone of the streamers. In addition, our results identify extracellular RNA (eRNA) as a modulator of the matrix network, contributing to both the structure and rheological properties of the eDNA backbone. Our findings reveal an instantaneous, purely physical mechanism enabling streamers to adapt to hydrodynamic stresses. Given the ubiquity of extracellular nucleic acids (eNA) in biofilms, this discovery prompts a re-evaluation of their functional role in biofilm mechanics, with potential implications for biofilm structural integrity, ecological resilience, and colonisation dynamics.
A microfluidic platform for characterizing the structure and rheology of biofilm streamers
Savorana, Giovanni; Słomka, Jonasz; Stocker, Roman; et al. (2022)
Soft Matter
Biofilm formation is the most successful survival strategy for bacterial communities. In the biofilm lifestyle, bacteria embed themselves in a self-secreted matrix of extracellular polymeric substances (EPS), which acts as a shield against mechanical and chemical insults. When ambient flow is present, this viscoelastic scaffold can take a streamlined shape, forming biofilm filaments suspended in flow, called streamers. Streamers significantly disrupt the fluid flow by causing rapid clogging and affect transport in aquatic environments. Despite their relevance, the structural and rheological characterization of biofilm streamers is still at an early stage. In this work, we present a microfluidic platform that allows the reproducible growth of biofilm streamers in controlled physico-chemical conditions and the characterization of their biochemical composition, morphology, and rheology in situ. We employed isolated micropillars as nucleation sites for the growth of single biofilm streamers under the continuous flow of a diluted bacterial suspension. By combining fluorescent staining of the EPS components and epifluorescence microscopy, we were able to characterize the biochemical composition and morphology of the streamers. Additionally, we optimized a protocol to perform hydrodynamic stress tests in situ, by inducing controlled variations of the fluid shear stress exerted on the streamers by the flow. Thus, the reproducibility of the formation process and the testing protocol make it possible to perform several consistent experimental replicates that provide statistically significant information. By allowing the systematic investigation of the role of biochemical composition on the structure and rheology of streamers, this platform will advance our understanding of biofilm formation.

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Eleonora Secchi

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