Adhesion and detachment mechanisms of sugar surfaces from the solid (glassy) to liquid (viscous) states

Boxin Zhao*, Hongbo Zeng*, Yu Tian*†, and Jacob Israelachvili*‡

*Department of Chemical Engineering, Materials Research Laboratory, University of California, Santa Barbara, CA 93106; †State Key Laboratory of , Department of Precision Instruments, Tsinghua University, Beijing 100084, People’s Republic of China

Contributed by Jacob Israelachvili, October 27, 2006 (sent for review August 25, 2006)

The viscosity of some sugars varies continuously by Ϸ10 orders of magnitude over a temperature range of 50°C. Sugars are therefore ideal model materials for studying the failure mechanism of ma- terials as they change from the solid to the liquid state. We have used a coupled to interference and optical microscopy imaging to study the way two sugar surfaces adhere and detach from adhesive contact. The sugar-coated layers on mica displayed significant loading–unloading hysteresis, and the adhe- sive strength and failure mechanism during a ‘‘loading–unloading cycle’’ depended on the loading rate, contact time, and unloading rate. The failure of two glassy sugar surfaces was manifested by the nucleation of many sharp microcracks at the external boundary that rapidly propagate along the original contact interface. At the other extreme, when the material is a liquid, ‘‘failure’’ occurs through the nucleation and inward growth of large rounded ripples, characteristic of a Saffman–Taylor fingering instability. In the transition from glassy to viscous failure, sharp crack tips and smooth rounded fingers coexist during the crack propagation. In addition, whereas the detachment geometry of two glassy surfaces Fig. 1. Viscosity of glucose as a function of temperature (18). The glass was characterized by a peeling-like mechanism along a plane, the transition temperature Tg was measured by differential scanning calorimetry ϭ fingers associated with the ‘‘snapping’’ of a liquid neck extended in at 10°C/min; the melting point is Tm 146°C. Also shown is the chemical all directions. These findings provide insights into the adhesion and structure of glucose, which has an irregular shape with five exposed hydro- gen-bonding hydroxyl groups. failure mechanisms of materials at the micro/nanoscales and are also relevant to the action of interparticle forces between sugar particles, such as those used in inhalation drug delivery. The viscosity of some sugars varies continuously over many orders of magnitude over a narrow temperature range. Fig. 1 shows adhesive failure ͉ fingering instability ͉ crack propagation ͉ the viscosity of glucose as a function of temperature (18). Unlike contact dynamics polymers that exhibit similar properties (8), glucose is a small molecule, and is therefore an ideal model material for studying the veryday experiences tell that solids fracture and liquids flow. transition between solid- and liquid-like materials and processes, EHowever, for many materials there is no clear boundary be- which has long been a challenge in materials science and engineer- tween the solid and liquid states (1). Also, a material can behave like ing. In addition to using sugar as a model material, the adhesion either a solid or a liquid depending on the time scale of the between sugar particles is important in its own right in the confec- observation. A typical material that does this is pitch, which, if hit tion industry (19) and in new biotechnologies such as the protein rapidly with a hammer, shatters like a brittle solid, but can flow like encapsulation by sugars and the inhalation of sugar-coated drug- a liquid over periods of years (2). Here, we report on a study of the carrying aerosols (20). adhesion and detachment mechanisms of a common sugar, glucose, as it transits from the solid (glassy) to the liquid (viscous) state. Results and Discussion The classic Hertz and Johnson–Kendall–Roberts (JKR) theories Adhesion and Detachment of Glassy Surfaces (T < Tg). When two (3, 4) describe the adhesion and contact mechanics of solids. These smooth glucose-coated surfaces of radius R Ϸ 2 cm are gently theories have been found to hold well for molecularly smooth and brought into contact under zero external load (L ϭ 0), they nonhysteretic surfaces both at macroscopic and microscopic length immediately deform because of the attractive intersurface forces scales. However, the adhesion mechanics of viscoelastic materials that pull the two curved surfaces into a flattened circular contact and sticky fluids are not so well understood (5–9). The adhesion or area of radius r0. On further pressing the surfaces together (L Ͼ 0) ϭ ␲ 2 ␲ 2 fracture energy of viscoelastic, such as some polymer, materials can the contact area A r increases above r0.Fig.2A shows a typical be four orders of magnitude greater than the thermodynamic value, (measured) contact diameter 2r as a function of the compressive which has been attributed to such energy-dissipating processes as molecular interdiffusion or entanglements across the contact junc- tion and macroscopic viscous/plastic deformations (10–12). In Author contributions: B.Z. and J.I. designed research; B.Z. and H.Z. performed research; B.Z., addition, detaching viscoelastic materials exhibit various interfacial H.Z., and Y.T. analyzed data; and B.Z. and H.Z. wrote the paper. instabilities and cavitation, which are also not yet understood The authors declare no conflict of interest. (13–16). It has been suggested (17) that conventional theories of Abbreviation: JKR, Johnson–Kendall–Roberts. Newtonian liquids could serve as a starting point for understanding ‡To whom correspondence should be addressed. E-mail: [email protected]. the instabilities associated with detaching viscoelastic materials. © 2006 by The National Academy of Sciences of the USA

