A conceptual thermal actuation system driven by interface tension of nanofluids

Abstract

In a system containing nanoporous materials and liquids, the well-known thermo-capillary effect can be amplified by the ultralarge specific surface area of the nanopores. With appropriate temperature change, the relative wetting-dewetting transition can cause the liquid to flow in or out of the nanopores, and part of the thermal energy is converted to significant mechanical output. A conceptual design of such a thermal actuation/energy conversion/storage system is investigated in this paper, whose working mechanism, i.e. the thermally dependent infiltration behaviors of liquids into nanopores, is analyzed using molecular dynamics simulations. The fundamental molecular characteristics, including the density profile, contact angle, and surface tension of the confined liquid molecules, are examined in considerable detail. The influences of pore size, solid phase and liquid species are elucidated, which couple with the thermal effect. The energy density, power density, and efficiency of the thermal actuation system are evaluated. An infiltration experiment on a zeolite/water system is performed to qualitatively validate these findings.

Fluid infiltration pressure for hydrophobic nanochannels

“…A lthough Eq. (1) may not be valid for nanochannels, it has been reported that the pressure needed to drive w ater into carbon nanotubes (CN Ts) at room tem perature ranges from around 50 to 300 MPa, depending on the size o f CN Ts [11][12][13]. For nanoporous zeolite with pore size of about 1 nm, the infiltration pressure o f w ater at room tem perature is about 8 0 -1 5 0 M Pa [14,15].…”

“…Both com puted and m easured infiltration pressures are higher than the predictions o f Eq. (1) ' mezli@ust.hk [11][12][13][14][15][16][17][18], O ther than the w ork in Refs. [1 1 -1 5 ], experim ents on w ater infiltration into nanoscale hydrophobic silica pores show that the infiltration pressures are about 1.5-2 tim es higher than the Young-Laplace equation [16].…”

“…To reduce the discrepancy betw een experim ents and theory, Eq. (1) has been m odified by incorporating a disjoining pressure [16] or colum n resistance [17], It has been shown that the infiltration pressure depends on the surface properties, tem perature, and geom etry o f nanopores [11,12,14,15,[19][20][21][22][23], M olecular dynam ics sim ulations have m ainly been used to study how the pore size and tem perature affect the fluid infiltration in CN Ts, w here the surface properties are invariant [11,12,21,22], The general effects o f surface properties have been exam ined by experim ents, w hich have dem onstrated that p\"{ can be changed through surface treatm ent; however, the detailed dependence o f p mt on the surface properties or contact angle still rem ains unknow n [19,20]. O ur previous w ork on the flows in nanochannels reveals that the m olecular interactions betw een fluid m olecules and wall atom s play a critical role and m ake the classic N avier-Stokes equation invalid in certain conditions [6,24,25].…”

A new working mode for molecular springs: water intrusion induced by cooling and associated isobaric heat capacity change of a {ZIF-8 + water} system

“…Similar difference of intrusion and extrusion curves can be seen on PV-diagrams at room temperature obtained by Patarin and co-workers 12 . The properties and characteristics determined for the investigated {ZIF-8 + water} molecular spring may be extended to numerous applications in the field of mechanical and thermal energy storage as well as for thermal to mechanical energy transformation 1,2,19 . Previously non-observed simultaneous and reversible effect of temperature and pressure on porosity of ZIF-8 ( Fig.1) is important for MOF applications 14 .…”

Graphene nanoribbon-guided fluid channel: a fast transporter of nanofluids

“…That thermal and electrical stimuli may serve as an additional way to control nanofluids to make the system highly responsive to multiple external fields deserves note. 20,21 Moreover, use of nanoribbons and graphene sheets in NRCs also permits the utilization of the remarkable mechanical, thermal, and electronic properties of these materials to create highly multifunctional systems. Normalized density profile, axial velocity profile, standard deviation of axial velocity, and in-plane velocity profile along the y-direction (thickness direction, vertical) for the steady-state transport of water through four NRCs at 50 m s À1 .…”