Contact Angle Hysteresis Research Articles

Wettability of saliva substitutes across various denture base fabrication techniques.

The present study evaluated the contact angles (CAs) of four denture base materials subjected to different surface treatments using deionized water and saliva substitutes. A total of 32 rectangular specimens were manufactured using four different denture base materials: heat-cured compression molded Lucitone 199 (C), milled Lucitone 199 (M), Formlabs Denture Base RP (P), and SR Ivocap High Impact (I). The CA of the surface of the unaltered, mechanically polished, and sandblasted surface specimens was evaluated after the application of five saliva substitutes: Biotene, VEGA, Spry, Moi-Stir, Dentilube, and ionized water. Ten droplet measurements were obtained for each group, with each droplet analyzed for advancing contact angle (ACA), receding contact angle (RCA), and the contact angle hysteresis (CAH) was calculated. The data of the experiment was analyzed using 2-way ANOVA, (α=0.05) with Tukey's test. CAH was demonstrated to have statistically significant differences among the denture bases (p<0.05), with unaltered 3D printed exhibiting the largest CAH, followed by unaltered milled. The unaltered denture bases exhibited higher CAH than the polished, and there were no significant differences in CAH among the polished denture bases (p>0.05). Sandblasting increased the ACA of the milled and conventional bases. The saliva substitutes exhibited differences in ACA, with Spry and VEGA having the highest ACA, and Biotene had the lowest CA of all the saliva substitutes evaluated. The manufacturing methods of denture bases influences the CAH, while the chemical composition of the denture base specimens does not appear to affect CAH. Sandblasting increases the ACA for the milled and conventional groups. Saliva substitutes do impact the ACA. Drawing from previous research, it is hypothesized that a 3D-printed denture base or sandblasting a milled denture base may offer greater resistance to dislodgement.

Smoothening Perfluoroalkylated Surfaces: Liquid‐Like Despite Molecular Rigidity?

AbstractThe rational design of surfaces at the molecular level is essential toward realizing many engineering applications. However, molecular‐scale defects affect processes such as triboelectrification, scaling, and condensation. These defects are often detectable via contact angle hysteresis (CAH) measurements. Liquid‐like surfaces exhibit extremely low CAH (≤5°) and rely on the use of highly flexible molecular species such as long‐chain alkyls or siloxanes. Their low glass transition temperatures lead to the so‐termed self‐smoothing behavior, reducing sensitivity to defects formed during fabrication. However, utilizing rigid molecular species such as perfluoroalkyl chains often results in higher hysteresis (10° to 60°) as defects are not self‐smoothed after fabrication. Consequently, state‐of‐the‐art perfluoroalkylated surfaces often show sub‐optimal interfacial properties. Here, a customizable chemical vapor deposition process creates molecularly‐thick, low‐defect surfaces from trichloro(1H,1H,2H,2H‐perfluorooctyl)silane. By implementing moisture‐exposure controls, highly homogenous surfaces with root‐mean‐square roughness below 1 nm are fabricated. CAH is achieved down to ≈4° (average: 6°), surpassing the state‐of‐the‐art by ≈5°. Reduction of CAH (26° to 6°) results in condensation suppression, decreasing surface droplet density by one order and surface droplet coverage by 40%. This work guides the synthesis of high‐quality surfaces from tri‐functional perfluoroalkylsilanes with liquid‐like properties despite their molecular rigidity.

Comparison of Contact Angle Models in Two‐Phase Flow Simulations Using a Conservative Phase Field Equation

ABSTRACTIn phase field methods based on a second‐order Allen‐Cahn (AC) equation, contact angles are prescribed mostly via a geometric formulation. However, it is of great interest to utilize the surface‐energy formulation, which is often employed in the Cahn‐Hilliard (CH) phase field method, in the AC phase field method. This article thus put forward a surface‐energy formulation of contact angles. The model was compared with the geometric one in a number of impact problems, including both normal and oblique impacts. The governing equations were discretized using a finite difference method on a half‐staggered grid. The Navier–Stokes equation was tackled using an explicit projection method. The major findings are as follows. First, the geometric model can maintain a fixed contact angle throughout contact line motion, while the surface‐energy one predicts a changeable contact angle, with a fluctuation of about 5°. In the oblique drop impact, contact angle hysteresis was captured even if a static contact angle was applied in the surface‐energy formulation.