19624–19629 ͉ PNAS ͉ December 26, 2006 ͉ vol. 103 ͉ no. 52 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0609529103 Downloaded by guest on September 29, 2021 SCIENCES APPLIED PHYSICAL

Fig. 3. Surface deformations during adhesion and detachment of glassy surfaces. (A) The top-view image right before pull-off after aging for 20 min at 23°C (point P in Fig. 2A). (B) Same as A showing the FECO fringe pattern just Fig. 2. Adhesion and detachment of glassy surfaces. (A) Typical plot of before pull-off. (C) Top-view image right after pull-off. (D) FECO fringe flattened contact diameter 2r as a function of load L for two initially curved pattern of second contact 2 min after separation. (E) SEM image of the ϭ glucose surfaces (R 2.0 cm) at 23°C during a loading-hold-unloading cycle. The external edge of the contact area after detachment following overnight aging ϭ ϭ ˙ ϭ contact time at Lmax was tc 22 min, and the unloading rate was dL/dt L 0.2 in contact (from a different sample). mN/s. The solid curve is the JKR fit (Eq. 1), using the initial contact diameter at zero ϭ ␮ ϭ ϭ ␲ 2␥ 3 ϭ load (2r0 80 matL 0), which gives K (because K 12 R /r0 at L 0). (B) ␥ ϭ ␲ Measured adhesive strength, ad Lad/3 R, as a function of contact time at Lmax and predicting the correct (thermodynamic) values for ␥ to ϭ 40 mN at two different unloading rates of L˙ ϭ 0.2 and 1 mN/s. better than 10%. The path predicted by the JKR theory for the sugar surfaces is load L for two glucose surfaces in the glassy state at 23°C. Once the shown by the solid curve in Fig. 2A. The measured loading data fit load had reached a certain value, L , the surfaces were allowed the theoretical prediction extremely well, indicating the (initially) max ␥ ϭ Ϯ to ‘‘age’’ for a certain contact time, t , after which the load was elastic nature of the materials and a surface energy of 46 5 c 2 ␥ ϭ 2 reduced until the surfaces abruptly jumped apart at a negative mJ/m , which is close to the thermodynamic value of 42 mJ/m (tensile) load L ϭϪL , commonly referred to as the adhesion or for glucose at this temperature (23). ad A ‘‘pull-off’’ force. It is clear from Fig. 2A that the loading and Fig. 2 further shows that the contact diameter did not change during the aging. This is reasonable considering the rigidity of unloading processes are not reversible: there is a strong adhesion glucose at this temperature. During unloading, instead of retracing hysteresis. the JKR path down to L ϭ 3␲R␥ ϭ 8 mN, the contact radius According to the JKR theory (21), an elastic sphere of radius R ad hardly changed until the surfaces detached abruptly at a much when pressed by a load L against a flat surface of the same material ␥ higher adhesion force. This ‘‘effective’’ adhesion force, Lad(eff) in of bulk elastic modulus K and surface energy will have a flat Fig. 2A, may be used in Eq. 2 to give the effective adhesive strength, contact area of radius r given by ␥ad (which is to be distinguished from ␥, the surface energy at true R thermodynamic equilibrium). For the system shown in Fig. 2A, the 3 ϭ ͫ ϩ ␲ ␥ ϩ ͱ ␲ ␥ ϩ ␲ ␥ 2ͬ adhesive strength was ␥ ϭ 234 mJ/m2, which is Ϸ5 times higher r L 6 R 12 R L (6 R ) . [1] ad K than the thermodynamic value. ␥ Furthermore, the loading and unloading paths should be revers- Fig. 2B shows the adhesive strength ad as a function of the contact time t at the same maximum load for two unloading rates ible, and separation or pull-off should occur when c defined in terms of the forces or loads, dL/dt or L˙ (to be distin- ϭϪ ϭϪ ␲ ␥ L Lad 3 R . [2] guished from separation rate in terms of the distance, defined by dD/dt or D˙). Within the time scales of our measurements, the The JKR theory has been found to work well for ‘‘ideal’’ (clean, adhesive strength increased roughly linearly both with the contact smooth, elastic) surfaces with known adhesion energies, e.g., time tc, and unloading rate dL/dt. Qualitatively, the observed aging mica coated with surfactant monolayers (22) or smooth layers of and rate-dependent effects are similar to those previously observed elastomeric polymers (10–12), showing no hysteresis, namely, for nonequilibrium (hysteretic) adhesion processes of, for example, the loading–unloading paths described by Eq. 1 are reversible, surfactant and polymer surfaces (7, 8, 10, 12, 22). However, the

