John Linkhorst

Bio: John Linkhorst is an academic researcher from RWTH Aachen University. The author has contributed to research in topics: Materials science & Electrode. The author has an hindex of 5, co-authored 16 publications receiving 120 citations.

Microfluidic colloid filtration.

Abstract: Filtration of natural and colloidal matter is an essential process in today’s water treatment processes. The colloidal matter is retained with the help of micro- and nanoporous synthetic membranes. Colloids are retained in a “cake layer” – often coined fouling layer. Membrane fouling is the most substantial problem in membrane filtration: colloidal and natural matter build-up leads to an increasing resistance and thus decreasing water transport rate through the membrane. Theoretical models exist to describe macroscopically the hydrodynamic resistance of such transport and rejection phenomena; however, visualization of the various phenomena occurring during colloid retention is extremely demanding. Here we present a microfluidics based methodology to follow filter cake build up as well as transport phenomena occuring inside of the fouling layer. The microfluidic colloidal filtration methodology enables the study of complex colloidal jamming, crystallization and melting processes as well as translocation at the single particle level.

3D nanofabrication inside rapid prototyped microfluidic channels showcased by wet-spinning of single micrometre fibres

Abstract: Microfluidics is an established multidisciplinary research domain with widespread applications in the fields of medicine, biotechnology and engineering. Conventional production methods of microfluidic chips have been limited to planar structures, preventing the exploitation of truly three-dimensional architectures for applications such as multi-phase droplet preparation or wet-phase fibre spinning. Here the challenge of nanofabrication inside a microfluidic chip is tackled for the showcase of a spider-inspired spinneret. Multiphoton lithography, an additive manufacturing method, was used to produce free-form microfluidic masters, subsequently replicated by soft lithography. Into the resulting microfluidic device, a three-dimensional spider-inspired spinneret was directly fabricated in-chip via multiphoton lithography. Applying this unprecedented fabrication strategy, the to date smallest printed spinneret nozzle is produced. This spinneret resides tightly sealed, connecting it to the macroscopic world. Its functionality is demonstrated by wet-spinning of single-digit micron fibres through a polyacrylonitrile coagulation process induced by a water sheath layer. The methodology developed here demonstrates fabrication strategies to interface complex architectures into classical microfluidic platforms. Using multiphoton lithography for in-chip fabrication adopts a high spatial resolution technology for improving geometry and thus flow control inside microfluidic chips. The showcased fabrication methodology is generic and will be applicable to multiple challenges in fluid control and beyond.

Soft temperature-responsive microgels of complex shape in stop-flow lithography

Abstract: Stop-flow lithography (SFL) has emerged as a facile high-throughput fabrication method for μm-sized anisometric particles; yet, the fabrication of soft, anisometric microgels has not frequently been addressed in the literature. Furthermore, and to the best of the authors' knowledge, no soft, complex-shaped microgels with temperature-responsive behavior have been fabricated with this technology before. However, such microgels have tremendous potential as building blocks and actuating elements in rapidly developing fields, such as tissue engineering and additive manufacturing of soft polymeric building blocks, bio-hybrid materials, or soft micro-robotics. Given their great potential, we prove in this work that SFL is a viable method for the fabrication of soft, temperature-responsive, and complex-shaped microgels. The microgels, fabricated in this work, consist of poly(N-isopropylacrylamide) (pNIPAm), which is crosslinked with N,N'-methylenebis(acrylamide). The results confirm that the shape of the pNIPAm microgels is determined by the transparency mask, used in SFL. Furthermore, it is shown that, in order to realize stable microgels, a minimum threshold of crosslinker concentration of 2 wt% is required. Above this threshold, the stiffness of pNIPAm microgels can be deliberately altered by adjusting the concentration of the crosslinker. The fabricated pNIPAm microgels show the targeted temperature-responsive behavior. Within this context, temperature-dependent reversible swelling is confirmed, even for fractal-like geometries, such as micro snowflakes. Thus, these microgels provide the targeted unique combination of softness, shape complexity, and temperature responsiveness and increase the freedom of design for actuated building blocks.

Direct Observation of Deformation in Microgel Filtration

Abstract: Colloidal filtration processes using porous membranes suffer from productivity loss due to colloidal matter retention and continuous build-up by the retained matter. Especially during filtration of soft matter, the deformation of the individual colloids that make up the filter cake may be significant; however, this deformation and its impact remain unresolved so far. Yet, understanding the deformation on the single colloid level as well as on the ensemble level is important to be able to deconvolute filter cake properties from resistance increase of the membrane either by simultaneous internal adsorption or blocking of pores. Here, we report on the compression of a filter cake by filtrating soft microgels in a microfluidic channel in front of a model membrane. To study the single colloid deformation amorphous and crystalline domains were built up in front of the membrane and visualized on-line using confocal fluorescence microscopy while adjusting the degree of permeation, i.e., the transmembrane flux. Results show locally pronounced asymmetric deformation in amorphous domains, while the microgels in colloidal crystals approached regular polyeder shape. Increasing the flux beyond the maximum colloid deformation results in non-isochoric microgel behavior. The presented methodology enables a realistic description of complex colloidal matter deposits during filtration.

