Compliant Surface Research Articles

Stability and the transition to turbulence in the flow through conduits with compliant walls

The mechanisms of destabilisation of the flow through soft-walled channels/tubes are qualitatively different from those in rigid-walled conduits. The stability depends on two dimensionless parameters, the Reynolds number $(\rho _f V_f h_f/ \mu _f)$ and $\varSigma = (\rho _f G h_f^{2}/\mu _f^{2})$ , where $\rho _f$ and $\mu _f$ are the fluid density and viscosity, $h_f$ and $V_f$ are the fluid length and velocity scale and $G$ is the wall elasticity modulus. There is an instability at zero Reynolds number when the dimensionless parameter $\varGamma = (\mu _f V_f / h_f G)$ exceeds a critical value. The low-Reynolds-number instability of the Couette flow past a compliant surface is well understood, and has been confirmed in experiments, but that in a pressure-driven flow is not completely understood. Two modes of instability at high Reynolds number have been predicted: the inviscid mode with an internal viscous layer, for which the transition Reynolds number scales as ${Re}_t \propto \varSigma ^{1/2}$ ; and the wall mode instability with a viscous layer at the wall, for which ${Re}_t \propto \varSigma ^{3/4}$ . The wall mode instability has been observed in experiments at Reynolds number as low as 300 in a soft-walled tube and as low as 100 in a channel with one compliant wall, though the scaling of the transition Reynolds number differs from the theoretical prediction due to substantial wall deformation. Though the flow after transition shares many of the characteristics of hard-wall turbulence, it differs in significant ways, suggesting that soft-wall turbulence is a separate class distinct from hard-wall turbulence.

Balance and vestibular function and survival in US cancer survivors

Cancer and its treatment damage the musculoskeletal system and induce neurotoxicity, affecting the key sensory inputs for maintaining balance. The present study describes the pattern of balance impairment and evaluated its association with mortality among US cancer survivors. Data on a nationally representative sample of cancer survivors from the US National Health and Nutrition Examination Survey from 1999 to 2015 was analyzed. Sensory-specific balance impairment was measured at baseline by the modified Romberg test of standing balance on firm and compliant support surfaces. The linked mortality data were updated through December 31, 2015. Among 511 cancer survivors, 282 (48.3%) had a balance impairment, predominantly attributing to vestibular dysfunction (251; 89.0% of 282 and 44.5% of 511). A higher prevalence of balance impairment was observed among cancer survivors with advanced age, lower socioeconomic status or educational attainment, body mass index <25 kg/m2 , and an inactive lifestyle. During up to 16.4 years of follow-up (median, 11.3 years; 5088 person-years), 253 cancer survivors had died. Cancer survivors with a balance impairment had a 63% higher risk of death from all causes (hazard ratio, 1.63; 95% confidence interval [CI], 1.12-2.38) after adjusting for sociodemographic factors, comorbidities, and cancer type. Specifically, those with vestibular dysfunctions had approximately 1.54 (95% CI, 1.05-2.27) times the risk of death compared to those without any balance impairment. These associations were stronger in males than in females. In a US nationally representative sample of cancer survivors, balance impairment and vestibular dysfunctions were prevalent and associated with heightened all-cause mortality.

Individual differences impacting skin deformation and tactile discrimination with compliant elastic surfaces.

Individual differences in tactile acuity are observed within and between age cohorts. Such differences in acuity may be attributed to various sources, including aspects of nervous system, skin mechanics, finger size, cognitive and behavioral factors, etc. This work considers individual differences, within a younger cohort of participants, in discriminating compliant surfaces. These participants exhibit a range of finger size and stiffness. Interestingly, both their finger size and stiffness well predict their discriminative performance, where softer/smaller fingers outperform stiffer/larger fingers. Stereo imaging captured biomechanical cues in the skin's deformation, including contact area and penetration depth, and their change rates. In those individuals with stiffer/larger fingers, who perceptually performed worse, we observed less distinguishable contact areas and eccentricities, compared to softer/smaller fingers. These particular cues well predicted individual differences observed in perceptual discrimination. In comparison, with two other cues, curvature and penetration depth, the imaging readily distinguished the compliant surfaces irrespective of finger stiffness/size, not aligned with discrimination. In conclusion, in passive touch, we find that individuals with softer/smaller fingers were better at discriminating compliances, and that certain skin deformation cues predict individual differences in perception.

