Nanoscale wetting and energy transmission at solid-liquid interfaces

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Nanoscale wetting and energy transmission at solid-liquid interfaces John Tomko, David Hans Olson, Ashutosh Giri, John Thomas Gaskins, Sean M. O'Malley, and Patrick E. Hopkins Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03675 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Nanoscale wetting and energy transmission at solid-liquid interfaces John A. Tomko,† David H. Olson,‡ Ashutosh Giri,‡ John T. Gaskins,‡ Sean M. O’Malley,¶ and Patrick E. Hopkins∗,† †Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA ‡Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904, USA ¶Department of Physics, Rutgers University - Camden, Camden, NJ 08102, USA §Department of Physics, University of Virginia, Charlottesville, VA 22904, USA E-mail: [email protected]

Abstract Understanding the effects and limitations of solid/liquid interfaces on energy transport is crucial to applications ranging from nano-scale thermal engineering to chemical synthesis. Up to now, the majority of experimental evidence regarding solid/liquid interactions has been limited to macroscale observations and experiments. The lack of experimental works exploring nanoscale solid/liquid interactions has been accentuated as the body of knowledge from theory and simulations at these scales has exploded in recent years. In this study, we expand on current nanoscale thermal measurement techniques in order to more fully understand solid/liquid interfacial energy transport. We use thermal ablation threshold measurements on thick Au films in various liquids as a metric to describe thermal transport at the Au/liquid interface. Furthermore, using ultrafast pump-probe experiments, we gain insight of this transport

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through picosecond ultrasonic coupling at solid/liquid interfaces with known macroscopic observations. We find significant variation in both ablation threshold and damping of the acoustic modes within the Au films depending on nanoscopic interactions at the solid/liquid interface rather than typical macroscale metrics such as acoustic mismatch, measured contact angle, or work of adhesion.

Introduction Nanoscale interactions and dynamics between solids and liquids are ultimately the defining factor in a range of applications, including, but not limited to, microfluidics, 1–3 chemical catalysis, 4–6 and microelectronic thermal management. 7,8 An increased understanding of such nanoscale dynamics provides insight into the mechanisms of various interfacial phenomena, such as protein folding and adsorption, 9–11 cell formation, 12,13 and other self-assembly processes. 14–16 While countless studies have focused on attempting to understand these solid/liquid interactions at the macroscale, common metrics such as contact angle become oversimplistic and fail to realize the complex phenomena occuring on the nanoscale. For example, works alluding to nanoscale effects at interfaces attempt to relate measurements of contact angle to chemical composition and molecular scale thermodynamic effects at the solid/liquid interface. 17,18 Additionally, other works have gone in the opposite direction: manipulating the molecular order at a solid/liquid interface, such as structure and composition, to control macroscale wetting. 16,19 While general empirical trends have been observed that relate macroscopic measurements of solid/liquid interactions to nanoscale interfacial properties, these trends assume that macroscopically observable interactions scale to the nanoscale interfacial region. However, only recently have the necessary models and theories been developed to enable a more fundamental understanding of solid/liquid interactions at the nanoscale. 20 Thus, it is critical to develop novel and feasible experimental probes that can interrogate these nanoscale solid/liquid dynamics to validate these theories. This will facilitate experimental insight into the true nature of the wetting of a liquid on a solid surface at length scales much smaller than typical contact angle measurements. 2


