Daria Monaenkova

Hydrophobic–hydrophilic dichotomy of the butterfly proboscis

Mouthparts of fluid-feeding insects have unique material properties with no human-engineered analogue: the feeding devices acquire sticky and viscous liquids while remaining clean. We discovered that the external surface of the butterfly proboscis has a sharp boundary separating a hydrophilic drinking region and a hydrophobic non-drinking region. The structural arrangement of the proboscis provides the basis for the wetting dichotomy. Theoretical and experimental analyses show that fluid uptake is associated with enlargement of hydrophilic cuticular structures, the legulae, which link the two halves of the proboscis together. We also show that an elliptical proboscis produces a higher external meniscus than does a cylindrical proboscis of the same circumference. Fluid uptake is additionally facilitated in sap-feeding butterflies that have a proboscis with enlarged chemosensory structures forming a brush near the tip. This structural modification of the proboscis enables sap feeders to exploit films of liquid more efficiently. Structural changes along the proboscis, including increased legular width and presence of a brush-like tip, occur in a wide range of species, suggesting that a wetting dichotomy is widespread in the Lepidoptera.

Butterfly proboscis: combining a drinking straw with a nanosponge facilitated diversification of feeding habits

The ability of Lepidoptera, or butterflies and moths, to drink liquids from rotting fruit and wet soil, as well as nectar from floral tubes, raises the question of whether the conventional view of the proboscis as a drinking straw can account for the withdrawal of fluids from porous substrates or of films and droplets from floral tubes. We discovered that the proboscis promotes capillary pull of liquids from diverse sources owing to a hierarchical pore structure spanning nano- and microscales. X-ray phase-contrast imaging reveals that Plateau instability causes liquid bridges to form in the food canal, which are transported to the gut by the muscular sucking pump in the head. The dual functionality of the proboscis represents a key innovation for exploiting a vast range of nutritional sources. We suggest that future studies of the adaptive radiation of the Lepidoptera take into account the role played by the structural organization of the proboscis. A transformative two-step model of capillary intake and suctioning can be applied not only to butterflies and moths but also potentially to vast numbers of other insects such as bees and flies.

Paradox of the drinking-straw model of the butterfly proboscis

Fluid-feeding Lepidoptera use an elongated proboscis, conventionally modeled as a drinking straw, to feed from pools and films of liquid. Using the monarch butterfly, Danaus plexippus (Linnaeus), we show that the inherent structural features of the lepidopteran proboscis contradict the basic assumptions of the drinking-straw model. By experimentally characterizing permeability and flow in the proboscis, we show that tapering of the food canal in the drinking region increases resistance, significantly hindering the flow of fluid. The calculated pressure differential required for a suction pump to support flow along the entire proboscis is greater than 1 atm (~101 kPa) when the butterfly feeds from a pool of liquid. We suggest that behavioral strategies employed by butterflies and moths can resolve this paradoxical pressure anomaly. Butterflies can alter the taper, the interlegular spacing and the terminal opening of the food canal, thereby controlling fluid entry and flow, by splaying the galeal tips apart, sliding the galeae along one another, pulsing hemolymph into each galeal lumen, and pressing the proboscis against a substrate. Thus, although physical construction of the proboscis limits its mechanical capabilities, its functionality can be modified and enhanced by behavioral strategies.

Elastocapillarity: Stress transfer through fibrous probes in wicking experiments

Current advances in the manufacture of nanoporous and nanofibrous materials with high absorption capacity open up new opportunities for the development of fiber-based probes and sensors. Pore structures of these materials can be designed to provide high suction pressure and fast wicking. During wicking, due to the strong capillary action, liquids exert stresses on the fiber network. In this paper, we discuss the effect of stress transfer in the direction of propagation of the wetting front. As an illustration, we first consider a single capillary and demonstrate the effect of a moving meniscus on stress distribution along capillary walls. We then analyze similar effects in yarns. We consider a yarn that can capture an aerosol droplet as a promising sensing element that could monitor the stresses caused by wetting fronts. We also discuss the elastocapillary effects occurring during upward and downward wicking. The distributions of stresses in these two cases are shown to differ significantly. We discuss how these effects might be exploited for designing fiber-based sensors that can probe very small amounts of liquids.

Wicking of liquids into sagged fabrics

The classical Bernoulli problem of a freely hanging fabric sagged between two posts is used for the analysis of wicking phenomena. We show that wicking of a wetting liquid into a Bernoulli catenary is an instructive nontrivial experiment illustrating an unusual coupling between mechanical and capillary forces. When the liquid wicks into the material, it causes the catenary to sway back and forth. We studied theoretically and experimentally the kinetics of wicking into sagged nonwoven materials. The proposed experiment can be used for the analysis of transport and tensile properties of thin porous films.

The butterfly proboscis as a fiber-based, self-cleaning, micro-fluidic system

The butterfly proboscis is a unique, naturally engineered device for acquiring liquid food, which also minimizes concerns for viscosity and stickiness of the fluids. With a few examples, we emphasize the importance of the scale-form functionality triangle of this feeding device and the coupling through capillarity.

Bernoulli catenary and elasto-capillary effect in partially wet fibrous materials

When a wetting liquid wicks into a fibrous material, it causes the material to deform. In this paper we discuss the elasto-capillary effect that leads to spontaneous internal stresses in the materials. The elasto-capillary effect produced by menisci in pores can be identified through a specific stress distribution in the fibrous matrix. We show that the classical Bernoulli problem of a freely hanging fabric can be used for the analysis of gravity-induced stresses in textile materials. These stresses change due to elasto-capillary effect. Wicking of a wetting liquid into a freely suspended fibrous material is shown to be an instructive nontrivial experiment illustrating interesting distinguishable stress distributions in the fibrous matrix.

Self-assembly of the butterfly proboscis: the role of capillary forces

The proboscis of butterflies and moths consists of two C-shaped fibres, the galeae, which are united after the insect emerges from the pupa. We observed that proboscis self-assembly is facilitated by discharge of saliva. In contrast with vertebrate saliva, butterfly saliva is not slimy and is an almost inviscid, water-like fluid. Butterfly saliva, therefore, cannot offer any viscoelastic adhesiveness. We hypothesized that capillary forces are responsible for helping butterflies and moths pull and hold their galeae together while uniting them mechanically. Theoretical analysis supported by X-ray micro-computed tomography on columnar liquid bridges suggests that both concave and convex liquid bridges are able to pull the galeae together. Theoretical and experimental analyses of capillary forces acting on natural and artificial proboscises show that these forces are sufficiently high to hold the galeae together.