Journal of Micromechanics and Microengineering
TOPICAL REVIEW
Carbon nanostructure based mechano-nanofluidics To cite this article: Wei Cao et al 2018 J. Micromech. Microeng. 28 033001
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Journal of Micromechanics and Microengineering J. Micromech. Microeng. 28 (2018) 033001 (16pp)
https://doi.org/10.1088/1361-6439/aaa782
Topical Review
Carbon nanostructure based mechano-nanofluidics Wei Cao1,2, Jin Wang3,4 and Ming Ma1,2,3,5 1
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, People’s Republic of China Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China 3 Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, People’s Republic of China 4 Department of Engineering Mechanics, Tsinghua University, Beijing 100084, People’s Republic of China 2
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[email protected] Received 26 October 2016, revised 3 January 2018 Accepted for publication 15 January 2018 Published 5 February 2018 Abstract
Fast transport of water inside carbon nanostructures, such as carbon nanotubes and graphenebased nanomaterials, has addressed persistent challenges in nanofluidics. Recently reported new mechanisms show that the coupling between phonons in these materials and fluids underconfinement could lead to the enhancement of the diffusion coefficient. These developments have led to the emerging field of mechano-nanofluidics, which studies the effects of mechanical actuations on the properties of nanofluidics. In this tutorial review, we provide the basic concepts and development of mechano-nanofluidics. We also summarize the current status of experimental observations of fluids flow in individual nanochannels and theoretical interpretations. Finally, we briefly discuss the challenges and opportunities for the utilization of mechano-nanofluidics, such as controlling the fluid flow through regulating the coupling between materials and fluids. Keywords: nanofluidics, carbon nanostructures, carbon nanotube, graphene, mechano-nanofluidics, vibration (Some figures may appear in colour only in the online journal)
1. Introduction
[15, 16]. Typical characteristics of the nano-, micro-, and milli-fluidic realms are shown in figure 1. Different from microfluidics, nanofluidics is influenced by interactions between the fluid and the walls of the system that lead to various unique transport phenomena. As the channel size shrinks from micrometers to nanometers, the surface properties of the channel walls play increasingly important roles in determining the mass transport in nanochannels, because of the high surface-to-volume ratio [3]. For example, water and ions can be transported across cellular membranes in a rapid, selective, and efficient way [17]. These biological nanofluidic channels are composed of narrow hydrophobic pores. To understand their functions, hydrophobic nanochannels such as carbon
Nanofluidics, introduced in 1995 as an analogue to the field of microfluidics, is defined as the study and application of fluid flow within a characteristic length scale down to 1–100 nm in at least one dimension [1–4]. It is recognized in a large number of classical disciplines, such as physiology, biology, chemistry, physics, colloid chemistry, and polymer science [5]. Major applications include nanofluidic energy conversion [6– 8], ultrafiltration and separation [9, 10], biomedical analysis [11, 12], seawater desalination [13, 14], and DNA sequencing 5
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Topical Review
J. Micromech. Microeng. 28 (2018) 033001
Figure 1. Typical characteristics of the nano-, micro-, and milli-fluidic realms. Insets from left to right show (a) the coupling between water and CNTs (reprinted by permission from Macmillan Publishers Ltd: [Nature Nanotechnology] [20], Copyright (2015)), (b) a large-scale and integrated chip to measure protein interactions (reprinted figure with permission from [21], Copyright (2005) by the American Physical Society), and (c) millidrops in aqueous bioreactors. [22] John Wiley & Sons. [© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].
nanotubes (CNTs) have been used to clarify the physics of rapid liquid flow and selectivity [18, 19]. Carbon nanomaterials have been widely applied in the study of nanofluidics for their unique structures and tunable physicochemical properties. Among the various carbon nanomaterials, graphene-based materials (graphene and graphene oxide (GO)) and CNTs possess ultrafast water transport properties, due to an ultralow potential energy barrier for water molecules sliding across the surfaces. It has been reported that water flow through CNTs is several orders of magnitude faster than that predicted by continuum models [23–25]. With the understanding of the transport mechanism, CNTs are widely applied in membrane separation and other related fields [26, 27]. Additionally, CNTs possess extraordinary mechanical, electronic, thermal, and chemical properties, making them attractive candidates for nanofluidic devices [28]. Graphenebased materials also offer a new approach to control mass transport at the nanoscale due to their ability to incorporate nanoscale pores and ultra-fast transport property [29–34]. Because of the minimum possible material thickness, high mechanical strength, and chemical robustness, these materials could be used to address persistent challenges in nanofluidics. Recently, theoretical studies have revealed that for CNTs and graphene, their transport properties for water are deeply related to their intrinsic mechanical properties such as phonons. For both materials, the coupling between water and phonons could lead to enhancement of the diffusion coefficient as large as 300% [20, 35]. Figure 2 shows the schematic of how the phonon modes of a CNT couple to water motion. It is believed that such coupling between phonons and liquid also holds for other carbon nanostructures. Since there are many methods to control the phonons, these results point to the development of mechano-fluidics in nanoscale objects as a new approach to couple nanofluidics and nano-electromechanics, especially for carbon nanostructures. These developments have led to an emerging field called mechano-nanofluidics, which studies the effects of mechanical actuation on the properties of nanofluidics. For mechano-nanofluidics, one important quantity used to characterize the coupling between the phonon modes of carbon nanostructures is the decay length δ. It can be estimated as δ = (2v/ω)1/2, where v is the kinematic viscosity of the
liquid and ω is the angular frequency of the surface oscillations. The decay length represents the depth within which the liquid responds to the surface oscillations. Taking CNTs as an example, by defining a dimensionless quantity J to show the balance of the decay length and the size of the CNT, J = Rδeff , (1)
we could calculate the fraction of confined liquid that responses to the oscillation as β = 2 J–J2, where Reff is the effective radius of the liquid inside the CNT. Since J
