Soft Matter
Soft Matter is an immense field of research that encompasses from Polumers and Colloids to liquid crystals, amphiphilic systems and biological systems. The following pentagon scheme shows some of the many areas of research inside this field
If you are interested in learning more about what is Soft Matter, please visit our dedicated page xx FALTAxxx.
In the ONL group we do research in the following areas of Soft Matter
THE ENDOTHELIAL GLYCOCALYX AND ITS IMPORTANCE
Behind these two words lies an organ whose existence was confirmed in mammals just before humans landed on the moon. This organ, in an adult human, weighs as much as the brain—approximately 1.4 kilograms—and if it were fully unfolded, it would easily cover more than three basketball courts. What differentiates it from other organs is that it is not located in any specific part of the body; on the contrary, it is found everywhere, in direct contact with the blood, resembling a soft velvet layer that internally covers all the arteries and veins of the body, from the largest ones to the smallest microcapillaries.
The thickness of this velvet covering we all have inside ranges between ten-thousandths and one-thousandth of a millimeter. But do not be deceived by its size: although it may seem like a minor thing not worthy of being considered a vital organ, the endothelial glycocalyx performs several critically important functions: it acts as a selective barrier, only allowing certain molecules to pass from the blood into the rest of the body, and protects us against fluid loss (edemas); it serves as a lubricating layer for transporting red blood cells, which is particularly important in the case of microcapillaries, where their openings can be smaller than the red blood cell itself; it prevents erosion of the walls of veins and arteries, and also greatly reduces the chances of other particles flowing through the blood adhering to the walls and causing clots and obstructions; furthermore, by capturing certain molecules, it controls the onset of thrombosis, inflammation, and oxidative stress. Another essential function of the glycocalyx, through the forces exerted by the blood upon it, is to send information to the cells forming the walls of the blood vessels (endothelium), so they modify their shape, size, and other properties, ensuring optimal blood flow in every moment and situation. Moreover, the glycocalyx also regulates the growth and migration of these endothelial cells throughout the body.
The vital role of the glycocalyx becomes evident when this covering disappears partially or totally: atherosclerosis quickly begins, and atheromatous plaques soon block the blood flow. Its loss has also been linked to strokes, hypertension, preeclampsia, and severe bacterial infections: some bacteria produce toxins that damage the glycocalyx as a strategy to penetrate every nook and cranny of the human body. Recent research in 2019 found that if the glycocalyx is damaged when a person contracts malaria, the chances of survival drop drastically. The glycocalyx also plays a major role in the growth and migration of tumor cells (metastasis) according to recent studies. Furthermore, strong evidence suggests that many of the complications that arise over time with diabetes (eye lesions that can lead to blindness, kidney damage, nerve damage, and small vessels that can lead to diabetic foot and gangrene) come from the fact that the disease significantly deteriorates the glycocalyx of the microcapillaries.
Thus, the glycocalyx has become an important therapeutic target in research aimed at curing or alleviating complications of certain diseases that afflict humanity. Despite the growing interest, the great challenge, fifty-five years after the discovery of the glycocalyx in mammals, is that, in many respects, the glycocalyx is still a great unknown. The initial neglect of its importance, its fragility, small size, and the difficulty of observing it in action in live studies have contributed to significant gaps in knowledge about its function, the mechanisms associated with its malfunction, and how these cause disruptions throughout the body. These gaps mean that medical advances are more the result of a slow learning process through trial and error than driven by a fundamental understanding of this complex organ. Undoubtedly, a better understanding of how the complex glycocalyx works would significantly accelerate medical advances.
