Structuration of the Surface Layer during Drying of Colloidal Dispersions

3D Printing Graphene Oxide Soft Robotics

We propose a universal strategy to 3D printing the graphene oxide (GO) complex structure with GO highly aligned and densely compacted, by the combination of direct ink writing and constrained drying. The constraints not only allow the generation of a huge capillary force accompanied by water evaporation at nanoscale, which induces the high compaction and alignment of GO, but also limit the shrinkage of the extruded filaments only along the wall thickness direction, therefore, successfully maintaining the uniformity of the structure at macroscale. We discover that the shrinkage stress gradually increased during the drying process, with the maximum exceeding ∼0.74 MPa, significantly higher than other colloidal systems. Interestingly, because of the convergence between plates with different orientations of the constraints, a gradient of porosity naturally formed across the thickness direction at the corner. This allows us to 3D print humidity sensitive GO based soft robotics.

Role of particle aggregation in the structure of dried colloidal silica layers

The process of colloidal drying gives way to particle self-assembly in numerous fields including photonics or biotechnology. Yet, the mechanisms and conditions driving the final particle arrangement in dry colloidal layers remain elusive. Here, we examine how the drying rate selects the nanostructure of thick dried layers in four different suspensions of silica nanospheres. Depending on particle size and dispersity, either an amorphous arrangement, a crystalline arrangement, or a rate-dependent amorphous-to-crystalline transition occurs at the drying surface. Amorphous arrangements are observed in the two most polydisperse suspensions while crystallinity occurs when dispersity is lower. Counter-intuitively in the latter case, a higher drying rate favors ordering of the particles. To complement these measurements and to take stock of the bulk properties of the layer, tests on the layer porosity were undertaken. For all suspensions studied herein, faster drying yields denser dry layers. Crystalline surface arrangement implies large bulk volume fraction (∼0.65) whereas amorphous arrangements can be observed in layers with either low (down to ∼0.53) or high (∼0.65) volume fraction. Lastly, we demonstrate via targeted additional experiments and SAXS measurements, that the packing structure of the layers is mainly driven by the formation of aggregates and their subsequent packing, and not by the competition between Brownian diffusion and convection. This highlights that a second dimensionless ratio in addition to the Peclet number should be taken into account, namely the aggregation over evaporation timescale.

Imbibition and structure of silica nanoporous media characterized by neutron imaging

HYPOTHESIS

Colloidal silica dispersions dried under controlled conditions form solid gels that display mechanical properties similar to those observed in several practical processes. An understanding of their structural characteristics and liquid flow properties can therefore help establish these gels as an alternative family of model materials to study practical porous systems.

EXPERIMENTS

Neutron radiography is a non-destructive technique well-adapted to study hydrogen-rich domains in porous materials due to the high attenuation power of hydrogen. We apply this technique to study gels prepared from silica nanoparticles of radii 5-40 nm.

FINDINGS

The water content in the gels have been quantified and different types of porosities have been determined: total porosity, effective porosity that contributes to liquid flow, and residual porosity that contains bound residual water. This residual water increases with decrease in particle size and constitutes an important fraction of the gel. The dynamics of water imbibition follows a √t law, from which the effective pore size and permeability are evaluated. We highlight the role of particle size on water retention, on particle organization and its impact on mechanical resistance. Quantitative analysis of the propagating liquid front shows front broadening that suggests elongated pores with reduced correlated liquid menisci.

Temporal evolution of concentration and microstructure of colloidal films during vertical drying: a lattice Boltzmann simulation study

We study the temporal and structural development of colloid films during vertical drying using the lattice Boltzmann (LB) simulation. The dispersed particles moving in Brownian motion have excluded volume and hydrodynamic interactions in the film. The concentrated colloidal film formed by solvent evaporation is modeled as an uniaxial compression of colloids with a planar moving interface. The simulation studies are carried out over a wide range of Péclet number (Pe), the relative ratio between the evaporation rate and the diffusion rate of colloids. The results clearly demonstrate a temporal variation of colloid concentration as the evaporation rate increases. In the case of high Pe, the increase of colloid concentration in the top layer creates structural features that can be distinguished along the height of the film, and eventually can induce a large tensile stress in the layer. However, surprisingly, the colloids are maximally crystallized in the case of moderate Pe. The LB simulation results are further compared with those from previous studies of the Brownian Dynamics (BD) simulation and the continuum model for the evaporation film. The LB and BD results match well both at low and high Pe limits. The qualitatively significant differences between LB and BD simulations at a moderate Pe indicate that hydrodynamic interactions (HIs) play an important role in this Pe. The presence of HIs induces a greater reduction of diffusion than under geometrical restriction alone, and the effect is conspicuous when particles are driven both by diffusion and by advection.