Validation of a phase-field approach for contact line hysteresis against a sloshing droplet case

AbstractUnderstanding liquid propellants behavior in microgravity conditions is critical for efficient spacecraft design. For a number of operations, ranging from engine restart to orbital propellant storage and transfer, insight is needed to characterize capillary-dominated flows. In such conditions, surface tension and wetting properties, including contact angle hysteresis, can greatly impact the fluid’s behavior and therefore spacecraft performance. Using experimental data from ESA Propulsion Laboratory, a contact line model for the Cahn–Hilliard phase-field method is validated. The case studied is that of a droplet confined between two oscillating plates, which aims to isolate and observe contact angle-driven physics, limiting the effect of gravity on the flow in a simple and reproducible way on ground. The contact line model allows for the prediction of contact line motion without requiring the computation of dynamic contact angles or contact line velocities, thus simplifying implementation and reducing computational overhead. For the validation, contact line pinning and motion under varying oscillation frequencies is investigated. Specifically, the length between the rear and front contact line edges, as well as the shape of the sloshing droplet are compared. The results show good agreement between simulations and experimental data, confirming the model’s accuracy in predicting contact line behavior and pinning.

Ultra-Slippery Hydrophilic Surfaces by Hybrid Monolayers.

Slippery solid surfaces with low droplet contact angle hysteresis (CAH) are crucial for applications in thermal management, energy harvesting, and environmental remediation. Traditionally, reducing CAH has been achieved by enhancing surface homogeneity. This work challenges this conventional approach by developing slippery yet hydrophilic surfaces through hybrid monolayers composed of hydrophilic polyethylene glycol (PEG)-silane and hydrophobic alkyl-silane molecules. These hybrid surfaces exhibited exceptionally low CAH (<2°), outperforming well-established homogeneous slippery surfaces. Molecular structural analyses suggested that the remarkable slipperiness is due to a unique spatially staggered molecular configuration, where longer PEG chains shield shorter alkyl chains, thus creating additional free volume while ensuring surface coverage. This was supported by the observation of decreased CAH with increasing temperature, highlighting the role of grafted chain mobility in enhancing slipperiness by self-smoothing and fluid-like behaviors. Furthermore, condensation experiments demonstrated the exceptional performance of the hydrophilic slippery surfaces in dew harvesting due to superior condensation nucleation, droplet coalescence, and self-sweeping efficiency. These findings offer a novel paradigm for designing advanced slippery surfaces and provide valuable insights into the molecular mechanisms governing dynamic wetting.

Facile, scalable and Substrate-Independent omniphobic surface

Omniphobic surfaces have a very wide range of applications. However, limited by substrate material and/or fabrication processes, scalable synthesis of robust omniphobic surfaces with universality and versatility remains challenging for both academia and industry. Here, we present a facile and scalable slippery omniphobic surface (FSSOS) based on the straightforward blending and dip/spray-coating of polysilazane (PSZ) and minute low surface energy silane under room temperature. Water shows a contact angle hysteresis (CAH) of 18°, and the overall trend across all tested solvents suggests a relatively low CAH (＜10°), further enhancing its surface omniphobicity. The one-step synthesis protocol is cost-effective, substrate-independent, and does not require curing aids such as UV irradiation or heat. The FSSOS achieves multi-liquid omni-repellency with chemical and mechanical durability under various harsh exposure conditions. The CAH remains stable even after exposure to 4 m/s water jet impact for 8 h, 130 W ultrasonic vibration for 250 min, 10 kPa pressure tape-peel test for 250 cycles, heating at 250 °C for 10 min, and 205 mW/cm2 UV irradiation for 28 days. This approach highlights a functional design of liquid-repellent surfaces for numerous real-world applications.

Wetting phenomena of droplets and gas bubbles: Contact angle hysteresis based on varying liquid-solid and solid-gas interfacial tensions.

Wetting phenomena have been widely observed in our daily lives, such as dew on lotus leaf, and applied in technical applications, e.g., ink-jet printing. In contrast to constant liquid-solid and solid-gas interfacial tensions in Young's law, we here focus on the wetting phenomena by considering varying fluid-solid interfacial tensions. We analyze the energy landscape, the map of Young's contact angle, the number and corresponding contact angle of local energy minima, and the contact angle hysteresis for a liquid droplet on a solid substrate in a gas phase. In addition, a gas bubble on a solid substrate in a liquid phase has been scrutinized from the aspect of surface energy minimization. The wetting effect has been regarded, where the liquid and gas species penetrate into the solid phase on the microscopic scale [F. Wang and B. Nestler, Phys. Rev. Lett. 132, 126202 (2024)]. We assume the liquid-solid and the solid-gas interfacial tensions to be a function dependent on the volume fraction of the liquid and the gas species and investigate the impact of the size ratio of the droplet to the solid surface that is overlooked in existing theories on the wetting phenomena. Our finding sheds light on the microscopic origin of the contact angle hysteresis and the droplet size effect on the wetting phenomena.

