Nanofluidics: a pedagogical introduction Simon Gravelle To cite this version: Simon Gravelle. Nanofluidics: a pedagogical introduction. 2016. hal-02375018 HAL Id: hal-02375018 https://hal.archives-ouvertes.fr/hal-02375018 Preprint submitted on 21 Nov 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Nanofluidics: a pedagogical introduction Simon Gravelle 01 MARCH 2016 1 Generalities reasonable expectation. Moreover, one can notice that most of the biological processes involving fluids 1.1 What is nanofluidics? operate at the nano-scale, which is certainly not by chance [8]. For example, the protein that regulates Nanofluidics is the study of fluids confined in struc- water flow in human body, called aquaporin, has got tures of nanometric dimensions (typically 1−100 nm) sub-nanometric dimensions [15, 16]. Aquaporins are [1, 2]. Fluids confined in these structures exhibit be- known to combine high water permeability and good haviours that are not observed in larger structures, salt rejection, participating for example to the high due to a high surface to bulk ratio. Strictly speak- efficiency of human kidney. Biological processes in- ing, nanofluidics is not a new research field and has volving fluid and taking place at the nanoscale attest been implicit in many disciplines [3, 4, 5, 6, 7], but of the potential applications of nanofluidics, and con- has received a name of its own only recently. This stitute a source of inspiration for future technological evolution results from recent technological progress developments. which made it possible to control what occurs at Hereafter, an overview of the current state of these scales. Moreover, advances have been made nanofluidics is presented. First, a brief state-of-the- in observation/measurement techniques, allowing for art, mainly focused on nano-fabrication and mea- measurement of the small physical quantities inher- surement techniques is given. Then some current ent to nano-sized systems. applications linked to nanofluidics are described. Even though nanofluidics is born in the footstep of microfluidics, it would be incorrect to consider it an 1.2 State-of-the-art extension of microfluidics. Indeed, while in microflu- idics the only scale which matters is the size of the Nanofluidics has emerged from the recent progresses system, nanofluidics has to deal with a large spec- of nanoscience and nanotechnology, such as pro- trum of characteristic lengths which induce coupled gresses made in developing nano-fabrication tech- phenomena and give rise to complex fluid behaviours nologies. Fabricating well-controlled channels is a [8]. Moreover, since nanofluidics is at the intersection major challenge for nanofluidics, and is a necessary between physics, chemistry and biology, it concerns condition for a systematic exploration of nanofluidic a wide range of domains such as physiology, mem- phenomena. This requires a good control of device brane science, thermodynamics or colloidal science. dimensions and surface properties (charge, roughness, Consequently, a multidisciplinary approach is often etc). For example, the improvement of lithography needed for nanofluidics' research. techniques (electron, x-beam, ion-beam, soft...) al- Some striking phenomena taking place at the lows the fabrication of slit nanochannels [17]. Focus nanoscale have been highlighted during the past few Ion Beam (FIB) allows to drill nanopores in solid years. For example, super-fast flow in carbon nan- membranes [18, 19]. There are also coatings and de- otubes [9, 10, 11], nonlinear eletrokinetic transport position/etching techniques that can be used to tune [12, 13] or slippage over smooth surfaces [14] have the surface properties [20, 21]. Siria et al. were able been measured. Those effects are indicators of the to manipulate a single boron nitride (BN) nanotube richness of nanofluidics. Accordingly, this field cre- in order to insert it in a membrane separating elec- ates great hopes, and the discovery of a large variety trolyte reservoirs and perform electric measurements of new interesting effects in the next decades is a [22]. Great developments of Scanning Tunneling Mi- 1 croscope (STM) or Atomic Force Microscopy (AFM) that ensure the flow of ions across cell membranes allow to characterize the fabricated devices. [33, 34]. Combined, those proteins allow the (human) In parallel, the efforts invested in nanofabrication kidney, which is an example of natural desalination have been combined to an improvement of measure- and separation tool, to purify water with an energy ment