The effects of mechanical noise bandwidth on balance across flat and compliant surfaces

Although the application of sub-sensory mechanical noise to the soles of the feet has been shown to enhance balance, there has been no study on how the bandwidth of the noise affects balance. Here, we report a single-blind randomized controlled study on the effects of a narrow and wide bandwidth mechanical noise on healthy young subjects’ sway during quiet standing on firm and compliant surfaces. For the firm surface, there was no improvement in balance for both bandwidths—this may be because the young subjects could already balance near-optimally or optimally on the surface by themselves. For the compliant surface, balance improved with the introduction of wide but not narrow bandwidth noise, and balance is improved for wide compared to narrow bandwidth noise. This could be explained using a simple model, which suggests that adding noise to a sub-threshold pressure stimulus results in markedly different frequency of nerve impulse transmitted to the brain for the narrow and wide bandwidth noise—the frequency is negligible for the former but significantly higher for the latter. Our results suggest that if a person’s standing balance is not optimal (for example, due to aging), it could be improved by applying a wide bandwidth noise to the feet.

Performance improvement of foil air journal bearing employing micro-pocket and textures on top compliant surface

Investigations to reduce the friction and improve the dynamic performance of a self-acting air foil journal bearing have been performed and presented herein employing the texture, micro-groove and micro-pocket on the top foil. Significant improvements in the static and dynamic performances have been found in presence of micro-pocketed/grooved and textured top foils in comparison to conventional plain foil. The micro-pocketed top foil yields better performance as compared to textured and conventional cases. In the presence of micro-pocket on top compliant foil, the bearing yielded a significant reduction in friction coefficient varying in the range of 8%–11% as compared to conventional one. Moreover, better dynamic stability has also been found even at lightly loaded conditions in the presence of texture and micro-pocket.

Vestibular Physical Therapy Treatment of Individuals Exposed to Directed Energy.

Following suspected sonic attacks on U.S. Embassies, a subset of individuals presented with a unique cluster of symptoms believed to have resulted from exposure to directed energy. Directed energy has been described as exposure to a unique sound/pressure phenomenon such as infrasonic or ultrasonic acoustic or electromagnetic energy. The Joint Force does not have an established protocol to guide vestibular physical therapy for individuals exposed to directed energy. Therefore, we have provided evidence-based guidance for the treatment of oculomotor- and vestibular-related impairments from similar populations. Published evidence was used to inform suggestions for clinical best practice. We offer resources for the management of non-oculomotor- and non-vestibular-related impairments, before discussing physical therapy interventions for dizziness and imbalance. The physical therapist should design a treatment program that addresses the individual's health condition(s), body structure and function impairments, activity limitations, and participation restrictions after suspected directed energy exposure. This treatment program may include static standing, compliant surface standing, weight shifting, modified center of gravity, gait, and gaze stabilization or vestibular-ocular reflex training. Habituation may also be prescribed. Interventions were selected that require little to no specialized equipment, as such equipment may not be available in all settings (i.e., operational environments). Evidence-based guidance for prescribing a comprehensive vestibular physical therapy regimen for individuals exposed to directed energy may aid in their rehabilitation and return to duty. This standardized approach can help physical therapists to treat complaints that do not match any previously known medical conditions but resemble brain injury or vestibular pathology.