Hot embossed micropatterned slippery Al 5083 alloys in stagnant and laminar saltwater

Slippery AA 5083 alloy surfaces were fabricated by using ground (mean roughness-Ra = 45±2 µm), polished (Ra = 9±3 µm) and hot embossed micro-patterned specimens, and coating them with silicone oils of varying viscosities (1000−100000 cSt). The surface energy of aluminium alloy (AA) 5083 was estimated to be 850±210 mJ·m−2 using Schultz model. The ground/polished specimens were coated with 12±1 µm and 24±0.3 µm thick oil. They showcased slippery character, wherein apparent water static contact angles (SCAs) of 94±1° to 104±1°, slide-off angles (SAs) of 4±0° to 21±2° and contact angle hysteresis (CAH) of <10° were observed. Also, CAH and SAs revealed an increasing trend with an increase in the oil viscosity, irrespective of the coating thickness or the substrate roughness. In addition, the water droplets bounced-off the oil coated (24 µm thick) surfaces up to Weber no. (We) of 3.91. Furthermore, micro-patterned specimens were fabricated by hot embossing at 300 °C, 17+1 MPa, 5 min and comprise of hexagonally arranged pillars and holes with 6.5−7.1 µm diameter and 9.5−13.6 µm pitch. A key finding is that micro-pillars offered the highest tolerance to shear drainage of oil under saltwater (3.5 wt% NaCl) for up to 7 days, in both stagnant and flowing laminar (Reynolds no.‐Re =247) conditions, as compared to micro-holes and unpatterned surfaces. Interestingly, the slippery properties of oil coated unpatterned AA5083 have retained after 7 days of submersion, showing SCAs ≥ 97±3° and SAs ≤ 14°, despite the negative spreading coefficient of oil on AA5083 under water. Such slippery durable micro-pillared and micro-holed AA5083 surfaces could be highly beneficial for mitigating biological corrosion of ship hulls in marine environments.

Superior anti-icing performance of a silicone-based slippery gel coating with a self-replenishing lubricant layer under airflow

Icing phenomena lead to energy consumption, mechanical failures, and potential human casualties. These issues often occur under dynamic airflow conditions. While superhydrophobic (SHPo) surfaces effectively delay ice formation under airflow, they cannot completely prevent it without external energy. This study proposes a long-chain entangled polydimethylsiloxane (LEP) gel coating infused with a lubricant, as an excellent anti-icing surface. The LEP gel has a self-regenerative, low-viscosity oil layer, and it shows remarkable droplet mobility, a very low contact angle hysteresis (CAH), and near-zero ice adhesion. X-ray microimaging shows that condensed droplets on the LEP gel move dynamically, while a bare silicone coating exhibits static behavior. This dynamic droplet behavior significantly delays droplet freezing and frost onset. The LEP gel’s anti-icing performance is assessed in a subsonic wind tunnel. Unlike control surfaces on which frost forms immediately, droplets condense on the gel and then slide off, reducing the frost-coverage rate. Notably, at a wind speed of 5.5 m/s, no frost is formed on the gel. The LEP gel’s superior anti-icing performance is attributed to synergistic effects of the lubricant layer and extremely low CAH. These results show that lubricant-infused slippery gel coatings could prevent icing in airflow environments.

Characterization of Mechanical Properties and Surface Wettability of Epoxy Resin/Benzoxazine Composites in a Simulated Acid Rain Environment

Despite the robustness of thermosetting coatings in various applications, prolonged exposure to acidic environments can cause gradual deterioration, leading to structural or functional damage. This study investigates composite materials comprised of cycloaliphatic epoxy resin (CER) and benzoxazine (BZ) at three different weight ratios: 50:50, 25:75, and 0:100. These composites were exposed to nitric acid, simulating acid rain, for durations ranging from 1 to 5 h. The specimens were characterized for weight change, mechanical properties (flexural strength and short beam strength), and surface properties (contact angle and contact angle hysteresis). Although minimal changes in the physical and mechanical properties of both homopolymer and copolymer composites were detected after short acid exposure (up to 5 h), surface wettability analysis via static contact angle and contact angle hysteresis revealed more pronounced deterioration. The static contact angle decreased by 24.96% and 28.32% for homopolymer BZ and copolymer BZ-CER composites, respectively. Contact angle hysteresis increased by 19.39% and 27.80% for 5 h acid-exposed homopolymer BZ and copolymer CER, respectively. This study underscores the utility of surface wettability analysis as a valuable tool for monitoring deterioration from acidic aging in polymers, particularly in BZ-CER systems used in structural and high-performance applications.

