Supporting Information

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1 Supporting Information Clustered Ribbed-Nanoneedle Structured Copper Surfaces with High- Efficiency Dropwise Condensation Heat Transfer Performance Jie Zhu, Yuting Luo, Jian Tian, Juan Li and Xuefeng Gao* Advanced Thermal Nanomaterials and Devices Research Group, Nanobionic Division, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, (P. R. China) * S1

2 Experimental Section Surface Nanoengineering: A facile electrochemical deposition method was used for in-situ growth of clustered copper hydroxide ribbed-nanoneedles on copper substrates. Commercial available copper foils (99.9 %, 30 mm 35 mm) and blocks (99.9 %, Φ26 mm 45 mm) were used as working electrode. They were cleaned by immersing in a 1 mol L -1 HCl aqueous solution for 10 min, rinsing three times with deionized water, and then drying with N 2. The counter electrode is a Pt foil with the diameter of 30 mm. The faint-blue nanoneedle films were obtained at a constant current density of 1.5 ma/cm 2 in a 2 mol/l aqueous solution of KOH for 1500 s (Chenhua, CHI660C, China). Fluorosilane Modification: The as-synthesized nanosamples and flat copper samples were modified with heptadecafluorodecyltrimethoxysilane (FAS, CF 3 (CF 2 ) 7 (CH 2 ) 2 Si (OCH 3 ) 3, Shin- Etsu chemical Co., Ltd., Japan). Typically, the samples were placed, together with a cup containing 10 µl FAS liquid, into a glass container (Φ145 mm 70 mm). The container was sealed with a cap and then heated for 2 h at 80 C. Morphological Characterization: The SEM images were taken using a field-emission scanning electronic microscope (Hitachi S4800, Japan) at 20 kv after sputtering an Au layer with thickness of 6 nm. Condensation Characterization: The condensation behaviors of the nanostructured and contrast flat surfaces at the ambient environment were captured by a high-speed Motion Analysis Microscope System (Keyence VW-9000, Japan) under magnification 50 ~ 500 with frame rates of 50~2000 fps. The samples were placed on a Peltier cooling stage with the substrate temperature of ~1 ºC, environment temperature of ~22 ºC and relative humidity of ~80 %. The details about the nucleation and growth of condensed microdrops on the nanostructured surfaces were further observed under high-resolution environmental scanning electron microscopy (ESEM, FEI Quanta250, USA). Here, the used vacuum pressure and vapor supersaturation in the ESEM chamber are ~800 Pa and ~1.2, respectively. At the beginning of the condensation experiment, a cold stage accessory was used to control the sample surface temperature at 284 K, slightly higher than the dew point (~276.8 K) to ensure a dry surface. Then, the sample surface temperature was slowly decreased to 274 K. To avoid the heating effect and surface chemistry damage caused by electron beam irradiation, the ESEM images were obtained under an electron beam potential of 20 kv, the spot size of 2 and diaphragm diameter of 30 µm, respectively. Thermal Characterization: The DCHT coefficients of copper blocks with polished surface and in situ grown nanostructures were tested in our customer-tailored heat exchanger. To ensure onedimensional axial steady-state heat transfer, a fin-integrated cylindrical copper block is inserted into a Teflon insulator that divides a steel chamber into a condensation chamber and a cooling chamber. Special measures have been taken to avoid the differences of the amounts of noncondensable gas in each trial so as to exclude their influences to heat transfer performance. Firstly, deionized water in the boiler was de-gassed by boiling for 30 min and then gradually cooled down to ~25 ºC, when the vapor inflow and release valves are kept in the closed and opened state, respectively. Then, the test chamber was vacuumed to ~600 Pa (closing to the limit of the vacuuming capability of our setup). After closing the vapor release valve and opening the vapor inflow valve, the whole system, including the test chamber, the boiler, the secondary S2