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The majority of these recent experimental advances have investigated the structural component of a liquid interacting with a solid. Work studying the atomic arrangement of liquids at an interface, 21–23 as well as the movement, 24–26 or “flow”, of such atoms has provided much of the framework for understanding of solid/liquid interfaces. Though the structural and mechanical nature of these interfaces are an undeniably important first-step in understanding phenomena driven by these interactions, insight into the exchange of energy across solid/liquid interfaces would be expected to provide complementary and novel information regarding the nature of wetting of surfaces at the nanoscale. As referenced prior, many of the macroscale phenomena driven by these nanoscale interactions are ultimately dependent on the ability for energy to couple from one phase to the next. With respect to thermal transport, recent work has pushed the theoretical understanding of vibrational scattering and energy exchange across solid/liquid interfaces. 32–36 In addition to this, fundamental theory, modeling, and simulations have helped frame a basis to suggest that the strength of the bond at the solid/liquid interface, which can determine the equilibirum contact angle, can be correlated to the efficacy of thermal transport. 37–46 Limited experimental results involving liquids other than water are available to validate and shape these works. In general, various experimental works have shown that greater hydrophobicity at solid/water interfaces, determined via contact angle experiments, leads to a decrease in thermal boundary conductance (TBC), a measure of nanoscale energy transport across the interface. 47–49 In general, these results imply that a smaller contact angle, and hence better “wetting”, leads to greater thermal boundary conductance. However, experimental works studying energy coupling across planar solid/liquid interfaces in which the liquid is not water, and relating this energy coupling to wetting, are scarce. The strengths of the intermolecular forces in a liquid relative to the interfacial bonding environment are the underlying mechanism that drives the manifestation of contact angles and wettability. 50 In the extreme case c c of weak intermolecular interactions in a liquid (e.g., fluorinatied fluids such as 3M Fluorinert ),

comparable liquid-liquid and solid/liquid interaction energies can lead to macroscopic observables and uncertainties (e.g., contact angle measurements) that are not indicative of the true nanoscale

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Figure 1: a) Sensitivity plot of the ratio of in-phase to out-of-phase signal in time-domain thermoreflectance (TDTR) at 8.8 MHz for a Au/water (blue) and Au/FC70 (red) interface when fitting for thermal conductivity (κ) and thermal boundary conductance (TBC). b) Experimental TDTR data with the line of best fit for SiO2 /Au/FC70 and Au/H2 O stacks. Note, a bidirectional model is used as we probe through the transparent SiO2 slide. c) Normalized residuals (i.e., the model’s fit relative to the best-fit thermal decay curve) for a single TDTR scan with an SiO2 /75 nm Au/FC70 stack. Assuming a 5% error in the Au metal transducer thickness, the minimum possible thermal boundary conductance for Au/FC70 varies from roughly 1 to 5 MW m-2 K-1 . d) Measured thermal boundary conductance for various Au/solid (Au/GaN, 27 Au/SiNx , 28 Au/SiO2 , Au/Cu, Au/Al2 O3 , Au/Ti, and Au/Si 29 ), Au/liquid (Au/Methanol, Au/Ethanol, Au/Toluene, Au/Hexane, 30 Au/H2 O 31 ) from literature (open circles), and Au/liquid (Au/H2 O, Au/Ethanol and Au/FC70), from this work, interfaces as a function of the ratio of the longitudinal sound speeds of the two media. The blue circle denotes the bounds of the lower limit of thermal boundary conductance for Au/FC70 interfaces assuming a 5% error in metal transducer thickness.

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interactions and resulting dynamics at the solid/liquid interface. This leads to the following major question regarding solid/liquid energy coupling on the nanoscale: How valid is the use of macroscale wettability (e.g., contact angles) in describing the nanoscale solid/liquid interactions that drive energy transport across solid/liquid interfaces? We seek to answer this question in our work. Thus far, the limited experimental studies probing the nanoscale thermal transport interactions at solid/liquid interfaces have shown that optical pump-probe techniques are particularly well-suited for this application. 47,49,51–54 Traditional thermoreflectance techniques, the current standard for measurement of interfacial energy transport, lack sufficient sensitivity to energy transport across solid/liquid interfaces as discussed further below. Thus, in this work, we develop an altered pump-probe schematic that provides enhanced sensitivity to the measurement of acoustic phonon propagation (i.e., picosecond acoustics) in a solid metal film in contact with a liquid. As acoustic phonons are the primary thermal carrier in metals that drive the thermal boundary conductance (TBC) across metal/non-metal interfaces, 55–57 the damping of such modes upon interaction with the solid/liquid interface provides a metric describing energy transport across this interface. 52,58–60 We perform these measurements on an array of gold-liquid interfaces; the results are compared to macroscale wettability determined via contact-angle measurements. In addition, we measure the thermal ablation threshold, a separate metric of interfacial energy transport, 29 of Au thin films submerged in these various liquids; the two methods are found to be in good agreement. In both cases, we find a deviation from our metric describing nanoscale energy transport from that suggested by macroscale wettability.