OUR RESEARCH
At the UIB, in our ONL research group, we have set out to
Contribute to improving the understanding of the glycocalyx and related diseases. Given the difficulty of studying and drawing conclusions from live studies, we have decided to address the problem using numerical simulations that model in great detail how the glycocalyx behaves when subjected to the flow of a fluid similar to blood. To achieve this detailed “in silico” model of the glycocalyx, the first task was to devise and build new algorithms that allow us to simulate such large and complex systems in a reasonable amount of time, with all the necessary detail to obtain reliable and quantitative results. This has been the first task completed in our “GLYCOSIM” project: we have developed the necessary computational tools, which will soon be available to other interested groups, and with these new tools, we are now investigating in detail two basic phenomena that are still poorly understood: how the glycocalyx modifies the properties of the fluids and red blood cells circulating inside microcapillaries, and the role of the glycocalyx in the initiation of obstructive deposits within microcapillaries. These are just the first steps towards fully understanding the glycocalyx, but they will undoubtedly bring us closer to the ultimate goal.
Through our “LEGOCITS” project we aim to use what we have learned from the glycocalyx to answer several key questions, from both basic science and applied science, in orde to be able to design optimized artificial coatings.
The ONL group thanks the funding provided via the following Spanish national projects:
PID2020-118317GB-I00 funded by MICIU/AEI /10.13039/501100011033
DPI2017-86610-P funded by MINECO/AEI/FEDER,UE.
Comming soon
What are magnetic filaments?
Artificial magnetic filaments can be obtained by mutually linking magnetic colloids to form a chain. These magnetic chains represent the equivalent to magnetic polymers but at supra- molecular scale. In difference to one-dimensional chemical magnetic polymers which only manifest their magnetic properties at temperatures below 100K, magnetic filaments can retain their magnetism at room temperature and zero field.
So far, we have focused on the equilibrium conformations of flexible and semiflexible magnetic filaments in different physical environments of relevance for forthcoming applications. In particular, we focus on the determination of the phase diagram at zero field for magnetic filaments which monomers exhibit short-range LJ attractive interactions (Stockmayer polymers, i.e. filaments in poor solvent conditions) in the limit of strong dilution, as well as filaments in good solvent conditions. We study the cases of magnetic chains in bulk and near an attractive surface. We find that the phase diagrams of magnetic systems exhibit a rich variety of new phases when compared with non-magnetic chains in similar environments.
The emerging interest in this relatively novel field is due to the fact that magnetic filaments are very appealing from the technological point of view. They can be thought as improved substitutes of current ferrofluids, or as elements for magnetic memories, chemical and pressure nanosensors, micro-propellers, non-permanent photonic crystals, and generation of unique patterns able to provide watermarks to authenticate cards or other documents, to just mention a few.
For a short presentation about magnetic filaments click here
What are Ferrofluids?
Dipolar magnetic fluids (also known as ferrofluids or ferrocolloids), are colloidal suspensions of ferromagnetic nanoparticles (typical sizes 10-20nm), usually stabilised by steric coatings (in non electrolyte carrier liquids) or by electrical double layers (in aqueous solutions). Sterical coatings are usualy made of a stabilizing dispersing agent (surfactant) which prevents particle agglomeration even when a strong magnetic field gradient is applied to the ferrofluid. The surfactant must be matched to the carrier type and must overcome the attractive van der Waals and magnetic forces between the particles. A typical ferrofluid may contain by volume 5% magnetic solid, 10% surfactant and 85% carrier.
Due to their size, ferrofluid particles can be considered as magnetic single-domains with a permanent magnetic moment proportional to their volume.
To know more about the basics of ferrofluids, see for instance the ferrofluids in the Wikipedia.
Synthesis of ferrofluids
The idea of making a liquid with magnetic properties seems to date back to the 1779 when Gowan Knight tried to make a magnetic fluid by introducing iron filings into water. Unfortunately, the iron filings sedimented very quickly. In the XX century the issue was retaken by F. Bitter (1932), and W.C.Elmore (1938), but the particles they obtained were quite large and not fully stable. It seems that Stephen Papell Solomon (US Patent 3215572 ) working for NASA in the 60s was the first to develop an easy and effective way of preparing such colloidal systems. The idea was to be able to use these particles to control the flux of fuel in a zero gravity environment. Since then, many synthesis have been developed to produce ferrofluid particles in both non-polar and polar solvents, increase their stability, improve the control over their size and degree of polidispersity, and produce particles with different materials and shapes. For recent reviews about the subject see (Lu2007) and (Tourinho1998).
