Surface chemistry of the ladybird beetle adhesive foot fluid across various substrates

A. Substrate characterization

Before vibrational spectra were collected, each substrate was characterized to determine polymer film thickness, wettability, and surface roughness. Static contact angle goniometry, with 10 μl ultrapure water as the test fluid, was used to measure the angle between the droplet and the substrate, with low angles correlating to hydrophilicity and vice versa. Contact angles of 49.0° ± 3.3°, 63.6 ± 2.3°, and 90.0 ± 1.5° were measured for PEO, CaF2, and polystyrene surfaces, respectively (insets in Fig. 3). A separate but simultaneously prepared set of surfaces was scanned using tapping mode atomic force microscopy with 1 × 1 μm scan size (see Fig. S3 in the supplementary material2727. See supplementary material at https://www.scitation.org/doi/suppl/10.1116/6.0001006 for surface chemistry of the ladybird beetle adhesive foot fluid across various substrates.). Root mean-squared roughnesses of 1.33 and 0.24 nm were observed for PEO and PS thin films, respectively. According to a recent study comparing adhesion on both smooth and rough surfaces, these RMS roughnesses fall within the “smooth” classification (1414. M. W. England, T. Sato, M. Yagihashi, A. Hozumi, S. N. Gorb, and E. V. Gorb, Beilstein J. Nanotechnol. 7, 1471 (2016). https://doi.org/10.3762/bjnano.7.139

B. SFG spectroscopy

Surface-sensitive vibrational spectra were collected in three separate stretching regions: C—D (1900–2300 cm−1), C=O (1600–1800 cm−1), and C—H (2800–3000 cm−1). Selectively deuterating the polymer surfaces (PEO and PS) allowed for the confinement of the SFG signal from the substrate side of the interface to the C—D stretching region. Thus, all signals measured in the C—H and C=O stretching regions must only come from molecular order within the interfacial layer of adhesive fluid.