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Failure of thermoreflectance techniques to measure TBC From the perspective of macroscale equilibrium thermodynamics, the interface between two media are described by the Dupré equation,

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where γ represents the surface energies of the solid/liquid interface (γSL ) between the solid medium (γS ) and the liquid medium (γL ), and WSL represents the work of adhesion between the two media. 61,62 These surface energies are defined as the energy per area required to form the surface of a given material. As interfaces form, the energy required to form these surfaces is reduced as the two media begin to form bonds; this is the work of adhesion. Many works determine this adhesive energy at the solid/liquid interface, quantified by WSL , through the Young-Dupré equation,

WSL ≈ γL (1 + cos(θ)),

(2)

where θ is the measured contact angle of the wetting liquid. 62 This formulation has held remarkably true down to atomic scales as demonstrated by Shenogina et al. 40 via molecular dynamics simulations. However, in situations where contact angles are hard to define, such as those at interfaces between solids and so-called ‘highly-wetting fluids,’ as determined via macroscopic techniques, this relationship is difficult to experimentally realize. Attempts to move beyond the use of Eq. 2 to quantify solid/liquid interaction strength and resulting energy transport have been made through methods such as liquid density depleted boundary layers and depletion lengths near solid/liquid interfaces. These quantities have been described in computational studies as fundamental parameters of interfacial thermal transport. 63 However, the experimental measurement of density depletion is non-trivial, requiring advanced methods such as neutron reflectivity measurements, 64 and has not been applied extensively to many interfaces, 64,65 creating a lack of validation between these computational studies and experimental

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works investigating energy transport at solid/liquid interfaces. More readily accessible thermoreflectance techniques, namely time-domain and frequency-domain thermoreflectance (TDTR and FDTR, respectively), have been used to measure interfacial energy transport in planar geometries, providing unique insight to nanoscale energy exchange in many material systems, 66,67 including select solid-liquid interfaces. 30,47,49,68 While this method would be optimal for probing the solidliquid interactions on the nanoscale due to its routine use in measurements of TBC, as thermal boundary conductance is intimately related to the interfacial bonding environment, 58,60,66,69–74 the experimental insensitivity to the interfacial resistance posed by a solid-liquid interface, due to the large thermal resistance of liquids relative to that of the interface, make this approach prone to large uncertianites or, in some cases, unmeasurable at interfaces involving low thermal effusivity fluids. We quantify this lack of sensitivity and resulting uncertainty in measuring solid-liquid thermal boundary conductance via TDTR measurements in Fig. 1. The relative sensitivity of the interfacial resistance is significantly lower compared to that of the liquid thermal conductivity, as shown in Fig. 1a for both water and the fluorocarbon-based liquid, FC70. Note, we calculate this measurement sensitivity based on the TDTR sensitivity analysis presented in Ref. 75 We experimentally confirm this predicted lack of sensitivity with TDTR measurements of 75 nm Au films on amorphous SiO2 substrates submerged in the respective liquids; the experimental data and thermal model fit are shown in Fig. 1b. For fitted values of both liquids, we find that only a lower limit to thermal boundary conductance can be obtained; this lower limit is found to be as low as ∼1.9 MW m-2 K-1 for an FC70/Au interface and ∼40 MW m-2 K-1 for H2 O/Au interfaces. To determine these bounds, we generate theoretical model curves with varying thermophysical parameters, namely the thickness of the metal transducer, around the best fit values obtained with least-square minimization of our thermal model to the measured data. We quantify the difference between the theoretical and experimental curves using a normalized residual. An example contour, where we consider 5% error in the Au thickness, shows the minimum range for which the Au/FC70 thermal boundary conductance allows no more than 1% error in the minimum value. As seen, a 1% residual encompasses TBC values no less than ∼1 to ∼5 MW m-2 K-1 over this possible range

