Surface Forces Apparatus Measurements of Interactions between Rough and Reactive Calcite Surfaces.

nm-Range forces acting between calcite surfaces in water affect macroscopic properties of carbonate rocks and calcite-based granular materials and are significantly influenced by calcite surface recrystallization. We suggest that the repulsive mechanical effects related to nm-scale surface recrystallization of calcite in water could be partially responsible for the observed decrease of cohesion in calcitic rocks saturated with water. Using the surface forces apparatus, we simultaneously followed the calcite reactivity and measured the forces in water in two surface configurations: between two rough calcite surfaces (CC) and between rough calcite and a smooth mica surface (CM). We used nm-scale rough, polycrystalline calcite films prepared by atomic layer deposition. We measured only repulsive forces in CC in CaCO3-saturated water, which was related to roughness and possibly to repulsive hydration effects. Adhesive or repulsive forces were measured in CM in CaCO3-saturated water depending on calcite roughness, and the adhesion was likely enhanced by electrostatic effects. The pull-off adhesive force in CM became stronger with time, and this increase was correlated with a decrease of roughness at contacts, the parameter which could be estimated from the measured force-distance curves. That suggested a progressive increase of real contact areas between the surfaces, caused by gradual pressure-driven deformation of calcite surface asperities during repeated loading-unloading cycles. Reactivity of calcite was affected by mass transport across nm- to µm-thick gaps between the surfaces. Major roughening was observed only for the smoothest calcite films, where gaps between two opposing surfaces were nm-thick over µm-sized areas and led to force of crystallization that could overcome confining pressures of the order of MPa. Any substantial roughening of calcite caused a significant increase of the repulsive mechanical force contribution.

Ca2+ Ions Decrease Adhesion between Two (104) Calcite Surfaces as Probed by Atomic Force Microscopy.

Solution composition-sensitive disjoining pressure acting between the mineral surfaces in fluid-filled granular rocks and materials controls their cohesion, facilitates the transport of dissolved species, and may sustain volume-expanding reactions leading to fracturing or pore sealing. Although calcite is one of the most abundant minerals in the Earth's crust, there is still no complete understanding of how the most common inorganic ions affect the disjoining pressure (and thus the attractive or repulsive forces) operating between calcite surfaces. In this atomic force microscopy study, we measured adhesion acting between two cleaved (104) calcite surfaces in solutions containing low and high concentrations of Ca2+ ions. We detected only low adhesion between calcite surfaces, which was weakly modulated by the varying Ca2+ concentration. Our results show that the more hydrated calcium ions decrease the adhesion between calcite surfaces with respect to monovalent Na+ at a given ionic strength, and thus Ca2+ can sustain relatively thick water films between contacting calcite grains even at high overburden pressures. These findings suggest a possible loss of cohesion and continued progress of reaction-induced fracturing for weakly charged minerals in the presence of strongly hydrated ionic species.

Adhesive forces between two cleaved calcite surfaces in NaCl solutions: The importance of ionic strength and normal loading.

The mechanical strength of calcite bearing rocks is influenced by pore fluid chemistry due to the variation in nano-scale surface forces acting at the grain contacts or close to the fracture tips. The adhesion of two contacting surfaces, which affects the macroscopic strength of the material, is not only influenced by the fluid chemistry but also by the surface topography. In this paper, we use Atomic Force Microscope (AFM) to measure the interfacial forces between two freshly cleaved calcite surfaces in CaCO3-saturated solutions with varying NaCl concentration. We show that calcite contacts become stronger with increasing NaCl concentration (>100 mM), as a result of progressively weaker secondary hydration and increasing attraction due to instantaneous ion-ion correlation. Moreover, we discuss the effect of normal applied force (Fn) and surface roughness on the measured adhesion forces (Fad). We show that the measured pull-off force (adhesion) is linearly correlated with the magnitude of Fn, where an increase in applied force results in increased adhesion. This is attributed to a larger number of contacting surface asperities and thus increase in real contact area and the contact-bond strength. We discuss that the possible variation in local topography at contacts, together with strong dependence on ionic strength of the solution, can explain the inconsistent behavior of calcite rocks in NaCl solutions.

The Association between Life Satisfaction and the Extent of Depression, Anxiety and Stress among Iranian Nurses: A Multicenter Survey.

OBJECTIVE: The determinants of satisfaction of life (SWL) are poorly described among Iranian employed nurses. This study aimed to assess the effect of various factors including age, gender, marital status, depression, anxiety and stress on SWL among Iranian nurses. METHOD: Employed nurses in three teaching hospitals were invited to participate in this study. Ninety-four nurses (65 women and 29 men) participated in this study. Depression, anxiety, stress scale (DASS-21) was used to measure the related variables. Satisfaction with Life Scale (SWLS) was used to assess SWL. Multivariate analysis was utilized to examine the relationship between multiple variables. RESULTS: A noticeable proportion of Iranian employed nurses were either dissatisfied or extremely dissatisfied with life (45%). Severe depression was related to lower scores of SWLS (P: 0.001, r = -0.32). The similar outcomes were detected between anxiety and stress scales, and SWLS (P: 0.023, r = -0.23 and P: 0.008, r = -0.27 for anxiety and stress, respectively). Although females were more vulnerable to depression (P: 0.010) and stress (P: 0.013), the overall effect of gender on SWL was insignificant (0.41). Satisfactions with financial power and work environment were associated with higher scores of SWLS (P: 0.030 and 0.042, respectively). Marital status was not related to severity of depression, anxiety, stress and SWLS (P: 0.39, 0.38, 0.80, and 0.61, respectively). CONCLUSION: This study revealed that poor satisfaction with financial status and work environment, depression, anxiety and stress are the major determinants of satisfaction with life among Iranian employed nurses.