Water drop transportation on wettability switchable surface via anisotropic molecules

Active control of the transportation of liquid drops on a horizontal surface is achieved using surfaces with switchable wettability via remote stimuli. However, the mechanism how the dynamic wettability influences drop dynamics is rarely reported. In this paper, we demonstrate that a surface with switchable wettability induces depinning of the contact line through re-orientation of anisotropic molecules. We investigated the dynamics of contact lines and contact angles during the initiation of drop movement by the advancing and receding angles of the surface. We found that imbalance between advancing and receding angles with respect to the dynamic contact angle provides the force needed to overcome the energy barrier due to contact angle hysteresis on the surface. We discovered that the driving energy is accumulated with oscillations in contact angle until it breaks the pinning energy barrier. Understanding the role of dynamic contact angles in drop movement on switchable surfaces paves the way for designing effective fluid manipulation devices, such as water harvesters, biosensors, and oil–water separators.

Achieving Super-Metallophobicity on Silicon-based Ceramics at High Temperature.

As a critical concept in physical chemistry, superwettability is widely concerned in both fundamental science and practical engineering in past few decades. Despite this, investigation on high temperature superwettability is still a void, which is significant both in scientific and industrial fields. Herein, a ceramic with specific high temperature non-wetting property, Si2N2O is proposed. Compared with other materials, Si2N2O is elucidated with better practical non-wetting property against various non-ferrous metals. Combining with micro-nanostructures, the metallophobicity is further improved (contact angle >150° and contact angle hysteresis ≈0°). The extraordinary metal repellency is defined as "super-metallophobicity", which is proved to be induced by distinctive thermodynamic and dynamic wetting behavior on the rough surface. The research of super-metallophobicity not only sheds light on superwettability at high temperature, but also offers worthy insights for future potential material design in a wide range of applications, such as metallurgy, 3D printing and semiconductor industry.

Adsorption, Adhesion, and Wettability of Commercially Available Cleansers at Dental Polymer (PMMA) Surfaces.

This study aims to evaluate the adsorptive, adhesive, and wetting energetic properties of five commercially available cleansers in contact with model dental polymer (PMMA). It was assumed that the selected parameters allow for determining the optimal concentration and place of key component accumulation for antibacterial activity in the bulk liquid phase and prevention of oral plaque formation at the prosthetic material surface. The adsorptive (Gibbs' excesses ΓLV, critical micellar concentration) and thermal (entropy and enthalpy) surface characteristics originated from surface tension γLV(T) and γLV(C) dependences. The surface wetting properties were quantified upon the contact angle hysteresis formalism on the advancing ΘA, receding ΘR contact angles, and γLV as the input data, which yield a set of wettability parameters: 2D adsorptive film pressure, surface free energy with its dispersive and polar components, work of adhesion, and adhesional tension, considered as interfacial interaction indicators. In particular, molecular partitioning Kp and ΓLV are indicators of the efficiency of particular active substance accumulation in the volume phase, while γSV, a = ΓSL/ΓLV, and WA point to the degree of its accumulation at the immersed polymer surface. Finally, the liquid penetration coefficient PC and the Marangoni temperature gradient-driven liquid flow speed were estimated.

An analytical approach for determining contact angle hysteresis on smooth, micropillared, and micropored homogeneous surfaces

HypothesisNumerous theoretical models have been developed from various research perspectives, including free energy analysis, force balance, and contact line dynamics, to elucidate the contact angle hysteresis on solid surfaces, especially for superhydrophobic surfaces. However, these models can produce inconsistent predictions, and few of them account for contact angle hysteresis on smooth and microstructured surfaces simultaneously. TheoryFormulas for advancing and receding free energy barriers of drops on different solid surfaces were derived, and then these formulas were simplified by incorporating the geometric constraint equation of drops, leading to an establishment of analytical models. FindingsThis study presented a unified approach for deriving analytical models of contact angle hysteresis for various wetting systems, including drops on smooth homogeneous surfaces, Cassie drops on micropillared and micropored homogeneous surfaces, and Wenzel drops on micropillared homogeneous surfaces. The established models revealed a significant impact of frictional tension, and of the change in free energy during drop motion, on the free energy barriers. These models were fully derived thermodynamically without subsequent theoretical modifications and found to agree well with experimental results. Some of the models in this study were validated using other existing models, despite the models having been developed using completely different approaches.

