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Research Papers: Micro/Nanoscale Heat Transfer

# Surface Functionalization Mechanisms of Enhancing Heat Transfer at Solid-Liquid Interfaces

[+] Author and Article Information
Javier V. Goicochea1

Zurich Research Laboratory, IBM Research GmbH, 8803 Rüschlikon, Switzerlandjgo@zurich.ibm.com

Ming Hu

Zurich Research Laboratory, IBM Research GmbH, 8803 Rüschlikon, Switzerland; Department of Mechanical and Process Engineering, Laboratory of Thermodynamics in Emerging Technologies, ETH Zurich, 8092 Zurich, Switzerlandhum@ethz.ch

Bruno Michel

Zurich Research Laboratory, IBM Research GmbH, 8803 Rüschlikon, Switzerland

Dimos Poulikakos

Department of Mechanical and Process Engineering, Laboratory of Thermodynamics in Emerging Technologies, ETH Zurich, 8092 Zurich, Switzerland

1

Corresponding author.

J. Heat Transfer 133(8), 082401 (Apr 26, 2011) (6 pages) doi:10.1115/1.4003533 History: Received March 11, 2010; Revised January 11, 2011; Published April 26, 2011; Online April 26, 2011

## Abstract

Two mechanisms that enhance heat dissipation at solid-liquid interfaces are investigated from the atomistic point of view using nonequilibrium molecular dynamics simulation. The mechanisms include surface functionalization, where –OH terminated headgroups and self-assembled monolayers (SAMs) with different chain lengths are used to recondition and modify the hydrophilicity of silica surface, and vibrational matching between crystalline silica and liquid water, where three-dimensional nanopillars are grown at the interface in the direction of the heat flux with different lengths to rectify the vibrational frequencies of surface atoms. The heat dissipation is measured in terms of the thermal conductance of the solid-liquid interface and is obtained by imposing a one-dimensional heat flux along the simulation domain. A comparison with reported numerical and experimental thermal conductance measurements for similar interfaces indicates that the thermal conductance is enhanced by 1.8–3.2 times when the silica surface is reconditioned with hydrophilic groups. The enhancement is further promoted by SAMs, which results in a 20% higher thermal conductance compared with that of the fully hydroxylated silica surface. Likewise, the presence of nanopillars enhances the interface thermal conductance by 2.6 times compared with a bare surface (without nanopillars). Moreover, for different nanopillar densities, the conductance increases linearly with the length of the pillar and saturates at around 4.26 nm. Changes in the vibrational spectrum of surface atoms and water confinement effects are found to be responsible for the increase in conductance. The modification of surface vibrational states provides a tunable path to enhance heat dissipation, which can also be easily applied to other fluids and interfaces.

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## Figures

Figure 1

Top panel: a snapshot of silica-SAMs-water model structure. The silica crystal is in the middle with SAMs (n=8) chemically bonded to the surfaces. Water is on the top of SAM headgroup. Bottom panel: steady state temperature profiles for JQ=6000 MW/m2 for the chain length of SAMs, n=8.

Figure 2

Top panel: three-dimensional view of nanopillars on quartz surface (water molecules are not shown). Bottom panel: a snapshot of quartz-pillars-water model structure.

Figure 3

Thermal conductance of silica/SAMs/water system as function of temperature. Triangular symbols represent the overall thermal conductance between silica and water (i.e., dotted-dash line, n=8; dotted line, n=16). For comparison, the conductance between fully hydroxylated silica and water is also shown (black squares).

Figure 4

Steady state temperature profiles for JQ=5400 MW/m2, 6×6 cross section, and nanopillars of increasing length from 0 to 12 unit cells (unit cell length ∼8.5 Å). Solid circles and squares represent quartz and water temperatures, respectively. Temperature profiles have been displaced 10 K in the vertical direction from each other for clarity. Original average values for all curves were about 300 K.

Figure 5

Thermal conductance of the quartz/water interface (base) as function of nanopillar length expressed in unit cells (unit cell length ∼8.5 Å)

Figure 6

Density of states of silicon atoms and oxygen atoms in water located at the base quartz/water interface for different nanopillar lengths (i.e., 0, 2, 5, and 12 unit cells)

Figure 7

Spectral behavior of density of states difference (ΔVDOS) determined for silicon and oxygen atoms located at the base quartz/water interface. ΔVDOSSi−Ow is defined as |VDOSSi−VDOSOw|.

Figure 8

Spectral behavior of ∫ω2ΔVDOSSi−Ow(ω)dω determined for silicon and oxygen atoms located at the base quartz/water interface

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