Zhao et al. PNAS ͉ December 26, 2006 ͉ vol. 103 ͉ no. 52 ͉ 19625 Downloaded by guest on September 29, 2021 Fig. 4. Adhesion and detachment of viscoelastic sur- faces. (A) Typical JKR plot for glucose at 40°C. The load- ing and unloading rates were both L˙ ϭ 0.2 mN/s, and the contact time at Lmax was tc ϭ 22 min. The solid line is the JKR prediction (Eq. 1), constructed as in Fig. 2A.(B and C) Top microscope views and FECO fringe images at point P in A.(D and E) Top microscope views and FECO fringe images at point Q in A.

unloading curves for these systems were still JKR-like, i.e., curved, Interestingly, many radial micro waves/ripples can be seen just rather than the straight, almost horizontal path until the failure outside the contact boundary. Similar features were also observed point. at higher temperature, as described below. Some details of the surface deformations in the fracture zone are shown in Fig. 3. Fig. 3 A and B show optical microscope top views Adhesion and Detachment of Viscoelastic Surfaces (T Ϸ Tg). A second and FECO images of the surfaces just before their first detachment set of experiments was conducted at 40°C, which is just above the from contact (point P in Fig. 2A), and Fig. 3 C and D shows the Tg of glucose. To put this temperature into perspective, the bulk surfaces after separation and bringing them back into contact again, viscosity of glucose at 40°C is higher than that of pitch (Fig. 1), respectively. It is apparent that the fracture occurred along the which behaves like a brittle solid that nevertheless flows like a liquid original interface, indicating that not enough time was given for the over periods of years (2). As with the viscosity, the bulk rigidity of contacting surfaces to coalesce (sinter or cold weld) into a contin- glucose has also been found to decrease rapidly above Tg (24). Fig. uous solid. However, at some stage during the contact and detach- 4A shows the JKR plot for loading and unloading this material. Now ment, the originally smooth surfaces have become rough at the the loading branch is less JKR-like, but the unloading branch is nano scale. Thus, Fig. 3D shows discontinuities in the previously more JKR-like. In other words, the loading behavior is less elastic- straight FECO fringes (compare Fig. 3B) coming from within the like, whereas the unloading is less brittle-like; and the cycle is overall flattened contact region, indicative of a local roughness of height less hysteretic. The JKR prediction is shown by the solid line, again Ϸ20 nm. Apparently, in the roughened regions, the surfaces have using the initial contact radius at L ϭ 0 as the reference point. (at least partially) coalesced into bulk material. More quantitative Notably, on first contact, the surfaces do behave like elastic (glassy) investigation is needed to clarify the coarsening of such fracture solids; this is due to the rapidity of the jump into contact and initial zones. Fig. 3E shows a scanning electron microscope (SEM) image flattening, which occurs in Ͻ1 ms. Clearly, the initial contact of a detached surface after aging the contacting surfaces overnight process occurs at a Deborah number, De Ͼ 1 (see below). before detachment. Fig. 3E Left is the contacted region showing On loading to Lmax, the contact area increased more steeply than some micro cracks; Fig. 3E Right is the noncontacted surface. the JKR prediction. This finding indicates that there is now some

