Saat mendengar istilah 'nano', kebanyakan orang akan terpikir mengenai hasil rekayasa canggih yang memerlukan teknologi terkini dan biaya yang besar. Padahal istilah nano, yang pada banyak kasus seharusnya ditampilkan lengkap sebagai 'nanometer', hanya menunjukkan skala panjang yang besarnya sepersemilyar meter, tidak lebih dari itu. Fenomena alami dengan skala nanometer di sekitar kita pun sebetulnya sangat banyak jumlahnya.

Sebagai contoh, kita bisa coba tengok fenomena nano-fluida, alias fenomena-fenomena mekanika fluida dengan skala nanometer. Fenomena seperti itu bisa mudah ditemui baik di permukaan daun teratai, maupun di dalam sel tubuh kita sendiri. Atau, bisa juga kita amati masalah apa yang dihadapi bakteri "E.coli" untuk bisa berenang (sesuatu yang terdengar mudah buat kita), dan apa solusi yang mereka pakai.

Mengamat-amati fenomena nano-fluida tentunya tidak hanya bermanfaat untuk memenuhi keingintahuan belaka. Contoh aplikasi praktis yang nyata meliputi perbaikan teknologi pelumasan di mesin-mesin, peningkatan efisiensi sel surya, hingga pengembangan mikro-divais untuk analisa DNA.

Di bawah, saya salin-tempel sepetik tulisan dengan judul "Life in a nanofluidic world". Mudah-mudahan ada manfaatnya, paling tidak untuk selingan di akhir pekan. Tulisan tersebut juga saya posting sebagai 'teaser' untuk publikasi disertasi saya, yang insya Allah akan dipertahankan dalam sidang terbuka pada awal tahun depan. Bagi yang tertarik baca-baca lebih lanjut mengenai nano-fluida di waktu senggang, bisa juga menengok kedua link berikut ini di internet:
http://en.wikipedia.org/wiki/Nanofluidics
http://www.qi.tnw.tudelft.nl/~gea/

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"Life in a nanofluidic world"

Nanofluidics describes the phenomena of fluids, which comprise liquids and gases, at the nanometer scale. These phenomena can be found abundantly in nature. For example, the cells that build our bodies work in a nanofluidic environment. Another example is the self-cleaning ‘Lotus effect’, where interactions between water and nanometer structures on the surfaces of Lotus leaves allow raindrops to roll off and take dust particles and micro-organisms away from the leaves.

Nanofluidic phenomena are not just *smaller* than fluidic phenomena at larger scales, but also *different*, and frequently seem counter-intuitive to us. To quantitatively portray one example of those differences, we can look at one particular dimensionless number: the Reynolds number, R. It is defined as:
R = (rho x v x L) / (mu) = (inertial forces) / (viscous forces)
with rho as the fluid density, v the bulk fluid velocity, L the characteristic length in the system, and mu the dynamic fluid viscosity. At the nanometer scale, where the value of L is very small, the Reynolds number also becomes very small. Consequently, the inertial forces become negligible compared to the viscous forces. In 1976, E.M. Purcell gave an enlightening talk on how we would live in a world with a very small Reynolds number. The significant viscous forces dictate that we would move as if we are immersed in highly viscous syrup, such that swimming would be an immensely hard task to perform. Any swimming style that uses reciprocal motion (that is, where we change our body shape into another shape, and then we go back to the original shape by going through the sequence in reverse) would be rendered useless at a very small Reynolds number, and we would not go anywhere because we have no inertial forces to use. The bacteria E. coli solve this problem by performing a type of nonreciprocal motion: their tails (which are called flagella and have a diameter of approximately 13 nm) are shaped as a helix and turn continuously around a rotary joint.

Another important difference arises as we shift from large-scale fluidics to nanofluidics, namely the increasing importance of surface-effects as compared to bulk-effects. Imagine a volume of fluid confined by the walls of a solid cube. In large-scale fluidics, only the parts of the fluid in the vicinity of the walls are significantly influenced by the existence of the cube, while the other parts are not. When the characteristic length of the cube is reduced to the nanometer scale, however, the confining walls influence most parts of the fluid. As the characteristic length is reduced, the surface-to-volume ratio gets higher, and the interfaces surrounding the volume of fluid become more important. The behaviour of fluids at interfaces is characterized by the surface tension, γ. The rising of water along a capillary column, for instance, can be explained by the surface tension in the water-air-solid interfaces. The pressure difference induced by the surface tension can reach more than ~10 bar for a capillary diameter of 100 nm. Hence if a liquid touches the entrance of a nanofluidic capillary, the liquid will spontaneously fill the capillary, assuming the capillary surfaces are not hydrophobic. Another important surface-effect originates in the existence of ions at the solid-liquid surfaces. The solid surfaces generally have a certain surface-charge, caused by ionization processes. This effect is very important in a nanofluidic environment, for example in biological cells, where ions and surfaces are ubiquitous. A gradient of electrical potential induces motions of the ions, which in turn drives the transport of fluid and objects. These electrical-driven transport phenomena are called electrokinetics.

Nanometer-scale objects immersed in fluids also behave differently than their large-scale counterparts. Due to the very small dimensions, gravitational forces on (and sedimentation of) such objects are negligible. On the other hand, the Brownian motion becomes much more significant for them. This motion is a random movement of small objects due to the constant bombardment from the thermally excited liquid molecules surrounding them. Due to the combination of the Brownian motion and the low Reynolds number in nanofluidics, mixing of two liquids inside nanofluidic environments depends strongly on diffusion rather than on inertial forces (e.g. by stirring the liquids).