You can also prepare your own ferrofluids and have fun at home! See for instance the educational article of Berger et al. (Berger1999), as well as (1), (2), (3), (4), (5), (6).
Why are Ferrofluids interesting?
Ferrofluids are interesting for a two fold reason:
Ferrofluids are systems exhibing anisotropic interactions leading to a very reach phase behaviour, as well as very interesting rheological and magnetic properties. Thus ferrofluids are a paradigma of physical systems with anisotropic interactions.
The singular properties of ferrofluids in external magnetic fields have found application in many areas, ranging from engineering, to biomedical applications (Popplewell1984,Rosensweig1985, Odenbach2002, Alexiou2003,Hilger2004, Scherer2005). Just to mention a few examples:
Magnetic Seals for instance in vaccum pumps and ferrofluid bearings.
Car dampers
nanoactuators
loudspeakers
adaptive optics (liquid mirrors)
biologically inspired robots.
Cancer treatment, cell separation, ultrasensitive analysis, MRI.
Art.
See also as an example the links (1), (2) …
Aggregating structures formed by Ferrofluids
Even in the absence of external fields, ferrofluids have a very complex microstructure, which is caused by the combination of interparticle interactions specific to magnetic fluids (see for instance figures 1 and 2). Ferrofluid particles are known to self-assemble into a variety of magnetic equilibrium structures which depend on several factors such as: system geometry, magnetic interactions, particle polydispersity, presence or absence of external fields, etc.
The number of probable cluster topologies in magnetic fluids is high: drops with high magnetic phase concentration (micron size), branched and fractal clusters (hundreds of nanometers in size), or chain- and ring-like structures (tens of nanometers). Signs of clustering process in ferrofluids at zero field are known for more than 40 years (Hess1966). Nonetheless, despite the large amount of clues obtained in subsequent studies (Shen2001, Donselaar1999, Cebula1983, Gazeau2002), a direct experimental proof of the existence of clusters like chains, rings, etc. has been elusive for many years, specially for those cases were dipole-dipole interactions were relatively week, like in magnetite nanoparticles (Fe3O4). Thus, due to the lack of conclusive experiments, the understanding of how dipole-dipole interactions influence the clustering process and determine the subsequent microstructure and phase behaviour of ferrofluids has become a challenge. In bulk ferrofluids, those aspects have been studied in detail through theoretical (Gennes1970, Jordan1973, Osipov1996, Zubarev1995, Tavares1997, Tavares1999, Roij1996, Tlusty2000, Morozov2002, Mendelev2004, Ivanov2004) and simulation (Weis1993, Levesque1994, Jund1995, Camp2000, Pshenichnikov2000, Wang2002, Wang2003, Holm2006) works. Comprehensive reviews on these subject for bulk systems are also available, see (Teixeira2000, Cabuil2000, Huke2004, Holm2005).
Ferrofluid Monolayers
The phase behaviour and microstructure of ferrofluid systems in reduced dimensions is not necessarily equivalent to that of 3D systems. In addition, thin-films and monolayers, have become recently a more successful experimental scenario to assert the existence of the clustering process (Klokkenburg2006, Butter2003, Puntes2001, Wen1999). In the experiments of Philipse and co-workers(Butter2003, Klokkenburg2006), images obtained by cryogenic transmission electron microscopy (cyro-TEM) give ample evidence of the existence of chain- and ring-like structures in ferrofluid monolayers, where all particles are trapped in one plane, but their magnetic moments are free to fluctuate in 3D (q2D monolayers).