First, we collected the C—D stretching region spectra for two reasons: to verify that polymer thin films matched standards and to interrogate any possible effect of the adhesive fluid on the organization of polymer side chains. PEO spectra contained a single feature near 2080 cm−1 corresponding to the CD2 stretching mode (see Fig. S4 in the supplementary material2727. See supplementary material at https://www.scitation.org/doi/suppl/10.1116/6.0001006 for surface chemistry of the ladybird beetle adhesive foot fluid across various substrates.).2121. T. W. Golbek, J. Franz, J. Elliott Fowler, K. F. Schilke, T. Weidner, and J. E. Baio, Biointerphases 12, 02D406 (2017). https://doi.org/10.1116/1.4982710 PS spectra also contained a single feature with a shoulder, this time near 2270 cm−1 (shoulder) and 2290 cm−1(peak) corresponding to the CD stretching modes of the pendant phenyl rings. The intensity of each mode was not significantly changed after the addition of adhesive fluid to the substrate, which indicated that the fluid had no impact on polymer film ordering.Next, spectra were collected at the C=O stretching region for fluid on each substrate to determine the order of any water, lipids, amino acids, proteins, or other carboxyl group containing molecules at the interface. Although these types of molecules have not been explicitly observed in the chemical analysis of ladybird beetle foot fluid to date, the analysis of locusts and other similar species has indicated the presence of lipids, sugars, and amino acids. Interestingly, the spectra of adhesive fluid on all three substrates produced no discernable signal, indicating that these molecules are either absent from C. septempunctata adhesive fluid or not surface active (Fig. S4 in the supplementary material).2727. See supplementary material at https://www.scitation.org/doi/suppl/10.1116/6.0001006 for surface chemistry of the ladybird beetle adhesive foot fluid across various substrates.Finally, the C—H stretching region was interrogated. Saturated and unsaturated hydrocarbons were the only types of molecules confirmed to be present within the bulk of C. septempunctata adhesive fluid by the existing chemical analysis. Methyl and methylene groups from the backbone of these molecules have been well studied with SFG and have several vibrations in this region. The resulting SFG spectra in SSP polarization combination (s-polarized SFG, s-polarized visible, p-polarized IR) contained three distinct modes near 2858, 2880, and 2930 cm−1 corresponding to CH2 symmetric, CH3 symmetric, and CH3 fermi stretches, respectively (Fig. 3).22–2422. J. E. Baio, M. Spinner, C. Jaye, D. A. Fischer, S. N. Gorb, and T. Weidner, J. R. Soc. Interface 12, 20150817 (2015). https://doi.org/10.1098/rsif.2015.081723. G. Ma and H. C. Allen, Langmuir 22, 5341 (2006). https://doi.org/10.1021/la053522724. M. T. L. Casford, A. Ge, P. J. N. Kett, S. Ye, and P. B. Davies, J. Phys. Chem. B 118, 3335 (2014). https://doi.org/10.1021/jp410401z The previously assigned three vibrations and two additional features were observed after switching to PPP polarization combination. These additional modes were located near 2900 and 2950 cm−1 corresponding to CH2 fermi and CH3 asymmetric stretches, respectively.22,2422. J. E. Baio, M. Spinner, C. Jaye, D. A. Fischer, S. N. Gorb, and T. Weidner, J. R. Soc. Interface 12, 20150817 (2015). https://doi.org/10.1098/rsif.2015.081724. M. T. L. Casford, A. Ge, P. J. N. Kett, S. Ye, and P. B. Davies, J. Phys. Chem. B 118, 3335 (2014). https://doi.org/10.1021/jp410401z The three SSP modes and five PPP modes (three overlapping modes) consistently appeared in all fluid-substrate C—H spectra as substrate chemistry was varied. However, the amplitude of each mode varied substantially between substrate chemistries.To quantitatively compare the ordering of molecules within the adhesive fluid interfacial layer, it was first necessary to fit each spectrum to determine the location, full width half maximum, and amplitude of each mode as well as nonresonant background and phase. From these values, the square root of the ratio of CH3 to CH2 symmetric stretch square root of the intensities was determined. This ratio is a well-known metric for evaluating the order of a layer of hydrocarbon chains, where large ratios correspond to a high degree of hydrocarbon ordering.2525. M. R. Watry, T. L. Tarbuck, and G. L. Richmond, J. Phys. Chem. B 107, 512 (2003). https://doi.org/10.1021/jp0216878 This is because hydrocarbon chains or molecules contain methyl groups at their ends followed by a backbone of methylene groups that are aligned centrosymmetric when these chains are well packed with similar tilt angles. SFG selection rules, as well as literature, make clear that the methylene symmetric mode should have very low amplitude in the case of well-packed, uniform layers. Thus, low CH2 symmetric amplitude produces a high value of CH3/CH2 symmetric ratio. Fits of resulting spectra generated ratios of 1.03 ± 0.07, 2.66 ± 0.16, and 3.46 ± 0.40 for fluid-PEO, -CaF2, and -PS substrates, respectively. Last, a significant difference in variation between these ratios was tested by performing a single-factor ANOVA against the null hypothesis that all amplitude ratios were equal. This produced a p-value of 0.002, which led to the rejection of the null hypothesis. A posthoc Tukey's test was then utilized to evaluate pairwise significant differences between the three ratios. This analysis indicated that the largest ratio, found in the spectra of the fluid-dPS interface, differed significantly from the ratios of both the fluid-PEO and -CaF2 interfaces. Contrarily, the PEO and CaF2 ratios were not found to differ significantly.Within the broader context of the mechanisms that dictate the success or failure of beetle adhesion, the positive relationship between substrate hydrophobicity and hydrocarbon ordering ratio follows the trend of increasing adhesion forces with increasing surface hydrophobicity (Figs. 4 and 5).55. N. Hosoda and S. N. Gorb, Proc. R. Soc. B: Biol. Sci. 279, 4236 (2012). https://doi.org/10.1098/rspb.2012.1297 The measured traction forces for female lady beetles interacting with a hydrophilic substrate are lower than those measured for increasingly hydrophobic surfaces (see red data points in Fig. 5). At a certain level of hydrophobicity (greater than the water contact angle of 70°), the adhesive forces level off.

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