Gender differences in postural balance, physical activity level, BMI, and body composition in athletes with visual impairment

The aim of this study was to examine the gender differences in postural balance, physical activity (PA), and body mass index (BMI) and body composition in visually impaired (VI) athletes. A total of 66 athletes took part in this study. After anthropometric measurements, the PA level was assessed by the International Physical Activity Questionnaire-Short Form. The modified Clinical Test of Sensory Integration and Balance tool was used to assess postural balance in four standing conditions: (1) eyes-open on a firm surface, (2) eyes-closed on a firm surface, (3) eyes-open on a compliant surface, and (4) eyes closed on a compliant surface. Findings revealed gender differences in anthropometric measurements ( p &lt; .05). No statistically significant differences were found in fat mass and fat free mass between sighted male and VI male athletes and between sighted female and VI female athletes ( p &gt; .05). Weekly, high and moderate PA levels as well as walking were found to be different across the four groups ( p &lt; .05). Assessment of postural balance in four standing conditions revealed no gender differences in sighted athletes ( p &gt; .05). Postural balance scores in conditions 1 and 3 showed significant gender differences ( p &lt; .05), whereas conditions 2 and 4 showed no gender differences in athletes with VI ( p &gt; .05). Gender-matched differences were also apparent in conditions 1 and 3, revealing that VI male and female athletes scored worse in these conditions than their sighted male and female counterparts ( p &lt; .05). There is a delay in balance parameter in VI athletes even if their fitness levels are similar with their sighted counterparts. The present findings could also be useful for gender-specific training session preparations in VI sports.

Investigation of Transition Delay on a Wing Section by Dynamic Surface Deformation

Numerical calculations were carried out in order to investigate the delay of transition to turbulence on a wing section by means of local dynamic surface deformation. Physically, the deformation may be produced by piezoelectrically driven actuators located below a compliant aerodynamic surface, which have been explored experimentally. One actuator was located in the upstream region of the wing, and it was oscillated at the most unstable frequency in order to develop small disturbances corresponding to Tollmien–Schlichting instabilities. A second controlling actuator was placed further downstream, and then it was oscillated at the same frequency but with an appropriate phase shift and modified amplitude in order to decrease the disturbance growth and delay the transition process. The configuration consists of a NLF(1) 0414F natural laminar flow wing section in subsonic flow at a chord-based Reynolds number of . Angles of attack of both 3.0 and 4.0 deg were considered. Large-eddy simulations were carried out via solution of the unsteady three-dimensional compressible Navier–Stokes equations using a high-fidelity computational scheme and an implicit time-marching approach. Two-dimensional simulations were used to develop an empirical process that was applied to determine the optimal phase shift and amplitude of the controlling actuator. Results of the simulations are described, features of the flowfields are elucidated, and comparisons are made between solutions for the uncontrolled and controlled cases in order quantify effectiveness of the control. It is shown that dynamic surface control can return approximately 20% of the upper wing surface to laminar flow that is lost to premature transition when disturbances are present.

Experiments and Modeling of a Compliant Wall Response to a Turbulent Boundary Layer with Dynamic Roughness Forcing

The response of a compliant surface in a turbulent boundary layer forced by a dynamic roughness is studied using experiments and resolvent analysis. Water tunnel experiments are carried out at a friction Reynolds number of Reτ≈410, with flow and surface measurements taken with 2D particle image velocimetry (PIV) and stereo digital image correlation (DIC). The narrow band dynamic roughness forcing enables analysis of the flow and surface responses coherent with the forcing frequency, and the corresponding Fourier modes are extracted and compared with resolvent modes. The resolvent modes capture the structures of the experimental Fourier modes and the resolvent with eddy viscosity improves the matching. The comparison of smooth and compliant wall resolvent modes predicts a virtual wall feature in the wall normal velocity of the compliant wall case. The virtual wall is revealed in experimental data using a conditional average informed by the resolvent prediction. Finally, the change to the resolvent modes due to the influence of wall compliance is studied by modeling the compliant wall boundary condition as a deterministic forcing to the smooth wall resolvent framework.