In recent years, several theoretical and computational works have been devoted to the study of ferrofluids in monolayers and thin-films. The thermodynamics, and magnetisation properties of quasi-two-dimensional (q2D) systems have been studied by Lomba et al. (Lomba2000), and Gao et al. (Gao1997). Weis and co-workers (see (Weis2002b, Weis2003), and references therein), have performed Monte Carlo simulations of monolayers and systems of finite thickness involving dipolar interactions. They have shown that q2D dipolar systems, alone or in combination with other interactions, present a rich variety of structures, phases and phase transitions. In a recent q2D Monte Carlo study on dipolar hard spheres (DHS), Tavares et al. (Tavares2006) have reported the structure of the fluid to be well described by an ideal mixture of self-assembling clusters at low and intermediate densities. Very recently, Duncan-Camp (Duncan2006) have studied the kinetics of aggregation in monolayers using stochastic dynamics simulations. The results obtained in that work suggest that the conditions for defect-driven condensation (Tlusty2000) could be met by kinetic trapping, giving rise to a metastable phase transition between isotropic fluid phases. The structure formation and magnetic properties of polydisperse ferrofluids in monolayers have been also recently addressed by theory and Monte Carlo simulations (Morimoto2003, Aoshima2004, Kristof2005).
Current Research
Despite the progress obtained in previous studies, the understanding of the phase behaviour and microstructure formation of ferrofluids in constrained geometries is only partial. The understanding of such features is of paramount importance in order to know better those systems and reduce the large amount of times devoted to trial-error in order to improve or design new applications based on ferrofluids.
Our current interest focuses on the peculiarities of the aggregation processes in both quasi-two dimensional (monolayers) and bulk ferrofluid systems. We make use of a combination of density functional theory, and molecular dynamics (MD) simulations. The microstructure formation and phase behaviour are studied thoroughly through a comparison of the theoretical, and computational results. The active areas of research about ferrofluids in our group at this moment are:
Ferrofluid monolayers: monodisperse particles.
Ferrofluid monolayers: bidisperse particles.
Structure factors of ferrofluids in the low-density low-aggregating limit.
Collaborators
Prof. Dr. Sofia Kantorovich (Vienna University)
Group of Prof. Dr. Alexey Ivanov at the (Department of Mathematical Physics, The Urals State University, Ekaterinburg, Russia.)
Prof. Dr. Christian Holm (Stuttgart University, ICP)
What is a Polyelectolyte Multilayer (PEM)?
Self-assembly processes of charged polymers known as polyelectrolytes (PE1, PE2 ) can be used to build-up multilayered materials (PEM) with unique properties. In the early 90’s Decher et al [1] demonstrated the feasibility of the self-assembly of polyelectrolyte multilayers (PEMs) using the so-called Layer-by-Layer (LbL) technique(animation,schematic plot). If you want to know more about PEMs generalities you can for instance visiti PEMs web page at Florida State University. Why Polyelectrolyte multilayers are interesting?
The versatility of the LbL process has allowed the fabrication of thin multilayer films made of synthetic polyelectrolytes, DNA, lipids and proteins, which has resulted in a boost of novel applications in recent years. For instance, PEMs are used as matrix materials for enzymes and proteins in sensor applications [2], and also as a matrix for active components in solar cells. PEMs are used as a coating for protecting and control the healing process of damaged arteries [3]. In addition PEM’s can be used as permeable membranes for nanofiltration [4], gas separation, and fuel cells. Furthermore, PEM’s are also used in the fabrication of non-linear optical materials [5], coloured electrochromic electrodes (future display devices), and to tailor the properties of photonic crystalls [6]. Other uses of PEM’s include analyte separation processes (chromatography) [7], and the fabrication of thin-walled hollow micro- and nanocapsules (see [8], and ref. therein). These capsules have great potential for drug carrier and nanoreactors.
Status of the PEMs research at a glance
Since the pionering work of Decher et al in the 90s, many scientists have been studying and characterizing the properties of Polyelectrolyte Multilayers. The research done about PEMs has been summarized in a few reviews [9-13]. But, just to mention a few relevant contributions to the field of PEM´s:
PEM experimental studies.
PEM theoretical studies.
Numerical simulations about PEMs.
