Precise drop dispensation on superhydrophobic surfaces using acoustic nebulization

Thach Vuong,Tuck Wah Ng,Oi Wah Liew,Aisha Qi,Jing Fu,James Friend,Cui Xia Ang,Leslie Yeo,Brandon Huey-Ping Cheong,Murat Muradoglu

doi:10.1039/c3sm00016h

Abstract

The adhesion forces of liquid drops on superhydrophobic surfaces are typically in the nano-Newton range which presents problems in their dispensation from pipettes. Furthermore, since the liquid adheres more strongly to the pipette tip, some portion of the liquid will tend to remain on the tip, causing inaccuracy in the volume dispensed. We advance a novel approach here, in which the spray from an acoustic nebulizer is sent to a superhydrophobic receptacle and the volume ascertained precisely using a weighing scale. The superhydrophobic surface was identified to develop via a galvanic displacement mechanism in an electroless deposition process. A time dependent morphology change from granular to dendritic with longer immersion into the silver nitrate solution was found which indicated that granular growth beyond a certain size was not feasible, although granular structures were more preferentially formed just after nucleation. The dendritic structure formation was likely due to the natural tendency of the process to maintain or increase the surface area to volume ratio in order not to limit the rate of deposition. An immersion for at least 7 seconds into the silver nitrate solution, when the granular structures were predominant, was all that was needed to ensure superhydrophobicity of the surfaces. Also, the superhydrophobic state required not just significant numbers of the granular structures to be present but also interrupted coverage on the surface. On using the technique, a single drop was created by subsequently covering the receptacle with a lid and shaking it gently. The volume dispensed was found to vary linearly with the operation time of the nebulizer. We elucidated the observed increased ability of drops to reside on inclines using wetting mechanics and presented an elementary mathematical description of the extent of aerosol coverage on the surface, which has implications for the mechanics of aerosol growth into drops. The structural changes in enhanced green fluorescent protein (EGFP) observed after acoustic dispensation necessitated all samples in a fluorimetric assay to involve equal nebulized volumes of the fluorescent protein marker for measurement consistency.

Highlights

Superhydrophobic surfaces are illustrated in nature through the well-known examples of lotus leaves and the legs of water striders.[1,2] There has been a recent proliferation in methods reported to arti cially mimic these surfaces.[3,4,5,6,7] While the original opportunity of superhydrophobicity was in self-cleaning,[8] there is substantial effort aimed at harnessing it forThe adhesion forces of liquid drops on superhydrophobic surfaces are typically in the nano-Newton range.[15]
A liquid supply chain was created out of a reservoir that delivers to a short capillary tube section, whose tip is placed in contact with a surface acoustic wave (SAW) nebulizer running at 30 MHz frequency using a small piece of tissue paper that constituted a capillary wick.[19]
When the energy is sufficient, destabilization of the liquid's free surface occurs. This leads to a breakup of capillary waves, generating a spray of aerosol droplets through a process known as SAW atomization or nebulization.[21,22]. When this spray of aerosol is channeled onto a semi-spherical superhydrophobic receptacle, larger drops develop on the receptacle surface from multiple coalescence events that are in uenced by gravity

Summary

Introduction

Superhydrophobic surfaces are illustrated in nature through the well-known examples of lotus leaves and the legs of water striders.[1,2] There has been a recent proliferation in methods reported to arti cially mimic these surfaces.[3,4,5,6,7] While the original opportunity of superhydrophobicity was in self-cleaning,[8] there is substantial effort aimed at harnessing it forThe adhesion forces of liquid drops on superhydrophobic surfaces are typically in the nano-Newton range.[15]. We will study the nature of how small aerosols form on these surfaces before evolving into single drops.

Results

Conclusion