Physics and Applications of Microfluidics in Fluids

Micro- and miniaturised fluid systems are at the forefront of both fundamental research and practical innovation in fluid mechanics. As characteristic length scales decrease, microfluidic systems offer particularly appealing advantages, including their substantially larger surface-to-volume ratios, rapid dynamic response times, and expanded opportunities for integrated functionality. These benefits, however, are accompanied by inherent challenges. Surface forces become dominant, classical continuum assumptions may no longer hold, and the coupling between fluid mechanics, heat and mass transfer, electrokinetics, and material interactions grows increasingly complex. These challenges reflect current trends in microfluidics research, where accurate interfacial modelling, rigorous multiphysics coupling, and improved material characterisation are receiving growing attention, although substantial knowledge gaps in these areas continue to limit progress.

Motivated by these considerations, this Special Issue, “Physics and Applications of Microfluidics”, brings together six contributions that advance the scientific understanding of microscale transport while highlighting opportunities for improved modelling, experimental validation, device optimisation, and material characterisation. The collection spans analytical developments, numerical simulations, experimental investigations, and a comprehensive review, collectively illustrating the diversity and strongly coupled, multiphysics nature of contemporary microfluidics research. Within this evolving scientific landscape, each contribution in this Special Issue addresses a distinct aspect of the physical mechanisms governing microfluidic transport.

Celli et al. [1] open the Special Issue with a careful examination of how random surface roughness alters fully developed flow in square microchannels. By combining simulations with experiments, they show that the common assumption of smooth walls can misrepresent shear stresses, pressure gradients, and velocity fields, especially in low-Reynolds-number situations. This conclusion is consistent with early findings in the microfluidics community, including the work of Gamrat et al. [2], which showed that wall roughness and manufacturing-induced surface features can substantially modify fluid behaviour in microscale channels. Complementing these insights, Lu et al. [3] showed that roughness effects can be even more pronounced in non-Newtonian and particle-laden flows, where surface topology influences shear-dependent viscosity and particle migration. Overall, these studies highlight the importance of incorporating realistic surface characterisation into microchannel modelling and optimisation.

The theme of non-continuum transport is introduced to the Special Issue in the work of Ajuda, Silva, and Semiao [4], who analyse the performance of multi-capillary Knudsen heat pumps. Their results map how device geometry, rarefaction, and thermal gradients interact to shape throughput and cooling power. These findings are expected to contribute to ongoing developments in both Knudsen compressors [5] and Knudsen heat pump [6] devices. As highlighted by Luo et al. [7], a deeper understanding of thermal transpiration remains essential for more robust progress in this field. The additional consideration of combined or complementary mechanisms may also prove to be worthwhile further investigation, for instance, based on hybrid pumping concepts capable of operating effectively across a wider range of Knudsen numbers.

Heat-transfer enhancement is considered by Zaw, Zhu, and Ma [8], who simulate nano-encapsulated phase-change slurries in wavy microchannels. The sinusoidal geometries generate secondary flows that improve mixing and increase the melting fraction of the particles, ultimately boosting heat-transfer coefficients. Their findings echo the directions suggested by Chandrasekaran et al. [9], who reported promising results for structured microchannel heat sinks that incorporate phase-change media. Together, these studies highlight the growing opportunity to unify numerical and experimental approaches in order to optimise thermal performance while avoiding excessive pressure-drop penalties.

Feng, Yi, and Liu [10] examine electroosmotic transport through an analytical solution, describing the transient coupling of pressure-driven and electroosmotic flow in cylindrical microtubes. Their work clarifies how velocity and shear evolve with different Debye lengths and slip conditions, and it introduces a useful retarded-time scaling. Stroock et al. work [11] on patterned surface charge distributions hints at future extensions of such models to more realistic channel surfaces. The incorporation of surface-charge heterogeneity, ion-selective coatings, or nonlinear electrokinetic effects should also be considered in future studies to better capture the complex interfaces increasingly employed in next-generation electrokinetic microdevices [12].

A direct biomedical application appears in the study by Dutta et al. [13], who simulate a three-pronged microfluidic device for separating breast cancer cells using alternating-current electroosmotic flow. By testing different voltages, frequencies, and inlet conditions, they show that electrical frequency plays a particularly important role in separation efficiency. In light of recent multiphysics cell-manipulation strategies (e.g., Jamshidi et al. [14]), the results suggest that ACEO-driven methods could become even more powerful when combined with dielectrophoresis or related forces. Future research may also benefit from integrating electrode miniaturisation, multi-frequency actuation, or machine-learning-assisted optimisation to enhance selectivity and transition these concepts toward clinically robust cell-sorting platforms [12].

The Special Issue concludes with the comprehensive review by Lima [15] on the role of PDMS in engineering, especially in microfluidic and biomedical platforms. The article summarises PDMS’s strengths, but also revisits persistent concerns such as hydrophobicity, small molecule absorption, and long-term mechanical changes. With the emergence of alternative elastomeric materials [16], the field is clearly moving toward hybrid and modified formulations that may better support demanding multiphysics environments. Taken together, these contributions point toward a future in which material innovation, coupled with rigorous characterisation and modelling, plays an increasingly central role in advancing reliable and application-specific microfluidic systems.

Collectively, the contributions in this Special Issue depict a research landscape in which micro- and miniaturised fluid systems are becoming increasingly sophisticated, spanning foundational transport physics, multiphysics interactions, and emerging engineering applications [17]. They also highlight persistent challenges, including the need for reliable interfacial and material characterisation; improved modelling of roughness, rarefaction, and electrokinetic effects; robust validation of phase-change-driven thermal strategies; and scalable biomedical implementations. Broader developments across the microfluidics community echo these themes: advances in electrokinetic transport and nanoscale interface engineering are refining our understanding of non-continuum behaviour [18]; thermal microfluidics continues to demonstrate the potential of structured channels and phase-change media for compact heat-management systems [19]; and rapid progress in biomedical microdevices is enabling increasingly precise cell sorting [20], organ-on-chip platforms [21], and physiologically relevant disease models. Continued integration of computational and experimental methodologies, alongside the adoption of next-generation materials and fabrication strategies, will be essential in overcoming these challenges and driving further innovation in the field [22].

Altogether, this Special Issue highlights a field evolving rapidly through the interplay of fundamental physics, computational modelling, experimental advances, and device engineering. By integrating these complementary perspectives, the collection brings emerging directions in microfluidics into clearer focus and highlights the substantial opportunities that remain for future scientific and technological progress.

The Guest Editor thanks all authors, reviewers, and the Editorial Office of Fluids for their contributions and dedication in realising this Special Issue.