Nonetheless, despite the amount of work done during the last 15 years, the understanding of the multilayer formation process and the knowledge about how slight differences during the growth process are able to strongly modify the properties of the multilayer materials is still in its infancy. The complex nature of PEMs possesses a challenge when one tries to choose a PEM system for a particular application. Therefore, one must first try to learn more about the fundamental properties of PEMs before it is possible to understand how to use these films for specific applications without a large and exhausting process of trial and error. Doubtless, the understanding of such issues is of paramount importance to improve current building-up methods and devices, tune finely the properties of such materials for specific purposes, and in turn devise new potential applications for such materials. Such knowledge will not be only of benefit for the Scientific Community but also for industry as well as society due to the huge potentiality of such materials for new devices and applications.
Our Research
Our current reserach on Polyelectrolyte Multilayers (PEMs) is aimed to help to shed light on some still not clearly understood aspects governing multilayer formation and the control of their properties. At this stage, numerical simulations that use the state-of-the-art algorithms to deal with charged soft matter offer a very valuable and useful tool in order to elucidate the mechanisms governing multilayering assembly and the properties of PEMs. These numerical simulations can build a bridge between the detailed experimental results and the relatively coarse grained analytical models. Our current aims in the area of PEM research are:
Clarify which factors contribute to stabilize multilayer films with special reference to weak polyelectrolytes.
Explain the mechanisms and the causes that induce the formation of exponential growing films instead of linear films.
Study how the stability and the properties of PEMs, as well as the kinetics of both linear and exponential buildup regimes as a function of the several factors which have been observed to be of relevance in experiments.
Study of hollow spherical PEM nanocapsules as drug carriers and chemical nanoreactors.
Refine current electrostatic methods in order to allow faster and more detailed simulations of large PEM systems.
Determine, in close collaboration with simultaneous experimental investigations (Specially groups of von Klitzing, T. Hugel, Helm), the inner structure and dynamics of a well defined, but small number of multilayer polymers on various substrates.
More specifically, we are currently studying the inner structure and dynamics of a small layer number of PEMs via all-atom (AA) and coarse-grained (CG) simulations. The AA level simulations have proved to be consistent with existing experimental data on chain conformation of adsorbed poly(styrene sulfonate) (PSS) in PSS monolayer systems, dielectric permittivity and diffusion constant of water in PSS/PDADMA polyelectrolyte complexes (PDADMA stand for poly(diallyldimethylammonium)).
We have built the PSS/PDADMA bilayers based on the previously obtained PSS monolayers and we are expecting to extract exciting information from these studies. The simulation of a bilayer represents our final goal for atomistic simulations of PMEs so far, due to the high requirement of computer resources (ca. 1000 000 cpu hours in total).
Due to the limitations of atomistic simulations, further insight into structure and dynamics of PEMs can be achieved only with simulations at CG level. Qualitative understanding and agreement with experiments has been obtained by us using the already existing generic bead-spring PE model. However, a refined CG PE model is needed in order to be quantitatively predictive. This is part of our next working program.
Some selected results obtained during these last years are:
- Bilayer thermodynamical instability
Our CG level simulations have shown that depending on the relative strength of the monomer-monomer and monomer-surface interaction energies, a progressive redissolution of the first bilayer or a partial dewetting resulting in a disordered melt can happen.
We have shown that a fast enough deposition of the third layer – before the aging process – can prevent such redissolution or partial dewetting and provide the stability needed to form a PEM. We have checked that the deposition of further layers is a stable process. This suggests that the first PE bilayer is not thermodynamically stable, while tri-layers and higher layers are stable, at least within the long run time of our simulations.
- Charge compensation mechanism
In our AA level simulations on PSS/PDADMA complexes, intrinsic (polyanions pair with polycations) and extrinsic (polyions pair with salt ions) charge compensation mechanisms have been found to co-exist, although the intrinsic one is predominant in the studied salt (NaCl) concentration range from 0.17 to 1.00 mol/L.
