Viscous) States

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 Tribology, 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 materials as they change from the solid to the liquid state. We have used a surface forces apparatus 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 adhesive 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˙).

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