We report an experimental investigation on the formation of liquid-core capsules having a thin hydrogel elastic membrane, named ‘liquid pearls’. These fish-egg-like structures are initially made of a millimetric liquid drop, aqueous or not, coated with an aqueous liquid film containing sodium alginate that gels once the double drop enters a calcium chloride bath. The creation of such pearls with a micrometer-thick membrane requires the suppression of mixing until gelling takes place. Here, we show that superimposing a two-dimensional surfactant precipitation at the interface confers a transient rigidity that can damp the shear-induced instability at impact. Based on this, pearls containing almost any type of liquid can be created. This opens the possibility to use such structures as a new tool for screening microorganism survival and growth in various three-dimensional environments.

The permeability of solids has long been associated with a diffusive process involving activated mechanism as originally envisioned by Eyring. Tensile stress can affect the activation energy but definitive experiments of the diffusion rate of species through a stressed solid are lacking. Here we use core-shell (liquid core–solid shell) colloidal particles that are sensitive to osmotic pressure to follow the permeation of encapsulated probes at various stresses. We unambiguously show that the tensile stress applied on colloidal shells linearly reduces the local energy barrier for diffusion.

We study the thermal expansion of chains formed by self-assembly of magnetic colloidal particles in a magnetic field. Using video microscopy, complete positional data of all the particles of the chains is obtained. By changing the ionic strength of the solution and the applied magnetic field, the interaction potential can be tuned. We analyze the thermal expansion of the chain using a simple model of a one-dimensional anharmonic crystal of finite size.

Biotechnological applications of emulsions, such as micro-reactors or drug carriers, demand accurate characterization techniques, able to measure the size and biochemical content of the droplets at the individual level. Since no available characterization technique completely fulfills these needs, we extended the use of flow cytometry, which was originally developed for cell studies, to the straightforward and quantitative characterization of micron-sized emulsions. Our method determines the size of soybean oil droplets from flow cytometric measurements of forward scattering and side scattering intensities combined with the theoretical scattered intensities exactly derived from Mie theory and numerically integrated with respect to the optical setup of the instrument. We evaluate the accuracy of our method by comparing the size distribution obtained for a monodisperse emulsion sample to the corresponding distribution measured with a commercial instrument. Applied to emulsion droplets functionalized with fluorescent streptavidin, our method allows for monitoring of the rate of grafted molecules on interfaces with a precision never obtained before.

By using microfluidic chips, we investigate the stability regarding coalescence of droplet pairs under an electric field as a function of drop separation and ac field intensity. Three different regimes are found: stable, coalescence, and partial merging. From this, we identify the two breaking scenarios of a one dimensional train of droplets: in one case the coalescence front propagates; in the other case, in which pairs belong to the partial merging regime, the coalescence front can become heterogeneous. From these findings, we can propose a destruction mechanism for a macroscopic emulsion, which includes the packing condition for which total and immediate destruction is effective.

In 1994 Leal Calderon et al. (Phys. Rev. Lett. 72, 2959 (1994)) introduced the magnetic chaining technique to directly probe the force-distance profile between colloidal particles. In this paper, we revisit this approach in two ways. First, we describe a new experimental design which allows us to utilize sample volumes as low as a few microliters, involving femtomoles of surface active macromolecules. Secondly, we extensively describe the characterization and preparation of the magnetic colloids, and we give a quantitative evaluation of performance and resolution of the technique in terms of force and interparticle separation.

 We have produced emulsion droplets of controlled size and composition coated by ligands, and studied the adhesion of these drops on a solid substrate coated by receptors and polymers. Using transmission, RICM and fluorescence microscopy we assess the size, contact angle and ligand density for each drop. We first show that non-specific interactions significantly enhance the proteins density within the adhesive patch. Then we show that binding within the patch is partially inhibited in good agreement with the hypothesis of an absence of translational diffusion. We confirm that the density of specific bonds sets the adhesive energy and therefore the final contact angle, and finally show that specific binding in our system is always associated with the existence of a positive line tension, which linearly increases with the density of receptors. These experiments describe a new scenario for specific wetting which raises the importance of the coupling between non-specific interactions and specific binding.

We discuss the theory of ligand receptor reactions between two freely rotating colloids in close proximity to one other. Such reactions, limited by rotational diffusion, arise in magnetic bead suspensions where the beads are driven into close contact by an applied magnetic field as they align in chainlike structures. By a combination of reaction-diffusion theory, numerical simulations, and heuristic arguments, we compute the time required for a reaction to occur in a number of experimentally relevant situations. We find in all cases that the time required for a reaction to occur is larger than the characteristic rotation time of the diffusion motion tau_rot. When the colloids carry one ligand only and a number n of receptors, we find that the reaction time is, in units of tau_rot, a function simply of n and of the relative surface alpha occupied by one reaction patch alpha = pi r_C²/(4pi r²), where r_C is the ligand receptor capture radius and r is the radius of the colloid.

Silicon biomineralization is a widespread mechanism found in several kingdoms that concerns both unicellular and multicellular organisms. As a result of genomic and molecular tools, diatoms have emerged as a good model for biomineralization studies and have provided most of the current knowledge on this process. However, the number of techniques available to study its dynamics at the cellular level is still rather limited. Here, new probes were developed specifically to label the pre-existing or the newly synthesized silica frustule of several diatoms species. It is shown that the LysoTracker Yellow HCK-123, which can be used to visualize silica frustules with common filter sets, presents an enhanced signal-to-noise ratio and allows details of the frustules to be imaged without of the use of ionophores. It is also demonstrated that methoxysilane derivatives can be coupled to fluorescein-5-isothiocyanate (FITC) to preferentially label the silica components of living cells. The coupling of labeling procedures might help to address the challenging question of the process of frustule exocytosis.