Universal droplet propulsion by dynamic surface-charge wetting

Controllable droplet propulsion on solid surfaces plays a crucial role in various technologies. Many actuating methods have been developed; however, there are still some limitations in terms of the introduction of additives, the versatilities of solid surfaces, and the speed of transportation. Herein, we have demonstrated a universal droplet propulsion method based on dynamic surface-charge wetting by depositing oscillating and opposite surface charges on dielectric films with unmodified surfaces. Dynamic surface-charge wetting propels droplets by continuously inducing smaller front contact angles than rear contact angles. This innovative imbalance is built by alternately storing and spreading opposite charges on dielectric films, which results in remarkable electrostatic forces under large gradients and electric fields. The method exhibits excellent droplet manipulation performance characteristics, including high speed (~130 mm/s), high adaptability of droplet volume (1 μL–1 mL), strong handling ability on non-slippery surfaces with large contact angle hysteresis (CAH) (maximum angle of 35°), significant programmability and reconfigurability, and low mass loss. The great application potential of this method has been effectively demonstrated in programmable microreactions, defogging without gravity assistance, and surface cleaning of photovoltaic panels using condensed droplets.

Superhydrophobicity, Photocatalytic Self-Cleaning and Biocidal Activity Combined in a Siloxane-ZnO Composite for the Protection of Limestone.

The erosion phenomena of the natural stone in cultural heritage are induced by various sources. Consequently, the development of multifunctional protective materials that combine two or more useful properties is an effective strategy in addressing the synergistic effects of various erosion mechanisms. A multifunctional coating, consisting of a silane-based precursor and zinc oxide (ZnO) nanoparticles (NPs), is produced and tested for the protection of limestone. The hybrid coating combines the following three properties: superhydrophobicity, including water-repellency, photocatalytic self-cleaning and biocidal activity. The relative concentration of the NPs (0.8% w/w), used for the suggested composite coating, is carefully selected according to wetting studies, colourimetric measurements and durability (tape peeling) tests. The non-wetting state is evidenced on the surface of the composite coating by the large contact angle of water drops (≈153°) and the small contact angle hysteresis (≈5°), which gives rise to a physical self-cleaning scenario (lotus effect). The photocatalytic chemical self-cleaning is shown with the removal of methylene blue, induced by UV-A radiation. Moreover, it is shown that the suggested coating hinders the incubation of E. coli and S. aureus, as the inhibitions are 94.8 and 99.9%, respectively. Finally, preliminary studies reveal the chemical stability of the suggested coating.

Spreading of rotating liquid annular ring with hysteresis effects

The spreading of a liquid, subjected to centrifugal forces or an air jet, leads to a lowering of the liquid height at the center. If the system is partially wetting, experiments show a break up of the liquid and the appearance of a dry patch at the center. In this article, the spreading of a liquid droplet or a liquid film, featuring a dry patch at the center, is investigated. Via a generalized Tanner’s law, we allow for a contact-angle hysteresis in partially wetting systems. By means of the lubrication approximation, an analytical quasi-steady solution can be derived in the limit of small capillary numbers. We find a power-law regime for the spreading, in which at least one of the contact lines follows a power law in time, almost independent of both static contact angles. The influence of the static contact angles remains restricted to the beginning of the spreading and to transition periods. Four different types of spreading can be identified, namely spreading (i) with a very thin central liquid layer, (ii) as annular ring with central dry patch, (iii) into a static equilibrium, and (iv) with closing of the central dry patch. In a flow map, these types of spreading are mapped as function of both static contact angles. Moreover, the dependency from gravitational and centrifugal forces is investigated and included in the flow map. For a perfectly wetting system, a disjoining-pressure correction in combination with Tanner’s law prohibits negative liquid heights at the center. Hereby, the magnitudes of the Hamaker constants have a negligible influence and arbitrary-small values can be used, since the dynamics of the contact line remains controlled by Tanner’s law.