19626 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0609529103 Zhao et al. Downloaded by guest on September 29, 2021 Fig. 6. Deborah and Weissenberg numbers, De and Wi, as functions of the separation time ts and velocity of surface separation dD/dt ϭ D˙ at 75°C, calculated based on Eqs. 3–-6.

peeling front. Fig. 4D shows the same location 11 s later, during the pull-off process. It can be seen that the outer ring is still at the

original position, whereas the inner ring had moved inward. The SCIENCES darker region outside the peeling front suggests some fine struc- tures that diffract light differently from the unaffected regions. The APPLIED PHYSICAL fine details of these structures could not be resolved in this study, and they relaxed after the surfaces detached. These structures may be similar to the viscous fingering and cavitation bubbles commonly seen in the peel zones of peeling adhesive tapes and viscoelastic polymer surfaces (13, 25).

Fig. 5. Detachment/failure of a glucose neck at 75°C (initial film thickness: 56 Adhesion and Detachment of Viscous Surfaces (T > Tg). The third set nm). (A) Diameter of the contact neck as a function of time during coalescence of experiments was conducted at 75°C, Ϸ27°C above the Tg of (under a low compressive load of 5 mN) and detachment. (B–E) Sequential glucose. Fig. 5A shows the contact diameter as a function of time. top-view microscope images of the surface deformations during neck thin- In this viscous state, the two surfaces coalesced immediately on ning leading to ultimate detachment. The substrate (mica) surfaces deformed coming into contact, with an outwardly growing meniscus (neck) in in a Hertzian fashion when under a compressive load, and relaxed to their which waves/ripples developed (even under zero external load). undeformed curved geometry on separation. Details of the coalescence process are given in ref. 26; here we focus on the separation process where, on the macroscopic scale, the plastic flow of the glucose under the action of the compressive load surface deformations were very much as expected for a thinning in addition to the purely elastic deformation predicted by the JKR liquid neck that eventually snaps. The adhesion strength was now Ϸ 2 theory. On unloading, the contact diameter decreased, first linearly, 80 mJ/m , about twice the thermodynamic value, and the time ts t ϭ then (after a certain tensile load had been exceeded) much more for detachment after the start of unloading ( s 20 s) was much shorter than for the viscoelastic (t ϭ 300sinFig.2A) and glassy sharply toward pull-off, but not as abruptly as in the glassy system s (t ϭ 370sinFig.3A) materials. (Fig. 2). The adhesion strength was now lower than that for the s Ϸ However, fine features at the boundary of the shrinking fluid glassy material, but still 3 times higher than the thermodynamic neck made this system (or temperature regime) particularly inter- value. esting. Fig. 5B shows a top view of the surface deformations at an Correlating the JKR plots with the changing shapes of the fringes early stage of separation, revealing micrometer-sized ripples or (Fig. 4 C and E), it appears that there are two distinct processes with waves growing into the fluid, and much finer secondary structures at least two characteristic time scales in the adhesion-detachment at the external neck boundary. These ripples are similar to the process: (i) elastic flattening and peeling (JKR-like), and (ii) viscous fingers observed at the peel front during the peeling of viscosity- and diffusion-controlled flow and molecular reordering. ‘‘pressure-sensitive adhesive’’ tape from a glass surfaces (27); and The loading–unloading process is similar to that observed with similar viscous fingers have also been reported in various dynamic certain viscoelastic polymers, which also display a complex hyster- studies of confined viscoelastic polymers (8, 28, 29). In the present esis on attachment and detachment (10, 25). experimental geometry, when the two coalesced fluid surfaces are The surface images of the detachment at 40°C show more pulled apart and the neck thins, fluid flows both axially and radially features than for the glassy surfaces. Fig. 4 B–E shows top-view inward toward the center (Fig. 5B). Such flow conditions give rise optical microscope images and FECO fringe images during the to Saffman–Taylor fingers (30), seen as the waves/ripples in Fig. 5 reduction of the contact diameter at points P and Q in Fig. 4A. The B and C. FECO fringes in Fig. 4 C and E were still smooth, indicating that The growth or evolution of these fingers with time is shown in Fig. the separation occurred along the same plane as the original 5 B–E. What is particularly interesting is that the initially rounded interface, i.e., that the two surfaces had not completely coalesced. ripples soon develop into two types of coexisting fingers: rounded Fig. 4B shows two concentric rings, the outer one defining the and sharp, where the rounded fingers moved in faster than the sharp boundary of the initial contact circle, the inner ring defining the crack-like tips, as illustrated in Fig. 5 D and E. Also shown in Fig.