Load-independent hydrogel friction

Biology largely manages tribological challenges either by eliminating sliding altogether or by protecting sliding interfaces with soft aqueous gels. In the body, aqueous gels are often thin (thickness, t < 100 μm), soft (elastic modulus, E < 10 kPa), lubricious (friction coefficients, μ < 0.01), and cover compliant surfaces, including cell membranes, pleura, cartilage, and the eye. These characteristics provide a natural defense against wide ranges of applied loads. In this work, hydrogel samples (7.5 wt% polyacrylamide, 0.3 wt% N,N′-methylenebisacrylamide) were prepared with spherically-capped shell probe geometries, which have been previously determined to provide constant contact pressures during indentation measurements against flat hydrogel disks. In a self-mated (“Gemini”) sliding configuration, this geometry is capable of load-independent friction over a range of low normal loads spanning 0.5 to 2.0 mN. This friction behavior is consistent with da Vinci-Amontons' friction law (Ff = μFn) due to the large compliance of the spherically-capped shell probe geometry enabling the area of contact to increase in proportion with the applied load and due to low shear stresses reacted across the sliding interface for high water content aqueous gels. Future bio-inspired lubrication strategies involving aqueous gels may benefit from leveraging contact geometry for constant, load-independent friction.

Middle-age people with multiple sclerosis demonstrate similar mobility characteristics to neurotypical older adults

BackgroundClinical trials often report significant mobility differences between neurotypical and atypical groups, however, these analyses often do not determine which measures are capable of discriminating between groups. Additionally, indirect evidence supports the notion that some mobility impaired populations demonstrate similar mobility deficits. Thus, the current study aimed to provide a comprehensive analysis of three distinct aspects of mobility (walking, turning, and balance) to determine which variables were significantly different and were also able to discriminate between neurotypical older adults (OA) and middle-aged people with multiple sclerosis (PwMS), and between middle-aged neurotypical adults and PwMS. MethodsThis study recruited 21 neurotypical OA, 19 middle-aged neurotypical adults, and 30 people with relapsing remitting MS. Participants came into the laboratory on two separate occasions to complete mobility testing while wearing wireless inertial sensors. Testing included a self-selected pace two-minute walk, a series of 180˚ and 360˚ turns, and a clinical balance test capturing a total of 99 distinct mobility characteristics. We determined significant differences for gait and turning measures through univariate analyses and a series of repeated measures analysis of variance in determining significance for balance conditions and measures. In determining discrimination between groups, the Area Under the Curve (AUC) was calculated for all individual mobility measures with a threshold of 0.80, denoting excellent discrimination. Additionally, a stepwise regression of the top five AUC producing variables was performed to determine whether a combination of variables could enhance discrimination while accounting for multicollinearity. ResultsThe results between neurotypical OA and middle-aged PwMS demonstrated significant differences for three gait and one turning variable, with no variable or combination of variables able to provide excellent discrimination between groups. Between middle-age neurotypical adults and PwMS a variety of mean and variability gait measures demonstrated significant differences between groups; however, no variable or combination of variables met discriminatory threshold. For turning, five 360˚ turn variables demonstrated significant differences and furthermore, the combination of 360˚ mean turn duration and variability of peak turn velocity were able to discriminate between groups. Finally, the majority of postural sway measures demonstrated significant group differences and the ability to discriminate between groups, particularly during more challenging balance conditions where participants stood on a compliant surface. ConclusionThese results offer a comprehensive analysis of mobility differences and measures capable of discriminating between middle-age neurotypical adults and PwMS. Additionally, these results provide evidence that OA and middle-age PwMS display similar movement characteristics and thus a potential indicator of advanced aging from a mobility perspective.

