Self-Generated Diffusioosmotic Flows from Calcium Carbonate Micropumps Joseph J. McDermott, † Abhishek Kar, † Majd Daher, † Steve Klara, † Gary Wang, † Ayusman Sen, ‡ and Darrell Velegol* ,† † Department of Chemical Engineering and ‡ Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States * S Supporting Information ABSTRACT: Calcium carbonate particles, ubiquitous in nature and found extensively in geological formations, behave as micropumps in an unsaturated aqueous solution. The mechanism causing this pumping is diffusioosmosis, which drives flows along charged surfaces. Our calcium carbonate microparticles, roughly ∼10 μm in size, self-generate ionic gradients as they dissolve in water to produce Ca 2+ , HCO 3 - , and OH - ions that migrate into the bulk. Because of the different diffusion coefficients of these ions, spontaneous electric fields of roughly 1-10 V/cm arise in order to maintain electroneutrality in the solution. This electric field drives the diffusiophoresis of charged tracers (both positive and negative) as well as diffusioosmotic flows along charged substrates. Here we show experimentally how the directionality and speed of the tracers can be engineered by manipulating the tracer zeta potential, the salt gradients, and the substrate zeta potential. Furthermore, because the salt gradients are self-generated, here by the dissolution of solid calcium carbonate microparticles another manipulated variable is the placement of these particles. Importantly, we find that the zeta potentials on surfaces vary with both time and location because of the adsorption or desorption of Ca 2+ ions; this change affects the flows significantly. ■ INTRODUCTION Increasing demand for the miniaturization of devices has led to a need for better control of pumping, mixing, and moving fluids to meet the desired needs. 1 Because pressure-driven mecha- nisms work poorly in tight or dead-end spaces, the need for an alternative mechanism to drive such flows is required. 2-5 The advent of colloidal motors 6-10 and micropumps 11-13 provides alternative ways of attaining flows in microchannels and nanochannels through chemistry-based mechanisms. In this article, we show that the simple dissolution of calcium carbonate microparticles, a material ubiquitous in natural geologic formations, can self-generate electric fields of roughly 1-10 V/cm that pump fluids and tracer particles over distances many times greater than the carbonate particle radius. We also find that even for simple model systems the interplay between the chemistry and fluid dynamics is complex, providing significant opportunities for designing flows and transport in regions that were previously inaccessible. Electroosmotic pumping for applications in narrow channels has been explored in the literature. 14-17 However, there arises a significant limitation: electroosmotic pumps need an external power source that is not feasible in many difficult-to-reach spaces. For example, placing electrodes in tight geometries can be challenging. However, diffusiophoresis is a transport mechanism that operates on the basis of the existence of a gradient of ion concentration; no electrodes are needed. This mechanism has seen relatively little technological application, although it has recently been used in connection with DNA translocation and entrapment 18 and colloidal transport. 19-21 Diffusiophoresis converts the free energy of dissolution, precipitation, or chemical reactions into a directed motion of fluid and tracers. The flow mechanism of diffusioosmosis has been studied using both modeling 22-24 and experiments 25-27 via imposed salt gradients in 1D systems at steady state. However, self-generated ionic gradients can be established when a solid dissolves into ions in an unsaturated solution. Such dissolution can occur when the thermodynamic equilibrium between the mineral and the surrounding water is disturbed, such as when new surfaces become exposed, allowing further dissolution of the minerals in surrounding aqueous regions. This physical phenomenon produces local ion gradients originating at the mineral surface. The gradients in turn drive microflows and particle movement along charged surfaces and pores by the mechanism of diffusiophoresis. In our systems, we observe mesoscale flows with speeds reaching as high as 40 μm/s. In short, the dissolution of the mineral particles provides a type of “localized battery”, and the charged surfaces provide the “pump”, even though surfaces act as a Received: August 23, 2012 Revised: October 13, 2012 Published: October 15, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 15491 dx.doi.org/10.1021/la303410w | Langmuir 2012, 28, 15491-15497