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of film thicknesses, yet the contour diverges and allows for infinitely high TBC values within the 1% residual bound for only slightly greater film thicknesses. Note, the best-fit value (∼68 MW m−2 K−1 ), as well as the bounds from error analysis, for the H2 O/Au interface remains within the range of previously reported measurements. 31,32,47 It is also worth noting that the TBC at the FC70/Au interface is the lowest value for TBC ever measured at room temperature. This result is rather counter intuitive given that FC70 is considered a “highly wetting fluid”, and traditionally highly wetted interfaces are thought to imply higher TBCs than non-wetted interfaces. However, the ability to only measure a lower bound to TBC with correspondingly large uncertainty restricts a more conclusive finding using TDTR. Note, this large uncertainty is due to the inherent limits of the thermal model used in TDTR; as the liquid thermal conductivity reduces, the uncertainty associated with the thermal boundary conductance at the solid/liquid interface increases. Thus, for low thermal conductivity materials or liquids, in a bi-directional heat transfer model, the uncertainty in TBC will be inherently large. In considering this TBC associated with Au/fluorocarbon solutions, it is useful to compare the measured value to the interfacial thermal resistances associated with other Au interfaces. Assuming a simple acoustic mismatch model (AMM) for interfacial resistances, 68 one would predict an increase in TBC with increasing overlap of the sound velocities, ν, of the two media, as the phonon density of states are proportional to these velocities from the perspective of the Debye model (i.e., as ν film /ν substrate approaches 1, TBC is expected to increase). As shown in Fig. 1d, the experimentally-measured thermal boundary conductances for various Au/solid and Au/liquid interfaces are plotted as a function of the ratio of the longitudinal sound velocities for the two media comprising the interface; we only consider Au interfaces as their TBCs are considerably lower than most metal/dielectric interfaces when no adhesion layer is utilized. 55,76,77 As expected, one can observe a general increase in TBC for liquids that have greater overlap in their vibrational density of states relative to the Au film. In the case of Au/FC70, the blue circle denotes the limits of the possible lower bounds in the thermal model. As the value of best fit in the model depends on the thermophysical parameters of the substrate and metal film, we consider a potential of 5% error

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in the film thickness, one of the model’s most sensitive parameters. As seen, the obtained lower bound for Au/FC70 interfaces is anomolously low compared to previously measured Au interfaces. While this lower bound is within the sensitivity/experimental limitations of TDTR, the ability to obtain a nominal value, or even meaningful upper bound, for TBC with various Au/liquid interfaces, that are poorly wetted as the nanoscale, is not plausible. The inability to define an upper bound is supported through consideration of the mean square deviation of the thermal model applied to the experimental TDTR data obtained for the two liquids. As shown in the contour plot in Fig. 1c, which represents this mean square deviation when fitting for various combinations of thermal boundary conductance of the solid/liquid interface and thermal conductivity of the liquid layer, any value greater than this lower limit associated with the TBC leads to the same quality of fit to our data. 78,79

Picosecond acoustics to quantify phonon transmission To overcome this lack of sensitivity, we seek alternative pump-probe experiments to quantify nanoscale energy transport at solid-liquid interfaces. Namely, we probe the damping of acoustic phonon modes (e.g., picosecond acoustics) in the solid layer upon interaction with the solidliquid interface. This measurement allows for optical detection of the propagation of acoustic modes through the piezo-optic effect of energy coupling between the solid film and contacting layers, 52,80,81 and can be applied outside of thin Au films, as demonstrated in various material systems including polymers and other amorphous materials, 80,82–84 semiconductors, 85–87 superlattices, 88,89 etc. The time-dependent intensity of these picosecond acoustic signals can be described via 58

I(t, T ) = A ∗ exp(−Γt) ∗ cos((2π/T )t − δ) − B ∗ exp(−t/τ ),

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where A, B, and δ are scaling factors, Γ is the damping due to interfacial energy transfer, T is the period of the pressure front, and the second term, exp(−t/τ ), accounts for the thermal decay rather than the signal associated with the acoustics. Fitting this equation to the picosecond acoustic signal 10