Zhao et al. PNAS ͉ December 26, 2006 ͉ vol. 103 ͉ no. 52 ͉ 19627 Downloaded by guest on September 29, 2021 5 D and E are vapor bubbles that nucleated (cavitated) within the ϭ ␶͞ Ϸ ␶ ͞ De tf D˙ D, [5] fluid and grew outward, eventually to meet and join up with the inward-moving fingers. which is not constant during the separation (detachment) process. Rounded Saffman–Taylor fingers and cavitation bubbles are Likewise, there is no single-valued shear rate ␭˙ that can be put into common when separating two connected viscous surfaces (30), but the above expressions for De or Wi. However, one can estimate the their appearance with sharp crack-like tips in the same detachment maximum values of De and Wi based on the maximum shear rate process was unexpected. We suggest that this temperature and ␭˙max, given by Eq. 3, during the detachment processes. To do this, unloading rate regime falls between solid like-fracture and liquid- we first estimate the molecular relaxation time ␶ using (1) like thinning and snapping. Previous studies on shearing Newtonian liquids have shown that solid-like fracture phenomena occur when ␶ Ϸ ␩͞G Ϸ ␩M͞␳N k T, [6] shear rates exceed a critical value that, for both polymer and simple A B

sugar fluids, is a function of temperature or viscosity (31, 32). where G Ϸ ␳NAkBT/M is the characteristic modulus, ␳ is the density At lower temperatures and/or higher unloading rates we observe of the fluid, NA is Avogadro’s number, and M is the molecular more crack-like tips, whereas at higher temperatures there are only weight. For glucose at 75°C, ␳ Ϸ 1.54 g/cm3, M ϭ 180 g/mol, ␩ Ϸ rounded fingers or no fingers at all (see below). Even finer Ϫ 104 Pa⅐s, so the characteristic relaxation time ␶ is Ϸ4 ϫ 10 4 s. Fig. structures are evident in Fig. 5 that provide further insights: thus, 6 shows plots of De and Wi as functions of the separation time ts Fig. 5 B–E shows smaller tips (secondary or residual cracks) at the ˙ outer boundary, which remained during separation, similar to the and the rate of surface separation, D at 75°C. It appears that De and ˙ sharp microtips seen during the detachment of two glassy surfaces Wi peak very sharply at a particular time ts and velocity D during in Fig. 3E. the separation (detachment) process. However, the peak in De Regarding the range of temperature (and/or viscosity) and occurs at De Ϸ 0.01, which is well below the expected value of 1.0. pulling rates where the liquid-to-solid (viscous-to-brittle) transition It is possible that the real value of the molecular relaxation time ␶ occurred, at the pulling rates used in our experiments the transition is higher than that estimated from Eq. 6, which was calculated in was observed to occur over the temperature range 50–80°C, terms of the bulk shear viscosity ␩ at zero shear rate. corresponding to bulk viscosities ␩ in the range 103 to 107 Pa⅐s. The

