|Topological Microfluidics|

Topological defects and hydrodynamics are intrinsically coupled, in both living (active) and synthetic (passive) systems. In nature, active and passive phases often co-exist. Topological Microfluidics - an emerging framework based on thermotropic and lyotropic liquid crystals in flow - captures topology-flow cross-talks which are characteristic of a range of active, passive and co-existing active-passive systems.

State-of-the-art microfluidic techniques rely usually on an isotropic carrier fluid, the flow of which is modulated using morphological patterns on the microchannels, or application of external fields. Topological Microfluidics , a domain in microfluidic research initiated by my doctoral thesis, replaces the isotropic fluid by an anisotropic liquid crystal, and harnesses the interaction between flow and molecular ordering. We have demonstrated novel applications beyond the conventional isotropic microfluidics. For instance, we have created a flexible but versatile approach to guided transport of microscopic cargo in microfluidic devices. By tailoring the boundary conditions and the overall geometry, topological defects in the liquid crystal bulk phase were created at will, and were guided towards a specific target. The structure of defects emerging in the system, which are otherwise considered nuisance in applications, was used as a self-assembled system of soft rails, along which droplets containing the materials of interest could be transported. Topological microfluidics introduces a unique platform for targeted delivery of single particles, droplets, or clusters of such entities, paving the way to flexible micro-cargo concepts in microfluidic settings. Currently, we are exploring different facets of Topological Microfluidics in a range of fluidic and liquid crystalline systems.

The research was embeddded in the Marie Curie Initial Training Network ’Hierarchy’ and carried out at the Max Planck Institute for Dynamics and Self-Organization, under the supervision of Dr. Christian Bahr and Prof. Dr. Stephan Herminghaus, in active collaboration with the teams of:

Prof. Dr. Jörg Enderlein (U.Göttingen), Prof. Julia Yeomans (U. Oxford), Prof. Dr. Miha Ravnik (U. Ljubljana), Prof. Luca Giomi (U. Leiden). and Dr. Marco G. Mazza (U. Loughborough).


(see below for key publications)

From thermotropic to lyotropic chromonic liquid crystals

Nematic and columnar phases of lyotropic chromonic liquid crystals (LCLCs) have been long studied for their fundamental and applied prospects in material science and medical diagnostics. Yet, microscale insights into confined LCLCs, specifically in the context of confinement geometry and surface properties, are lacking. We study the emergence of time dependent textures in static LCLC solutions, confined in PDMS-based microfluidic devices, using a combination of soft lithography, surface characterization, and polarized optical imaging. Our results demonstrate how a combination of microfluidic confinements and anchoring conditions enable spatial and temporal control of the lyotropic phase transitions.

Time-dependent emergent textures in confined lyotropic LCs (Crystals 11, 35, 2021)

Harnessing liquid crystal microfluidics

Liquid crystals offer an experimentally tractable system within which the fundamentals of anisotropic interactions can be precisely tuned and studied. In our lab, we harness this setting to understand the fundamentals of anisotropic cross-talks between LCs and external cues, and further apply this knowledge to uncover the dynamics of real living systems. Topological defects emerging within such partially ordered systems regulate their micro-environment, and thereby tune the local material, transport, and mechanical attributes. The ability to harness anisotropy of LCs – across disparate material fields – equips us with a novel toolkit to discover new phenomena and processes ; and design smart and responsive (bio-) materials, and non-invasive characterization techniques.

Optofluidic velocimetry using liquid crystal microfluidics. See, Applied Physics Letter 101, 164101, 2012

Key publications:

11) Spatio-temporal programming of lyotropic phase transition in nanoporous microfluidic confinements: V. Ulaganathan and A. Sengupta*, (under review, arXiv preprint, 2209.02151, 2022)

10) Time dependent lyotropic chromonic textures in microfluidic confinements: A. Sharma, I. L. H. Ong, A. Sengupta*, Crystals 11, 35, 2021

9) Cross-talk between topological defects in different fields revealed by nematic microfluidics: L. Giomi, Z. Kos, M. Ravnik, & A.Sengupta*, PNAS 114, E5771, 2017

8) Hydrodynamic cavitation in Stokes flow of anisotropic fluids: T. Steiger, H. Agha, M. Schoen, M. G. Mazza*, & A. Sengupta*, Nature Communications 8, 15550, 2017

7) Topological Microfluidics - Present and Prospects: A. Sengupta., Liquid Crystals Today 24, 70, 2015

6) Liquid crystal microfluidics: surface, elastic and viscous interactions at micro-scales: A. Sengupta*, S. Herminghaus, & C. Bahr,* Liquid Crystal Reviews 2, 73, 2014

5) Topological Microfluidics: A. Sengupta., Springer International Publishing, ISBN: 978-3-319-00857-8, Switzerland, 2013

4) Tuning fluidic resistance via liquid crystal microfluidics: A. Sengupta, Int. J. Mol. Sci. 14, 22826, 2013

3) Topological microfluidics for flexible micro-cargo concepts: A. Sengupta*, C. Bahr, & S.Herminghaus, Soft Matter 9, 7251, 2013

2) Liquid crystal microfluidics for tunable flow shaping: A. Sengupta*, U. Tkalec, M. Ravnik, J. Yeomans, C. Bahr, & S. Herminghaus, Physical Review Letters 110, 048303, 2013

1) Functionalization of microfluidic devices for investigation of liquid crystal flows: A. Sengupta*, B. Schulz, E. Ouskova, and C. Bahr, Microfluidics and Nanofluidics 13, 941, 2012