One-dimensional microfluidic drying as a platform for complex fluids

To disentangle the role of molecular interactions and thermodynamic non-ideality in the drying of complex fluids, we have developed a simple one-dimensional microfluidic approach based on a unidirectional drying channel. The device consists of a low-Reynolds-number glass capillary connected to a reservoir and exposed at one end to air of controlled humidity, imposing fixed and well-defined boundary conditions on chemical potentials. Evaporation at the open end induces the spontaneous formation of extended hydration gradients, while continuous feeding from the reservoir prevents complete drying and allows gradients to propagate over long times.

This geometry, conceptually close to a semi-infinite Hele–Shaw cell, deliberately simplifies the hydrodynamics while retaining the essential couplings between transport, thermodynamics, and structure. As a result, it provides exceptional experimental access to non-equilibrium phenomena that are otherwise difficult to probe. The platform is compatible with a wide range of characterization techniques, including optical microscopy, Raman spectroscopy, and spatially resolved SAXS, enabling high-resolution measurements of concentration, structure, and mesophase evolution along the gradient.

A key outcome of this approach is the emergence of robust diffusion-like scaling laws, even in strongly non-ideal, advection–diffusion-dominated systems. These scaling properties allow data acquired at different times to be collapsed onto master profiles and, in some cases, enable analytical solutions of transport equations. Beyond its simplicity and low cost, the strength of this 1D microfluidic strategy lies in its generality: the mechanisms revealed are generic and transferable to thin films, membranes, droplets, and more complex drying geometries encountered in biology, soft matter, and technology.

Representative problems addressed with this approach

• Non-ideal drying and transport
  Quantification of mutual diffusion coefficients in complex mixtures, directly as a function of composition.

• Structure–transport coupling
  Direct observation of how self-assembly and mesostructuration feed back on mass transport during drying.

• Humidity-insensitive evaporation
  Identification of universal self-regulation mechanisms explaining why some soft and biological materials evaporate almost independently of ambient humidity.

• Soft colloids and microgels
  Separation of molecular-scale and colloidal-scale control during drying, revealing spatially distinct regimes within a single system.

• Aerosol and droplet drying
  Extension of the framework to respiratory fluids, enabling realistic descriptions of aerosol lifetime and virus survival.

• Biological and biomimetic membranes
  Reconstruction and characterization of lipid films under controlled hydration gradients, linking composition, structure, and permeability.

• Multicomponent mixtures under gradients
  Demonstration that hydration gradients act as a control field driving selective partition, segregation, and non-equilibrium organization.

Building competitive color conversion pixels for microdisplays made of semiconductor nanocrystals requires reaching a deposition thickness high enough to absorb all the blue light from the backlight unit. The dielectrophoretic directed assembly of such nanocrystals has been modeled and simulated to understand what the intrinsic limitations of the process are, and allowed to propose new assembly routes. A theoretical model of dielectrophoretic interactions between polarizable nano-spheres and an electrostatically patterned substrate has been developed. Monte Carlo simulations have been run using this model to rationalize the effects of parameters driving the dielectrophoretic directed assembly and to find optimal deposition conditions for reaching a maximal thickness of nanocrystal pixels. Experiments with CdSe quantum plates and with alumina spheres embedding quantum plates (micro-pearls) have been carried out and compared to the model. We found that that the directed assembly of semiconductor nanocrystals is limited essentially by the small object size, which sets the maximum dielectrophoretic force they can undergo. They indicate that using larger objects should allow reaching unprecedented assembly heights, but will induce lateral extension of the assembly. This trade-off has been illustrated with diagrams in the parameter space and confirmed experimentally with micro-pearls.

Due to their unique quantum yield and photostability performances, quantum nanoplatelets are very promising building blocks for future generations of displays. The directed assembly of such colloidal nano-objects in the shape of micro-pixels is thus the next mandatory step to reach this goal. Selectively trapping them on electrostatically charged patterns by nanoxerography is a versatile and appealing strategy but requires a full understanding of the assembly mechanisms in order to make the most of their integration.

We proposed an experimental platform based on a smart resealable microfluidic chip coupled to an inverted optical fluorescence microscope and a high-speed camera for in situ access of such assembly mechanisms, using CdSe/CdZnS quantum nanoplatelets as model nano-objects. The photoluminescence signal of the nanoplatelet patterns was thus recorded in real time during their assembly and data extracted after image processing.

The coupling of experimental results and numerical simulations evidenced the main role of advection at the origin of this directed nanoparticle trapping. Deep understanding of the involved mechanisms and tuning of experimental parameters allowed to make high resolution quantum nanoplatelet based micro-pixels with a fine control of their lateral and vertical dimensions.

Atomic force microscopy (AFM) nanoxerography was successfully used to direct the assembly of colloidal nanodiamonds (NDs) containing nitrogen-vacancy (NV) centres on electrostatically patterned surfaces. This study reveals that the number of deposited NDs can be controlled by tuning the surface potentials of positively charged dots on a negatively charged background written by AFM in a thin PMMA electret film, yielding assemblies down to a unique single-photon emitter with very good selectivity. The mechanisms of the ND directed assembly are attested by numerical simulations. This robust deterministic nano-positioning of quantum emitters thus offers great opportunities for ultimate applications in nanophotonics for quantum technologies.

Silica precipitation is a ubiquitous but deleterious phenomenon occurring in many hydrometallurgical processes. Indeed, silicon is often released during the dissolution of minerals under acidic leaching conditions. Eventually, it precipitates into a hard-to-filter silica gel, which has prompted some efforts to hinder silica precipitation through pre-treatment or extra dilution. However, these approaches are usually either mineral-specific or costly. Here, we propose a disruptive strategy based on controlling the gel's mesostructure, and therefore its filterability. We designed an alternative precipitation pathway consisting of adding extra silicate ions, but at basic pH. Using small-angle Xray scattering, we show that this pathway transforms the network of polymeric silica ('polymer gel') obtained in the very acidic leaching conditions into a network of dense silica particles ('particle gel'). This structural compaction at the mesoscopic length scales cascades to the macroscale and leads to a drastic improvement of filterability by two orders of magnitude. Furthermore, we demonstrate that this method is generic by applying it successfully to both a model and real ore systems.