reason for the broad, rather than sharp, transition is that sharp crack Adhesion (Coalescence) and Detachment of Liquid Surfaces (T ϾϾ Tg). tips and smooth rounded fingers coexist within the rupturing As the temperature increases further, the viscosity of glucose junction over this temperature range. continues to decrease until it behaves like a simple low-viscosity During a separation the fluid flow within the rupturing neck Newtonian liquid at the shear rates used in these experiments. changes as a function of time and position. Thus, as a function of Because of limitations of our apparatus, we could not study the time, because the system starts and ends at rest (there is no flow at high-temperature behavior of liquid glucose (at T close to Tm ϭ t ϭ 0 and t Ն ts), the shear rate ␭˙ must reach a maximum at some point of the process. As a function of position, Chan and Horn (33) 146°C). However, previous studies on liquids such as n-dodecane ϭϪ ϭ have shown that for a sphere of radius R separating from a flat (Tm 9.6°C) and n-hexadecane (Tm 18°C) have shown that the surface in bulk liquid, at any instant when the surfaces are at a liquid necks thin and eventually snap at the theoretically expected ϭ ␲ ␥ separation D and are moving normally apart at velocity dD/dt ϭ D˙ adhesion force of Lad 4 R , with no ripples or fingers (Table 1, the maximum shear rate in the fluid is given by row 5, columns B and C). These phenomena are all describable by simple macroscopic equations in terms of the surface tension ␥, i.e., ␭˙ ϭ ͞ ͞ 5͞2 1͞2͞ 3͞2 ˙ max (1 2)(3 2) (R D )D, [3] independent of the viscosity (35, 36). and occurs at the surfaces at a radial distance of r ϭ (2RD/3)1/2 from the center (the shear rate is always zero at the center, at r ϭ 0). Note Table 1. Surface deformations and flows characterizing the ␭ that, during the detachment process, ˙max increases from zero transition from solid to liquid (rows 1 3 5) showing the (when D˙ ϭ 0) to a maximum value then falls to zero again. Thus, deformations associated with adhesion͞loading (column A) ␭˙ max itself reaches a maximum value at some point during the and separation͞detachment͞unloading (columns B and C) detachment process. No such equations have been derived for finite-sized liquid necks or bridges between two surfaces, but we may anticipate that at any instant the shear rate is highest and the fluid is therefore more solid-like close to the outer edge, and more liquid-like close to the center. The critical shear rate that describes the transition from one relaxation process/ mechanism to another is usually given by the Deborah number (De) or Weissenberg number (Wi) (1) defined as ϭ ␶͞ ϭ ␶␭ De tf, and Wi ˙, [4] where ␶ is the characteristic relaxation time of the material at the temperature of the measurement, tf is the characteristic time of the flow, and ␭˙ is the shear rate (also sometimes defined as the inverse of the experimental ‘‘observation’’ time, ␭˙ Ϸ 1/tf). The Deborah and Weissenberg numbers are traditionally used in (1): at a high Deborah number (De Ͼ 1), the flow is fast compared with the fluid’s ability to relax, and the fluid then responds more like a solid, whereas for De Ͻ 1 the fluid is more liquid-like. Shull and Creton (17) proposed that the Deborah number can also be used as a quantitative parameter to describe the transition from liquid-like to solid-like behavior of thin polymer films. In our system, the situation is much more complicated because both De and ␭˙max are different at different times and locations within the neck. For our geometry, the Deborah number can be approximated by (8, 34)