Quantitative characterization of walking on sand inecological conditions: Speed, temporal segmentation, and variability

BackgroundWalking on compliant surfaces, on sand in particular, is now recommended for training in both elderlies and injured subjects/individuals, allowing to perform high intensity exercises (i.e. augmented energy expenditure) in safe conditions (i.e. minimizing the impact on the joints and the risk of fall). Nevertheless, despite the assessment of energetics of walking on sand, the quantitative biomechanical characterization of walking on sand in ecological conditions is largely lacking. Research questionWhich is the effect of sand surface on gait speed, gait temporal segmentation and their variability as related to surface compliance in ecological condition? MethodsEighteen healthy adults were assessed while walking on solid ground, dry-, and wet sand in ecological conditions by means of wearable inertial sensors (Miniwave, Cometa s.r.l., Italy). The best performing algorithm for the segmentation of walking on sand was selected among 17 algorithms designed for solid ground. Gait timing (i.e. speed, temporal segmentation, variability) was analysed, for the first time, with respect to sand compliance, and compared to walking on solid ground. ResultsSelf-selected speed on a 60 m distance increased when walking on sand with respect to solid ground (Median 1.02 m/s), with the highest speed on wet sand (Median 1.15 m/s). A stabilizing strategy on the uneven surface provided by sand was highlighted by i) increased stance and double support durations with respect to speed on wet sand, and ii) increased short-term variability of stride, corresponding to continual adjustments of the lower limbs due to shifting surface provided by sand. SignificanceThis study represents the first step in the objective characterization of walking on compliant surfaces as sand, necessary for the definition of training and rehabilitative programs.

Association of Balance Function With All-Cause and Cause-Specific Mortality Among US Adults

Difficulty maintaining balance is common among individuals aged 40 years or older and increases the risk of falls. However, little is known about the association of balance function with long-term mortality outcomes in adults. To investigate the association of balance function with all-cause and cause-specific mortality among US adults. A prospective, population-based cohort study of a nationally representative sample of 5816 adults (weighted population, 92 260 641) from the US National Health and Nutrition Examination Survey was conducted from 1999 to 2004. Individuals aged 40 years or older who completed the modified Romberg Test of Standing Balance on Firm and Compliant Support Surfaces were included. Participants were linked to mortality data from the test date through December 31, 2015. Data analysis was conducted from February 1 to June 1, 2020. The modified Romberg Test of Standing Balance on Firm and Compliant Support Surfaces was used to measure balance function and define balance disorder according to sensory input. Mortality associated with all causes, cardiovascular disease (CVD), and cancer. A total of 5816 adults (weighted mean [SE] age, 53.6 [0.2] years; 2897 [49.8%] women) were included in this cohort study. During up to 16.8 years of follow-up (median, 12.5 years; 68 919 person-years), 1530 deaths occurred, including 342 associated with CVD and 364 associated with cancer. Participants with balance disorder were at a higher risk of death from all causes, CVD, and cancer. After adjusting for sociodemographic characteristics, lifestyle factors, and chronic conditions, the hazard ratios (HRs) among participants with balance disorder compared with those without balance disorder were 1.44 (95% CI, 1.23-1.69) for all-cause mortality, 1.65 (95% CI, 1.17-2.31) for CVD mortality, and 1.37 (95% CI, 1.03-1.83) for cancer mortality. Furthermore, vestibular balance disorder was associated with increased mortality from all causes (HR, 1.31; 95% CI, 1.08-1.58), CVD (HR, 1.59; 95% CI, 1.12-2.27), and cancer (HR, 1.39; 95% CI, 1.04-1.86). In this nationally representative sample of US adults, balance disorder was associated with an increased risk of all-cause, CVD, and cancer mortality. Further studies are needed to confirm these findings and evaluate whether the observed associations represent a causal biological phenomenon and, if so, whether the effect is modifiable with a multicomponent exercise program.

An investigation on ice adhesion and wear of surfaces with differential stiffness

Ice adhesion is known to be higher on stiff metallic surfaces and lower on compliant polymeric surfaces. In designing icephobic surfaces, while one approach is to use surfaces with lower stiffness, such surfaces typically exhibit poor wear resistance. The aim of this study is to create a patterned surface with differential stiffness such that there is always a shared contact between stiff and compliant surfaces, and to study the possibility of such surfaces having an optimal balance between low ice adhesion and favorable wear resistance. Patterned surfaces with alternating layers of stiff aluminum and compliant polyurethane elastomer were fabricated, and ice adhesion and wear behavior of such patterned surfaces are reported. Ice adhesion as a function of the metal-polymer contact area ratio was studied, and patterned surfaces exhibited reduced ice adhesion compared to a plain aluminum surface. Reciprocating ball-on-flat sliding tests were used to study the wear behavior of the surfaces. Wear resistance of the patterned surfaces is presented in terms of wear depth, and the patterned surfaces exhibited higher wear resistance than plain polyurethane surface. Simple finite element modeling was used to arrive at stress states of the patterned surfaces and to understand their wear behavior. The novel surface engineering approach outlined in this paper shows promise for realizing durable icephobic surfaces under contact conditions.