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Figure 3: a) Measured transmission of acoustic phonon modes at the solid-liquid interface as a function of the work of adhesion calculated via Young-Dupré equation for various liquids on 75 nm Au films. Note, the values are normalized by the nominal value of air for 75 nm Au films and the dashed line is a guide to the eye for an acoustic transmission value equivalent to the value of an Au/air interface. In these experiments, the nominal transmission coefficient can only be considered a qualitative measure due to the brillioun scattering/interference induced by a pressure-front in the underlying substrate leads to an inability to quantify the transmissivity of acoustic modes. b) Measured transmission of acoustic phonon modes as a function of the measured contact angle. As seen, there remains a deviation between the measure of nanoscale energy transport and macroscale metrics for solid/liquid wetting. c) Measured ablation threshold of 200 nm Au films as a function of the work of adhesion calculated via Young-Dupré equation for various liquids on Au surfaces. d) Measured ablation threshold as a function of the measured transmission coefficient in various media; the two measures of nanoscale energy transport show great agreement.

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over the first nanosecond provides us with two important variables: the period, T, and damping, Γ, of the acoustic modes. After obtaining these values, the transmission of these acoustic modes across the solid-liquid interface can be determined as

T ransmission = 1 − exp(−ΓT ),

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Although these signals inherently exist in TDTR, as the pump always creates an acoustic wave propagating through the metal film, its detection is non-trivial, moreso when using an Au transducer due to its low piezoreflectance response at the commonly used TDTR probe wavelength of 800 nm. 90,91 Typically, this can be overcome by performing transient transmission experiments with the use of thin (i.e., partially transparent) metal films, which produce much larger responses. These gains in signal from testing in a transmission configuration can potentially be obscured by non-linear interactions between the beam and liquid as the beam passes through the liquid. Thus, we alter our typical TDTR layout to enhance the picosecond acoustic signal in a reflection geometry with optically opaque films, allowing for quantitative analysis of acoustic wave transmissivity across Au/liquid interfaces. To enhance the picosecond acoustic signal, we make two major adjustments: the film is moved out of the Rayleigh length of the focused probe beam waist and a partially-closed iris is implemented just before the photodiode. In moving the sample to the linear regime of a focusing beam, as the strain wave propagates to the surface of the transducer, a larger change in spot-size occurs at the sample surface. This deviation in spot-size is more easily detected with the addition of an iris prior to the photodiode; as the back-reflected probe beam is focused to the photodiode surface, the iris partially clips the edges of the beam. With changing spot size, the spatial variation leads to increased (or decreased) portions of the beam being cut-off by the iris. 60 This deviation is then translated to a change in signal at the photodiode. These modifications are depicted in Fig. 2a. This alteration leads to a drastic increase in sensitivity to picosecond acoustic signals during TDTR measurements on 75 nm Au films supported by amorphous SiO2 substrates. The signals before and after these adjustments are demonstrated. It should be noted that this technique simultaneously en12


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hances the change in signal produced from brillouin scattering within the substrate as shown in Fig. 2b. At our probe wavelength of 800 nm, Brillouin scattering leads to optical interference between the partial reflection of the beam with the pressure front propagating through the substrate and the primary reflection at the Au/SiO2 interface with an oscillation frequency of ∼21.19 GHz. 92 To avoid the creation of beat frequencies and distorted signals, our choice of a 75 nm Au film ensures the picosecond acoustic signal of the film is in resonance with this interference frequency; the resonance of the two effects allows us to determine relative decay between the varying media. Thus, all transmission coefficients are normalized to that of the 75 nm Au film in air, as we can not decouple the two phenomena. As can be seen in Fig. 3a, the measured acoustic transmission at across various solid/liquid interfaces shows no discernible trend. For example, Fomblin provides the same acoustic attenuation as that of air, whereas Ethanol exhibits nearly a 60% increase, despite the two liquids having nearly the exact same value for their work of adhesion. Additionally, we repeat these acoustic-damping experiments in a transmission geometry with thin, 20 nm Au films submerged in the varying media; As the optical response associated with the acoustic mode propagation is much greater in this thickness regime, we do not require the use of an iris or operating off-focus to enhance our signal, and thus are able to eliminate the contribution of interference due to reflection in both the Au film and the pressure-front in the underlying SiO2 substrate 90,92 within our scans. We find the two methods, and thus two film thicknesses, are in excellent agreement, as shown in Fig. 3a. Furthermore, as shown in Fig. 3b, in considering only the contact-angle associated with the interface, rather than accounting for the liquid’s inherent interaction energy (γL ), there is a distinct lack of trend with energy transport at the interface. This clearly demonstrates that macroscopic wetting metrics are not sufficient to describe nanoscale dynamics in many fluids, particularly those with low contact angles. Given the agreement between methods, with both finding the Au/air and Au/fluorocarbon interfaces to have similar transmission coefficients, we consider other measurements at solid-liquid interfaces to further explore the differences between contact angle measurements and nanoscale energy transport.