Furthermore, the relative scale of the interaction energy of the ion-pairs in such PSS/PDADMA mixture is calculated to follow (in kJ/mol): Na-Cl (-520) > PSS-Na (-420) > PDADMA-Cl (-280) ~ PSS-PDADMA (-270). The relative scale of the interaction energy can be very useful to explain some experimental finding, such as PSS is found to be in a higher concentration than PDADMA in PSS/ PDADMA complexes [5]. This information is also valuable to properly model the interactions between ion-pairs in the upcoming refined CG model.
- PSS adsorption monolayer
The PSS monolayer is diposited from a PSS solution via atomistic simulations. Our results demonstrate that short-range interactions of van der Waals origin from the adsorbing substrate play a significant role in the layer structure of the adsorbed PSS, and they alone are already sufficient to induce a stable PSS adsorption layer. The PSS chains are found to behave as hydrophilic PEs, two kinds of conformations of which are observed: flat PSS adsorption layer dominates with some adsorbed PSS chains dangling into the above PSS solution.
- PE chain pulling experiment
The present, non-refined CG model yields a qualitative agreement with the experiments by the Hugel group. This makes us confident that maybe even a quantitative comparison might be obtainable once the refined coarse-grained model will be ready.
In the computer simulations on PE pulling experiments, a PE chain, which is similar to the PE chains of the capping layer, is introduced with the corresponding counterions. The averaged force, that is needed to keep one of the chain ends fixed at a given point Z_{tip}, is measured by performing several independent runs. The position of the chain tip is slowly increased to a new value where a new measurement was performed.
Collaborators
Prof. Dr. Christian Holm (Stuttgart University, ICP)
Baofu Qiao
Long Range Interactions & the root of the problem
Formally a potential is defined to be short ranged if it decreases with distance r quicker or similar than where d is the dimensionality of the system. Electrostatic, gravitatory and dipolar interactions, present in many physical systems, are examples of long range interactions. When long range intgeractions are present in a system, the weight of the interactions comming from far particles is non negligible. This is due to the type of decay of the interaction with the distance: despite the particle-particle interaction decreases with the distance, the number of interactions increases in such way that the total contribution of the far particles may have a weight as large as the one due to the interaction of neighbouring particles.
The limited power of current computers makes impossible simulate macroscopic bulky systems. Small systems have a large surface vs volume ratio and therefore surface effects may govern the physics of the system. When long-range forces are present, the scenario to mimic bulky systems is even worse because we will neglect a substantial part of the long-range interaction.
Then, why we don’t wait a little bit until computers become more powerful? Even if Moore´s law was able to hold on indefinitely, we would still need around two centuries to be able to tackle with systems of the size of about one cubic centimeter. Therefore, it is clear that we need to do some sort of approach in order to mimic bulky systems right now. How to mimic bulky systems with long range interactions
The straight cut-off (sometimes including a shift) of the long-range interactions have been observed to lead to many unphysical artifacts in the simulations of bulky systems. Although no perfect solution has been found, there exist some approaches to tackle with the problem:
Reaction Field Methods.
Periodic Boundary Conditions (artificial periodicity): Lattice-Sum Methods
Hybrids of the previous two approaches, eg. LSREF (Heinz2005).
MEMD – Maxwell Equations Molecular Dynamics (see ref.2) Our Research: Periodic Boundary Conditions
Frequently, periodic boundary conditions are the chosen approach. When periodic boundary conditions are used, an artificial periodicity is introduced in order to emulate the bulky system. The cell system is replicated and the interactions between the particles in the main cell and the particles located in the replica cells is taken into account and added to the interactions between particles of the main cell. For this reason, this kind of methods are also known as Lattice Sum Methods. When one performs this kind of sums by brute force, the method is known as Direct Sum.
Despite it seems very easy to perform a Direct Sum, it is in fact very tricky because this kind of sums have a conditional and very slow convergence, which implies that many terms must be included to obtain a reasonable accuracy for the value of the interactions.
Collaborators
Dr. Vincent Ballenegger, CNRS, Institut UTINAM, Besancon, France