3 condenser and their connected pipes, was further vacuumed to the vapor pressure (3.17 kpa), corresponding to the saturated vapor pressure of water at 25 ºC. On the basis of the above vacuuming procedures, we can exclude the influences of the difference of amounts of noncondensable gas to the condensation heat transfer performance. Subsequently, the temperatures of the saturated water vapor were regulated to the presetting value of 25 ºC, 30 ºC, 35 ºC or 40 ºC, which corresponds to the saturated vapor pressure of 3.17 kpa, 4.24 kpa, 5.63 kpa and 7.00 kpa, respectively. Note that our as-prepared nanostructured films are chemically-stable under these low-tempature working conditions, frequently met in the case of heat-pipe-based electric chip cooling. Via regulating the pressure (P) of saturated vapor (with the corresponding temperature, T v ) and the temperature of coolant (with a fixed flow rate), we can measure the temperature gradient ( T) within the copper block using four equidistant K-type thermocouples (OMEGA TJ36-CASS-020U-6, USA), which can be used for calculating the surface temperature (T s ). At the steady state, all experiment data were collected by a data acquisition unit (Agilent 34970A, USA). All experiments were repeated three times at the identical operation conditions to ensure the repeatability. Usually, we can obtain the curves of the heat flux (q) and DCHT coefficients (h) varied with the degree of subcooling under different saturated vapor pressures according to the equations: q = k T and h = q/ T, where k is the thermal conductivity of copper and T is the degree of wall subcooling (i.e., the difference between T v and T s ). Figure S3b and S3c show the exemplified heat flux and DCHT coefficient curves corresponding to the saturated vapor pressure of 7.00 kpa, respectively. In view of common feature that the h values decrease with the increase of T and the mutually converted nature of the q and h values, we only present and compare the h values of the nanostructured and flat samples under varied P while fixing T = 1.3 ± 0.1 K (Figure 3b), which is the minimium avaliable for accurately measuring the h values in this case. S3

4 Figure S1. Time-lapse optical top-view images of the vertical nanostructured surface (a) and flat surface (b) under variable magnification, corresponding to prolonged condensation period. These tests are conducted at the controlled condensation condition of T s ~ 1 ºC, T air ~ 22 ºC and RH ~ 80%, which is the same with Figure 1. To observe the later growth and departure event of millimeter-scale condensate drops on the flat surface, the optical imaging is operated at the lower magnification ( 50), as shown in panel b. In contrast, to highlight the continuous self-propelling of condensed microdrops on the nanostructured surface during corresponding condensation period, their optical imaging are set at 500, as shown in panel a. Compared with the flat surface, the nanostructured surface can not only control the sizes of condensate drops at microscale but maintain higher drop number density during the whole condensation period, which is highly desirable to enhance condensation heat transfer. S4

5 Figure S2. a-c) The time-laspe environmental scanning electronic microscopy (ESEM) topviews showing the typical self-expansion growth mode of tiny condensates forming atop the crossed regions of clustered nanoneedles. S5

6 Figure S3. a) Optical images showing no apparent stickiness of the nanosample to the suspended water droplet (4 µl) during the contacting-compressing-releasing experiment. b) The recorded adhesive force curves showing that the nanosample almost has no detectable adhesive force even under severe compression. It is well-known that the adhesive force of condensed microdrops on the nanosample surface cannot be directly measured due to the limitation of characterization technique and the complexity of condensation events. Even so, it is still easily understood that the adhesive force of either the inner condensate or the suspended microdrops on the nanosample surface should be extremely low, especially as compared with the case of the flat surface, which is the key why these small-scale condensate can realize self-transport and self-departure under the weak surface tension. S6

7 Figure S4. a) Optical image of a nanosample used for characterizing the DCHT coefficient. b,c) The measured heat flux (q) and DCHT coefficients (h) of the nanosample (red) and the contract flat sample (black) varied as the degree of subcooling ( T) under the saturated vapor pressure of 7.00 kpa. S7