Anti-icing transparent coatings modified with bi- and tri-functional octaspherosilicates for photovoltaic panels

In recent years, to progress towards carbon emissions reduction, photovoltaic technology has become one of the most significant methods of generating electricity using renewable sources. However, the accumulation of ice and snow during the winter season affects the decrease in the power generation efficiency of photovoltaic modules. As a promising solution, coatings that exhibit anti-icing properties can be used. To date, no efficient ice-phobic coating has been developed for use on photovoltaic panels. In this paper development of transparent silicone-epoxy coatings modified with bi- and tri-functionalized octaspherosilicates was presented. The used organosilicone-based modifiers reveal both types of functional groups: one increases the surface properties and the second compatibility with the polymer matrix. The icephobic properties were discussed by determining an ice adhesion (IA) and a freezing delay time of water droplets (FDT). The coating exhibiting the highest anti-icing performance achieved the IA of 102 kPa and the FDT of 210 min, a 43 % reduction and a 70 times increase in value compared to the unmodified coating, respectively. The performed study included also the characterization of hydrophobic properties: water contact angle (WCA) at room temperature and below 0°C, contact angle hysteresis(CAH), and roll-off angle (RoA) as well as surface roughness and coatings thickness. In addition, the correlations between them and ice adhesion/freezing delay time were determined. It was found that roughness as well as dynamic parameters of the wettability of CAH and RoA have a significant effect on the obtained IA values. Moreover, the developed coatings can be potentially applied to photovoltaic panels, since the conducted modification did not affect the optical properties of the investigated coatings.

Ultrahigh Subcooling Dropwise Condensation Heat Transfer on Slippery Liquid-like Monolayer Grafted Surfaces.

Rapid and continuous droplet shedding is crucial for many applications, including thermal management, water harvesting, and microfluidics, among others. Superhydrophobic surfaces, though effective, suffer from droplet pinning at high subcooling temperature (Tsub). Conversely, slippery liquid-like surfaces covalently bonded with flexible hydrophobic molecules show high stability and low droplet adhesion attributed to their dense and ultrasmooth water repellent polymer chains, enhancing dropwise condensation and rapid shedding. In this work, linear poly(dimethylsiloxane) chains of various viscosities are covalently bonded onto silicon substrates to form thin and smooth monolayer coated surfaces. The formation of the monolayer is characterized by cryogenic transmission electron microscopy. On these surfaces a very low contact angle hysteresis is reported within wide surface temperature ranges as well as continuous dropwise condensation at ultrahigh Tsub of 60 K. In particular, one of the highest condensation heat fluxes of 1392.60 kW·m-2 and a heat transfer coefficient of 23.21 kW·m-2·K-1 at ultrahigh Tsub of 60 K is reported. The experimental heat transfer performance is further compared to the theoretical heat transfer via the individual droplets with the droplet distribution elucidated via both macroscopic observations as well as environmental scanning electron microscopy. Finally, only a mild decrease in the heat transfer coefficient of 20.3% after 100 h of condensation test at Tsub of 60 K is reported. Slippery liquid-like surfaces promote droplet shedding and sustain dropwise condensation at high Tsub without flooding empowered by the lower frictional forces, addressing challenges in heat transfer performance and durability.

Droplet Friction on Superhydrophobic Surfaces Scales With Liquid-Solid Contact Fraction.

It is generally assumed that contact angle hysteresis of superhydrophobic surfaces scales with liquid-solid contact fraction, however, its experimental verification has been problematic due to the limited accuracy of contact angle and sliding angle goniometry. Advances in cantilever-based friction probes enable accurate droplet friction measurements down to the nanonewton regime, thus suiting much better for characterizing the wetting of superhydrophobic surfaces than contact angle hysteresis measurements. This work quantifies the relationship between droplet friction and liquid-solid contact fraction, through theory and experimental validation. Well-defined micropillar and microcone structures are used as model surfaces to provide a wide range of different liquid-solid contact fractions. Micropillars are known to be able to hold the water on top of them, and a theoretical analysis together with confocal laser scanning microscopy shows that despite the spiky nature of the microcones droplets do not sink into the conical structure either, rendering a diminishingly small liquid-solid contact fraction. Droplet friction characterization with a micropipette force sensor technique reveals a strong dependence of the droplet friction on the contact fraction, and the dependency is described with a simple physical equation, despite the nearly three-orders-of-magnitude difference in liquid-solid contact fraction between the sparsest cone surface and the densest pillar surface.