Numerical study of pattern formation in compliant surfaces scraped by a rigid tip.

The emergence of surface patterns on the surfaces of compliant materials subject to plowing wear is a complex problem which can be quantitatively characterized, e.g., on polymer surfaces scraped by an atomic force microscope (AFM) tip. Here we explore the applicability of a phenomenological model recently introduced to describe this phenomenon. Based on the competition between the viscoplastic indentation and the elastic shear stress caused by the tip, the model is able to reproduce the wavy features (ripples) observed when the tip is scanned along a series of parallel lines. For low values of the driving velocity v and the spacing b between scan lines, the existence of dotted areas formed by variously oriented pit alignments is observed. Moreover, coexistence of rippled with dotted domains is also observed at suitable parameter values. The formation process of the ripples is also described in detail. The amplitude, period, and orientation of these features are estimated numerically for different values of v and b parameters. We have also revisited the formation of the wavy patterns formed when a single line is scanned, and derived an equation which correctly describes their period and depth, and the static friction as well. This equation is not applicable when several lines are scanned one after the other and the ripples emerge as result of a cooperative process which involves the scanning of several lines.

Real-time conscious postural control is not affected when balancing on compliant surface by young adults

Previous research has illustrated that real-time conscious postural control (i.e., reinvestment - shifting from movement automaticity to a more consciously controlled and monitoring of movement) increased with standing task difficulties among healthy older adults. However, such association has not been investigated in the younger population. This study attempted to examine real-time conscious postural control among healthy young adults when performing different standing tasks on a compliant (foam) surface. T3-Fz EEG (electroencephalography) coherence, indicative of real-time conscious postural control, was recorded during the standing tasks (i.e., wide base on foam (WBF), narrow base on foam (NBF) and tandem stance on foam (TAF)). Body sway was also recorded by a motion capture system. Participants’ perceived difficulty on the different standing tasks was evaluated by the Visual Analogue Scale (VAS). Results revealed that while body sway and perceived difficulty increased significantly with task difficulties, T3-Fz EEG coherence did not differ among standing tasks. In addition, no differences of any measures were found between young adults with high and low trait reinvestment propensity. Our findings indicate that young adults do not pose higher real-time conscious postural control when task difficulty increases. We also add support to the existing literature; the between-group effect of trait reinvestment appears to be minimal in real-time.

Control Modification of Grasp Force Covaries Agency and Performance on Rigid and Compliant Surfaces

This study investigated how modifications in the display of a computer trace under user control of grasp forces can co-modulate agency (perception of control) and performance of grasp on rigid and compliant surfaces. We observed positive correlation (p < 0.01) between implicit agency, measured from time-interval estimation for intentional binding, and grasp performance, measured by force-tracking error, across varying control modes for each surface type. The implications of this work are design directives for cognition-centered device interfaces for rehabilitation of grasp after neurotraumas such as spinal cord and brain injuries while considering if grasp interaction is rigid or compliant. These device interfaces should increase user integration to virtual reality training and powered assistive devices such as exoskeletons and prostheses. The modifications in control modes for this study included changes in force magnitude, addition of mild noise, and a measure of automation. Significant differences (p < 0.001) were observed for each surface type across control modes with metrics for implicit agency, performance, and grasp control efficiency. Explicit agency, measured from user survey responses, did not exhibit significant variations in this study, suggesting implicit measures of agency are needed for identifying co-modulation with grasp performance. Grasp on the compliant surface resulted in greater dependence of performance on agency and increases in agency and performance with the addition of mild noise. Noise in conjunction with perceived freedom at a flexible surface may have amplified visual feedback responses. Introducing automation in control decreased agency and performance for both surfaces, suggesting the value in continuous user control of grasp. In conclusion, agency and performance of grasp can be co-modulated across varying modes of control, especially for compliant grasp actions. Future studies should consider reliable measures of implicit agency, including physiological recordings, to automatically adapt rehabilitation interfaces for better cognitive engagement and to accelerate functional outcomes.