19628 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0609529103 Zhao et al. Downloaded by guest on September 29, 2021 Conclusions surface forces apparatus (SFA) (8, 10–12, 37) coupled to interfer- We have used the simple sugar glucose as a model material to study ence and optical microscopy imaging. the differences between solid- and liquid-like adhesion and failure D-glucose from Sigma-Aldrich (St. Louis, MO) was used as processes, and the transition between them. Our results also received. The melting point of glucose is 146°C. Its glass transition provide insights into the adhesion and failure mechanisms of temperature was measured by differential scanning calorimetery Ϸ materials at the micro/nano scales. The surface deformations and and found to be 38°C. Uniform glucose films, 40 nm thick (the flows characterizing the transition from the solid state to the liquid range of film thicknesses used was 20–60 nm), were fabricated on (viscous) state are illustrated in Table 1. The two extremes are molecularly smooth mica surfaces by first dissolving glucose in shown in rows 1 and 5, and the transitional behaviors are shown in deionized water to make 1–10 wt% solutions, then depositing a rows 2–4. drop of one of the solutions onto two molecularly smooth mica As mentioned in the introduction, elastic and nonhysteretic surfaces, then removing the free water from the surface by wicking or spinning. The sugar surfaces were characterized by AFM contacts (Table 1, row 1) are well described by the classic JKR (Veeco-Digital Instruments, Santa Barbara, CA; dimension 3000), theory for two purely elastic materials. The detachment force is L ad giving an rms roughness of Ϸ1 nm. The coated surfaces were kept ϭ 3␲R␥. At the other extreme of pure Newtonian liquids, two in a desiccator under reduced pressure overnight. Two such sur- surfaces coalesce into a liquid bridge right after contact (Table 1, faces were then installed into the SFA, facing each other in a crossed row 5, column A), and detachment or ‘‘failure’’ occurs by the liquid cylinder configuration (cylinder radii R Ϸ 2 cm), which is locally the bridge thinning and snapping (Table 1, row 5, columns B and C), ϭ ␲ ␥ same as a sphere-on-sphere or a sphere-on-flat geometry. Once with a detachment force of Lad 4 R , which is only 33% larger inside the SFA, the glucose films were further dried by purging with than the pull-off force of two elastic solids. Our experimental results dry N gas. The SFA box was well sealed with a small container of fall in between the two extreme cases, where the pull-off forces are 2 P2O5 inside to maintain dryness. very much greater than either the mechanical (JKR) or thermo- Loading–unloading cycles were carried out by pressing the dynamic values. surfaces together under a load, keeping them in contact for The adhesion of two glassy surfaces was describable by a JKR different times, then separated at various unloading rates, dL/dt or process, but only on loading (Fig. 2A and Table 1, row 2, column L˙, until they completely detached. During these loading–unloading A), whereas separation (unloading) lead to abrupt brittle-like processes, the film thickness and contact diameter were measured failure, preceded by the nucleation of many sharp microcracks at (recorded) in real time using the FECO optical interference the boundary (Table 1, row 2, columns B and C). technique, which also allows one to observe the surface shape SCIENCES Around the glass transition temperature, the adhesion of the changes, laterally on the microscale and normally on the nanoscale. APPLIED PHYSICAL surfaces on loading was intermediate between that of a JKR elastic The experimental procedure involved driving the bottom surface deformation and plastic flow (Fig. 4A and Table 1, row 3, column toward the top surface until the two surfaces came into contact. The A), whereas the deformation on detachment was more JKR-like surfaces were then pressed together and kept in contact for (compare Table 1 row 3, column B with row 1, column B). different times, tc. After this ‘‘contact time,’’ the adhering (and now As the material becomes more liquid-like at T Ͼ Tg, the surfaces partially fused) surfaces were separated at various rates until they coalesce right after contact (Table 1, row 4, column A), and detached. The experiments were conducted at three temperatures: ‘‘viscous failure’’ occurs via the nucleation and inward growth of T ϭ 23, 40, and 75°C. At these three temperatures, bulk glucose (Tg large rounded ripples (Table 1, row 4, column B), characteristic of ϭ 38°C) is in the glassy, viscoelastic, and fluid states, respectively a Saffman–Taylor fingering instability. In this regime of the tran- (compare Fig. 1). sition, sharp crack tips and smooth rounded fingers coexist during The FECO fringes and top-view microscope images were con- the crack propagation. In addition, whereas the detachment geom- tinuously recorded at all stages of the approach, coalescence, etry of the more solid-like surfaces was characterized by a JKR-like loading, contact, separation, and detachment of the surfaces. The peeling mechanism along a plane (Table 1, column B, rows 1–3), the contact diameters 2r were measured both by the FECO fringes and fingers associated with the snapping of a liquid neck extended in all by direct visualization of the surfaces using the top-view micro- directions (Table 1, rows 4 and 5, columns B and C). The transition scope. Both methods gave the same results within the errors of the from solid-like to liquid-like failure occurs through a broad regime measurements (Ϸ5%). where cracks and viscous fingers coexist, rather than a sharp, discontinuous transition where one type of failure mechanism is We thank James Langer and Robert McMeeking for helpful discussions replaced by the other. and Greg Carver for technical assistances. This work was supported by the Materials Research and Science Engineering Center Program of the Experimental Methods National Science Foundation under Award No. DMR05-20415. B.Z. was supported by a Natural Sciences and Engineering Research Council of The adhesion (detachment) forces and associated surface defor- Canada postdoctoral fellowship award, and Y.T. was supported by a mations of glucose-coated mica surfaces were studied using a Tsinghua University Huaxin Distinguished Scientist Scholarship.

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