Free-surface pumping by an undulating carpet
Examples of fluid flows driven by undulating boundaries are found in nature across many different length scales. Even though different driving mechanisms have evolved in distinct environments, they perform essentially the same function: directional transport of liquid. Nature-inspired strategies have been adopted in engineered devices to manipulate and direct flow. We have developed a mechanism that drives liquid flow by undulating a thin solid boundary beneath the liquid surface. This device generates large-scale pumping of a thin layer of liquid near the free surface. Two dimensional traveling waves on the undulator, a canonical strategy to transport fluid at low Reynolds numbers, surprisingly lead to flow rates that depend non-monotonically on the wave speed. Through an asymptotic analysis of the thin-film equations that account for gravity and surface tension, we predict the observed optimal speed that maximizes pumping. Our findings reveal a novel mode of pumping with less energy dissipation near a free surface compared to a rigid boundary.
Related article: A. Pandey, Z. Chen, J. Yuk, Y. Sun, C. Roh, D. Takagi, S. Lee and S. Jung, Optimal free surface pumping by an undulating carpet, Nature Communications 14, 7735 (2023). [arXiv]
Morphology & mechanics of creases and folds
Surfaces of soft polymers develop self-folded regions known as creases under constrained growth, swelling/ de-swelling, or mechanical compression. Stresses tend to diverge around the tip of these sharp folds. Our interest lies in uncovering the singular morphology and mechanics of these interfacial structures. Through confocal microscopy and theoretical modeling we have revealed the micro-morphology of sticky creases. Interestingly, folds on poro-elastic interfaces redistribute the solvent around them. We predict that obtuse folds squeezes the solvent out of the fold-tip whereas an acute fold aspirates the solvent towards the tip. These predictions could explain recent observation of spontaneous phase-separation of solvent at the edge of adhesive contacts.
Related articles: M. Essink, M. van Limbeek, A. Pandey, S. Karpitschka, and J. H. Snoeijer, Adhesive creases: bifurcation, morphology, and their (apparent) self-similarity, Soft Matter, 19, 5160, 2023.
M. M. Flapper, A. Pandey, M. Essink, E. H. van Brummelen, S. Karpitschka, and J. H. Snoeijer, Reversal of solvent migration in poroelastic folds, Physical Review Letters, 130, 228201, 2023. [arXiv]
M. van Limbeek, M. Essink, A. Pandey, J. H. Snoeijer, and S. Karpitschka, Pinning-induced folding-unfolding asymmetry in adhesive creases, Physical Review Letters, 127, 028001, 2021. [arXiv]
Elastocapillary interactions beyond the 'cheerios effect'
Most of us have noticed that paperclips, breakfast cereals, and small bubbles tend to float in clusters on a liquid surface. This phenomenon is a classical example of capillary interaction governed by surface tension of the liquid. If we replace the liquid with a bowl of soft solid e.g. jello and place tiny liquid drops on top of it, the drops starts to interact. We found that the nature of this interaction depends on size of the underlying substrate. For a very thick substrate, droplets attract and coalesce, whereas very thin substrates lead to a repulsive interaction between the droplets. This 'inverted cheerios effect' could potentially lead to the design of self cleaning soft surfaces and smart fabrics.
Related articles: S. Karpitschka, A. Pandey, L. A. Lubbers, J. H. Weijs, L. Botto, S. Das, B. Andreotti, and J. H. Snoeijer, Liquid drops attract or repel by the inverted cheerios effect, Proceedings of the National Academy of Sciences, 113, 7403, 2016. [arXiv]
A. Pandey, C. L. Nawijn, and J. H. Snoeijer, Hydrogel menisci: Shape, interaction, and instability, EPL (Europhysics Letters), 122, 3, 2018. [arXiv]
Soft wetting & surface elasticity
Functionality of soft, polymeric solids crucially rely on their surface properties. We have demonstrated that these solids possess an intricate surface elasticity which governs the contact angle of liquid drops deposited onto the surface. A detailed understanding this surface mechanics potentially open up routes to design soft surfaces that switch between hydrophilic and hydrophobic states on demand by sequential deformation. In fact, recent studies have shown that the mobility of droplets these surface can be tuned on-demand by mechanical deformation.
Related articles: A. Pandey, B. Andreotti, S. Karpitschka, G. J. van Zwieten, E. H. van Brummelen, and J. H. Snoeijer, Singular nature of the elastocapillary ridge, Physical Review X, 10, 031067, 2020. [arXiv]
Snapping, curling, & bending of soft, slender structures
Soft, slender structures bend, crease, snap, and wrinkle in response to a wide range of external stimuli like pH, humidity, electric field or swelling. These large yet reversible deformations offer an exciting pathway towards soft, designer materials. My interest is to investigate the fundamental dynamics of these deformations, and how liquid flow can alter the dynamics these structures. As such, my work has revealed how fast a curved strip snaps between two stable configurations that are far apart within milliseconds, how two curling fibers transport a water drop in a ratchet like mechanism, and how a structure transitions from global bending to local creasing when swelled in a solvent.
Related articles: D. P. Holmes, P. T. Brun, A. Pandey, and S. Protiere, Rising beyond elastocapillarity, Soft Matter, 12, 4886, 2016.
A. Pandey and D. P. Holmes, Swelling-Induced Deformations: A materials defined transition from structural instability to surface instability, Soft Matter, 9, 5514, 2013.