techniques. Most of them are based on the cost far below current artificial desalination plants measurement of electric currents, and have been de- [8]. veloped since the early days of physiology. But for Desalination { At the same time, some of the most a full understanding of nanofluidic properties, other used (man-made) desalination techniques, consisting quantities have to be made accessible. For exam- in the separation of salt and water in order to produce ple, local values of a velocity field can be obtained fresh water, are using nanofluidic properties [35, 36]. using nano-Particle Image Velocimetry (nano-PIV). This is the case of membrane-based techniques, such Surface Force Apparatus (SFA) have been used to as Reverse Osmosis (RO) [37], Forward Osmosis (FO) explore the hydrodynamic boundary condition and [38] or ElectroDialysis (ED) [39]. The improvement measure forces that play an important role in nanoflu- of the membrane technology has made it possible to idics, such as van der Waals or electric forces. desalinate with an energy consumption close to the In addition, a current challenge concerns water minimum energy set by thermodynamics. flow measurements. The main difficulty is due to Extraction of mixing energy { Another inter- the magnitude of typical flows through nanochannels: esting application of nanofluidics concerns the ex- ∼ 10−18 m3/s (it would take several years to grow traction of the energy of mixing from natural wa- a drop of 1 nl with such a flow). In order to detect ter resources. This so-called blue energy is the en- a water flow through a nanochannel, some poten- ergy available from the difference in salt concentra- tial candidates have emerged during the last decade. tion between, for example, seawater and river water. One can cite, for example, Fluorescence Recovery Af- Pressure-Retarded Osmosis (PRO) converts the huge ter Photobleaching (FRAP), confocal measurements pressure difference originating in the difference in salt or coulter counting measurements that have been concentration (∼ bars) between reservoirs separated reported to detect respectively 7 · 10−18 m3/s [23], by a semipermeable membrane into a mechanical 10−18 m3/s [24] and 10−18 m3/s [25]. However, the force by the use of a semipermeable membrane with inconvenient of most of the existing measurement nanosized pores [40]. Siria et al. proposed another techniques is that they are indirect and require the way to convert blue energy based on the generation of use of dyes or probes. an osmotic electric current using a membrane pierced Meanwhile, numerical progresses combined with with charged nanotubes [22]. calculation capacity improvement allow for the theo- Nanofluidic circuitry { The recent emergence of retical exploration of a large variety of nanofluidic nanofluidic components benefiting of the surface ef- properties. For example, the friction of water on solid fects of nanofluidics leads naturally to an analogy surfaces can be investigated using ab initio methods with micro-electronics. Indeed, some nanofluidic com- [26], while molecular dynamics simulations are good ponents imitate the behaviour of over-used micro- candidates for fluid transport investigations [27, 28]. electronic components such as the diode or the tran- sistor [12] 2. Even if a complete analogy between both 1.3 Applications fields fails due to the physical differences between ions and electrons, controlling/manipulating nano- Some important applications of nanofluidics are listed flows the same way we control electric currents would hereafter. allow for regulating, sensing, concentrating and sep- Biology { First of all, most of the biological pro- arating ions and molecules in electrolyte solutions cesses that involve fluids take place at the nanoscale [42] with many potential applications in medicine, [15, 29, 30, 31]. For example, the transport of water such as drug delivery or lab-on-a-chip analyses. through biological membranes in cells is ensured by An overview of the full complexity of nanofluidics aquaporins, a protein with subnanometric dimen- is highlighted in the following by the description of sions. Aquaporins appear to have an extremely high some theoretical bases. The first part provides the water permeability, while ensuring an excellent salt definitions of the characteristic lengths that separate rejection 1. Another example of proteins with nano- the different transport regimes and lead to a large metric dimensions are ion pumps and ion channels, variety of behaviours. The second part describes 1Note that the shape