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Ablation threshold as metric for TBC As shown in our previous work, the thermal ablation threshold associated with short-pulse, singleshot measurements provides an accurate metric for changes in thermal transport at interfaces, and correlates well with TDTR measurements of thermal boundary conductance. 29 This threshold quantifies the minimum laser fluence, or energy density, required to remove mass from a material’s surface; the nominal value for this threshold is dependent on the laser pulse duration and thermal properties of the material system. In our previous work, we found that the ablation threshold for 25 picosecond pulses, where the electrons and lattice reach equilibrium during the pulse and diffusive heat transfer is the characteristic process, the assumption of homogeneous thermal properties of a bulk target break down in thin film systems. Instead, the thermal boundary conductance between the Au film and substrate becomes a limiting factor. In fact, there is a monotonic, linear increase in ablation threshold with increasing TBC; 29 the nominal value for the ablation threshold was measured to increase 6.46 J m−2 per MW m−2 K−1 increase in thermal boundary conductance. Thus, as a probe of energy transport at solid-liquid interfaces and means of quantifying the thermal boundary conductance for interfaces with large uncertainties, we measure the ablation threshold for 200 nm Au films with an overlying liquid layer; these results are depicted in Fig. 3c. The ablation conditions and analysis procedure are detailed in the Supporting Information.Given the aforementioned change in laser fluence necessary to induce ablation for a given increase in thermal boundary conductance (6.45 J m−2 per MW m−2 K−1 ) and the measured TBC for a Au/water interface (∼68 MW m−2 K−1 ), one would predict a TBC for the Au/FC70 interface to be ∼1-3 MW m−2 K−1 ; this prediction is in good agreement with the range of lower bound from uncertainty analysis on our TDTR measurements which spans ∼1-5 MW m−2 K−1 . Although the ablation threshold for the thin Au films follow a nearly linear trend with the bulk wetting and thermophysical properties associated with the overlying liquid, where the threshold is plotted as a function of the liquid’s measured work of adhesion, a clear deviation from any such trend is again observed in the fluorocarbon-based liquids, similar to our picosecond acoustic measurements of transmissivity, where fluorocarbon liquids have nearly the same ablation threshold 14


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as that found in air. In comparing these ablation threshold values to the picosecond ultrasonic measurements of acoustic transmissivity, as shown in Fig. 3d, we find that the two methods are in excellent agreement, where an increase in phonon transmission corresponds to a nearly monotonic increase in fluence necessary to induce ablation of thin Au films. This agreement between ablation threshold and phonon transmission further indicates that acoustic phonons are indeed the primary thermal carriers contributing to thermal boundary conductance at Au/liquid interfaces and electronic transport does not play a significant role of interfacial heat transfer in these systems. Our posit regarding a phonon-dominated TBC is further supported by a two-temperature model 93,94 and thermoreflectance coefficient analysis 95 of the pump-probe reflectivity signals, which shows no change in the electron-phonon coupling factor of Au, regardless of the liquid layer, and thus indicates a lack of interfacial electron-phonon coupling or charge transfer. 96 Clearly, both the picosecond acoustic transmission measurements and ablation threshold measurements show that the Au/fluorocarbon interactions do not follow similar trends as the other liquids with respect to the Young-Dupré equation. In fact, the experiments suggest that the interaction energy between Au and these fluorocarbon liquids are similar to that as Au and air (based on the acoustic phonon transmission and ablation thresholds being similar for both Au/air and Au/fluorocarbon liquid). Indeed, the bounds of our analysis for the TBC of Au/fluorocarbon interfaces of ∼1-5 MW m−2 K−1 from our various experiments is on par with the previously measured TBC of Au/air interfaces by Park et al., where a value of 4 ± 4 MW m−2 K−1 was obtained. This defies the macroscopic observation that FC70, and other fluorinated fluids, are “highly wetting,” since a highly wetting fluid would imply relatively strong solid/liquid interaction energies. Considering the relatively weak intermolecular bonds of fluorocabon fluids relative to other fluids, it is not surprising that molecules in a fluid that have inherently weak intermolecular interactions with each other will also not want to interact with a surface. Although on the macroscale, fluorinert and other fluorocarbons are known, and measured here, to have both low surface energy and a correspondingly low work of adhesion, our metrics display a clear discrepency relative to the macroscopic definitions of wetting. The comparatively large