Functional Microgels and Microgel Systems

Abstract: ConspectusMicrogels are macromolecular networks swollen by the solvent in which they are dissolved. They are unique systems that are distinctly different from common colloids, such as, e.g., rigid nanoparticles, flexible macromolecules, micelles, or vesicles. The size of the microgel networks is in the range of several micrometers down to nanometers (then sometimes called “nanogels”). In a collapsed state, they might resemble hard colloids but they can still contain significant amounts of solvent. When swollen, they are soft and have a fuzzy surface with dangling chains. The presence of cross-links provides structural integrity, in contrast to linear and (hyper)branched polymers. Obviously, the cross-linker content will allow control of whether microgels behave more “colloidal” or “macromolecular”.The combination of being soft and porous while still having a stable structure through the cross-linked network allows for designing microgels that have the same total chemical composition, but different propert...

Clogging of microfluidic systems

Abstract: The transport of suspensions of microparticles in confined environments is associated with complex phenomena at the interface of fluid mechanics and soft matter. Indeed, the deposition and assembly of particles under flow involve hydrodynamic, steric and colloidal forces, and can lead to the clogging of microchannels. The formation of clogs dramatically alters the performance of both natural and engineered systems, effectively limiting the use of microfluidic technology. While the fouling of porous filters has been studied at the macroscopic level, it is only recently that the formation of clogs has been considered at the pore-scale, using microfluidic devices. In this review, we present the clogging mechanisms recently reported for suspension flows of colloidal particles and for biofluids in microfluidic channels, including sieving, bridging and aggregation of particles. We discuss the technological implications of the clogging of microchannels and the schemes that leverage the formation of clogs. We finally consider some of the outstanding challenges involving clogging in human health, which could be tackled with microfluidic methods.

Selection of Non-Equilibrium Over-Limiting Currents: Universal Depletion Layer Formation Dynamics and Vortex Instability

Abstract: The ion flux dynamics across a straight nanoslot is imaged to understand the nonequilibrium phenomenon of overlimiting current density across a nanoporous membrane. With a slow ac field, an ion-depletion front is generated intermittently from one end of the nanochannel, and a vortex instability first predicted by Rubinstein, Staude, and Kedem [Desalination 69, 101 (1988).10.1016/0011-9164(88)80013-4] is found to arrest the self-similar diffusive front growth. This electrokinetic instability evolves into a stationary interfacial vortex array that specifies the overlimiting current, independent of external stirring or convective flow.

Multifunctional Micro/Nanoscale Fibers Based on Microfluidic Spinning Technology

Abstract: Superfine multifunctional micro/nanoscale fibrous materials with high surface area and ordered structure have attracted intensive attention for widespread applications in recent years. Microfluidic spinning technology (MST) has emerged as a powerful and versatile platform because of its various advantages such as high surface-area-to-volume ratio, effective heat transfer, and enhanced reaction rate. The resultant well-defined micro/nanoscale fibers exhibit controllable compositions, advanced structures, and new physical/chemical properties. The latest developments and achievements in microfluidic spun fiber materials are summarized in terms of the underlying preparation principles, geometric configurations, and functionalization. Variously architected structures and shapes by MST, including cylindrical, grooved, flat, anisotropic, hollow, core-shell, Janus, heterogeneous, helical, and knotted fibers, are emphasized. In particular, fiber-spinning chemistry in MST for achieving functionalization of fiber materials by in situ chemical reactions inside fibers is introduced. Additionally, the applications of the fabricated functional fibers are highlighted in sensors, microactuators, photoelectric devices, flexible electronics, tissue engineering, drug delivery, and water collection. Finally, recent progress, challenges, and future perspectives are discussed.

Interpenetration of polymeric microgels at ultrahigh densities

Abstract: Soft particles such as polymeric microgels can form ultra-dense phases, where the average center-to-center distance a s can be smaller than the initial unperturbed particle diameter σ 0, due to their ability to interpenetrate and compress. However, despite of the effort devoted to microgels at ultrahigh densities, we know surprisingly little about their response to their environment at effective volume fractions ϕ eff above close packing (ϕ cp ), and the existing information is often contradictory. Here we report direct measurements of the size and shape of poly(N-isopropylacrylamide) microgels at concentrations below and above ϕ cp using the zero average contrast method in small-angle neutron scattering. We complement these experiments with measurements of the average interparticle distances using small-angle x-ray scattering, and a determination of the glass transition using dynamic light scattering. This allows us to unambiguously decouple interaction effects from density-dependent variations of the particle size and shape at all values of ϕ eff . We demonstrate that the microgels used in this study significantly interpenetrate and thus change their size and shape only marginally even for ϕ eff ≫ ϕ cp , a finding that may require changes in the interpretation of a number of previously published studies on the structural and dynamic properties of dense soft particle systems.