Features of panel flutter response to shock boundary layer interactions

The compounding effects of panel flutter and oblique shock impingement are of great concern to the development of light-weight, high-speed vehicles. Shock-induced panel flutter response is investigated at M=2 and Re=120,000 using the Navier–Stokes equations closely coupled to the von-Kármán equations. A naturally occurring laminar inflow boundary layer and incident oblique shockwave are specified through the boundary conditions. The shockwave impinges at the midpoint of a flexible panel. Several incident shock strengths and non-dimensional dynamic pressures are examined. The resulting fluid–structure interaction is significant and is described in detail. In general, the incident shockwaves lead to higher frequency, higher mode flutter when compared to the configuration without an impinging shockwave or with an impinging shockwave in inviscid flow. Additionally, the presence of a compliant surface is shown to promote turbulent transition downstream of the shock boundary layer interaction.

Surface textures suppress viscoelastic braking on soft substrates

A gravity-driven droplet will rapidly flow down an inclined substrate, resisted only by stresses inside the liquid. If the substrate is compliant, with an elastic modulus G < 100 kPa, the droplet will markedly slow as a consequence of viscoelastic braking. This phenomenon arises due to deformations of the solid at the moving contact line, enhancing dissipation in the solid phase. Here, we pattern compliant surfaces with textures and probe their interaction with droplets. We show that the superhydrophobic Cassie state, where a droplet is supported atop air-immersed textures, is preserved on soft textured substrates. Confocal microscopy reveals that every texture in contact with the liquid is deformed by capillary stresses. This deformation is coupled to liquid pinning induced by the orientation of contact lines atop soft textures. Thus, compared to flat substrates, greater forcing is required for the onset of drop motion when the soft solid is textured. Surprisingly, droplet velocities down inclined soft or hard textured substrates are indistinguishable; the textures thus suppress viscoelastic braking despite substantial fluid-solid contact. High-speed microscopy shows that contact line velocities atop the pillars vastly exceed those associated with viscoelastic braking. This velocity regime involves less deformation, thus less dissipation, in the solid phase. Such rapid motions are only possible because the textures introduce a new scale and contact-line geometry. The contact-line orientation atop soft pillars induces significant deflections of the pillars on the receding edge of the droplet; calculations confirm that this does not slow down the droplet.

Direct Simulation of Fluid–Structure Interaction in a Hypersonic Compression-Ramp Flow

Sustained flight at hypersonic speeds presents an enduring challenge to vehicle design and control. An extreme aerothermal environment acting on geometrically thin, multifunctional structures can result in the significant structural response of key aerodynamic surfaces. Specifically, the oscillations of a compression corner shock-wave/boundary-layer interaction, which models a control surface deflection, can couple with the lowest natural frequencies of a proximal compliant surface, leading to a range of undesirable outcomes from reduced service life to structural failure. The present work investigates these phenomena through time-accurate high-fidelity aeroelastic simulations of a laminar two-dimensional flow at over a 35 deg compression ramp featuring an embedded compliant panel. Surface pressure, skin friction, and heat transfer associated with the corner shock-wave/boundary-layer interaction are directly compared between rigid and compliant configurations. The upstream separation zone is observed to synchronize with the compliant-panel deflection. A reduction in surface heat flux is observed for a majority of compliant cases relative to the rigid case, whereas entirely new flow features are observed in response to large-scale panel deflections. A modification is proposed to the local piston-theory aerodynamic model to improve the accuracy of surface pressure predictions.