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hydrophobicity, for example, of fluorocarbons is attributed to their ‘fatness,’ or large molecular cross-sectional area, leading to their reduced surface energy, 97 and their previously measured hindrance to thermal conductance when measured as self-assembeled monolayers. 98 Furthermore, the low polarizability of both fluorine and carbon bonds leads to reduced London dispersion forces, which only exist due to momentary dipoles, leading to their inherent, repulsive nature. 99 Such features may lead to a greatly reduced, if even existant, stern layer at the solid-liquid interface compared to the case of other liquids, such as water. Finally, given the high solubility of gases in fluorocarbons due to these weak intermolecular forces, 100,101 it is possible that a vapor layer of such gases forms at the solid-liquid interface, greatly inhibiting the energy transport across the interface; as determined by Takata et al., 102 these vapor layers are not found to exist in solid-liquid interfaces involving water, including under super-hydrophobic conditions. This vapor layer could explain the similarity in thermal boundary conductance between Au-fluorocarbon interfaces and Au-air interfaces. As density-depletion is known to occur at solid-water interfaces, yet the macroand nanoscale metrics for wetting provide similar results in such cases, this effect is not a likely explanation for disparities in thermal transport at the fluorocarbon-solid interface. These microscopic features lead to deviations in energy transport on the nanoscale, but similar macroscopic properties to other solvents. Interfacial electronic transport,

Summary These results show a clear deviation between macro- and nanoscale measures of liquid wetting a solid surface based on energy transport mechanisms at the interface of the two phases, as the relative acoustic impedance, contact angle, and work of adhesion between of the solid/liquid interface fail to agree with various measurements of nanoscale thermal transport. Thus, these results raise the question: what metrics should be used to quantify solid/liquid interaction energies on the nanoscale that are universal to all solid/liquid interfaces? Comparing the transmission of acoustic phonon modes across this interface to our ablation threshold experiments, we find excellent agree-

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ment (Fig. 3d), not only reinforcing the notion that macroscale observables are poor indicators of nanoscale energy transport at an interface composed of varying phases, but also that these experimental methods provide a means to individually, yet accurately, probe such energy transfer. With the improved detection outlined in this work and the non-destructive nature of the measurement, picosecond acoustics is thus shown to be a versatile method for insight towards energy transport at interfaces. This clear discrepency between macro- and nanoscale phenomenon at solid-liquid interfaces provides insight to the need for better metrics of interactions between various phases of matter at the nanoscale.

Associated Content Supporting Information. Sample preparation; picosecond acoustic measurements; wetting measurements; ablation threshold measurements

Acknowledgement We would like to thank the Office of Naval Research for support (N00014-15-12769). S.M.O acknowledges award CMMI-1531783 from the National Science Foundation.

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Table of Contents Figure

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Acoustic Phonon Transmission (a.u.)

Page 29 of 29

20 nm 75 nm

2.0

EtOH H2O

1.5

EG 1.0 Air 0.5

0.0

FC40

FC70 Fomblin

0

20

40

Cyclohexane

60

Work of adhesion (mN m-1)

29


80

100

Nanoscale wetting and energy transmission at solid-liquid interfaces

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