The destabilization process of an emulsion under flow is investigated in a microfluidic device. The experimental approach enables us to generate a periodic train of droplet pairs, and thus to isolate and analyze the basic step of the destabilization, namely, the coalescence of two droplets which collide. We demonstrate a counterintuitive phenomenon: coalescence occurs during the separation phase and not during the impact. Separation induces the formation of two facing nipples in the contact area that hastens the connection of the interfaces prior to fusion. Moreover, droplet pairs initially stabilized by surfactants can be destabilized by forcing the separation. Finally, we note that the fusion mechanism is responsible for a cascade of coalescence events in a compact system of droplets where the separation is driven by surface tension.

Recent advances in micro-machining allow very small cargos, such as single red blood cells, to be moved by outfitting them with tails made of micrometre-sized paramagnetic particles yoked together by polymer bridges. When a time-varying magnetic field is applied to such a filament, it bends from side to side and propels itself through the fluid, dragging the load behind it. Here, experimental data and a mathematical model are presented showing the dependence of the swimming speed and direction of the magnetic micro-swimmer upon tunable parameters, such as the field strength and frequency and the filament length. The propulsion of the filament arises from the propagation of bending waves between free and tethered ends: here we show that this gives the microswimmer a gait that is intermediate between a eukaryotic cell and a waggled elastic rod. Finally, we extract from the model design principles for constructing the fastest swimming micro-swimmer.

In this work, the kinetics of coupled aggregation and sedimentation processes arising in magnetic fluids has been studied. Aggregation was induced applying a constant uniaxial magnetic field. The time evolution of the cluster-size distribution and the weight-average chain length was monitored using optical microscopy and digital image analysis. The experimental results are compared with the corresponding solutions of Smoluchowski’s equation. For this purpose, a recently proposed aggregation kernel was employed. When sedimentation effects are taken into account, the fits improve especially at long aggregation times.

We introduce a general methodology based on magnetic colloids to study the recognition kinetics of tethered biomolecules. Access to the full kinetics of the reaction is provided by an explicit measure of the time evolution of the reactant densities. Binding between a single ligand and its complementary receptor is here limited by the colloidal rotational diffusion. It occurs within a binding distance that can be extracted by a reaction-diffusion theory that properly accounts for the rotational Brownian dynamics. Our reaction geometry allows us to probe a large diversity of bioadhesive molecules and tethers, thus providing a quantitative guidance for designing more efficient reactive biomimetic surfaces, as required for diagnostic, therapeutic, and tissue engineering techniques.

, 483), is based on the encapsulation of silica spheres in emulsion droplets. The originality of our work lies in the preparation of monodisperse emulsions, which allows us to obtain some high yields of small aggregates over a wide range of conditions. Using optical microscopy and disk centrifugation, we show that the relative fractions of 2, 3, and 4 particle aggregates are controlled by the emulsification conditions, particularly the concentration of silica in the dispersed phase. Our best yields are obtained using low to moderate shear rates, a highly viscous continuous phase, and intermediate amounts of silica. The sedimentation of the colloidal solution into a gradient of concentration leads to aqueous suspensions of identical clusters. Since the overall process can easily be scaled up, large quantities of identical clusters may be prepared, which should allow the thermodynamic properties of these new colloidal objects to be measured for the first time. These nonspherical particles could serve as building blocks for more complex assemblies, such as colloidal crystals which could find applications as photonic materials.

We have studied the effect of shear on the stability of suspensions made of non-Brownian solid particles. We demonstrate the existence of an irreversible transition where the solid particles aggregate at remarkably low volume fractions (Phi~0.1). This shear-induced aggregation is dramatic and exhibits a very sudden change in the viscosity, which increases sharply after a shear-dependent induction time. We show that this induction time is related exponentially to the shear rate, reflecting the importance of the hydrodynamic forces in reducing the repulsive energy barrier that prevents the particles from aggregating.

Following a novel realization of low-Reynolds-number swimming (Dreyfus et al., Nature, vol. 436, 2005, p. 862), in which self-assembled filaments of paramagnetic micron-sized beads are tethered to red blood cells and then induced to swim under crossed uniform and oscillating magnetic fields, the dynamics of magnetoelastic filaments is studied. The filament is modelled as a slender elastica driven by a magnetic body torque. The model is applied to experiments of Goubault et al. (Phys.Rev. Lett., vol. 91, 2003, art. 260802) to predict the lifetimes of metastable static filament conformations that are known to form under uniform fields. A second experimental swimming scenario, complementary to that of Dreyfus et al. (2005), is described: filaments are capable of swimming even if not tethered to red blood cells. Yet, if both ends of the filament are left free and the material and magnetic parameters are uniform along its length then application of an oscillating transverse field can only generate homogeneous torques, and net translation is prohibited by symmetry. It is shown that fore–aft symmetry is broken when variation of the bending stiffness along the filament is accounted for by including elastic defects, which produces results consistent with the swimming phenomenology. 

When ligands and receptors are both attached on surfaces, because of the restriction of configurational freedom, their recognition kinetics may be substantially reduced as compared with freely diffusing species. In nature, this reduction may influence the efficiency of the capture and adhesion of circulating cells. Here we show that similar consequences are observed for colloids grafted with biomolecules that are used as probes for diagnostics. We exploit Brownian magnetic colloids that self-assemble into linear chains to show also that the resulting one-dimensional confinement considerably accelerates the recognition rate between grafted receptors and their ligands. We propose that because confinement significantly augments the colliding frequency, it also causes a large increase in the attempt frequency of the recognition. This work gives the basis of a rapid, homogeneous, and highly sensitive bioanalysis method.