About

Robert Tilton is the Chevron Professor of Chemical Engineering at Carnegie Mellon University, where he has been a faculty member since 1992. He is recognized as a Fellow of both the American Chemical Society and the American Institute for Medical and Biological Engineering. His research group focuses on complex fluid interfacial phenomena, conducting fundamental investigations into the structure and dynamics of macromolecules, surfactants, and composite nanoparticles in suspension or adsorbed at solid or fluid interfaces. The group's work supports application-driven research in areas such as environmental nanotechnology, aerosolized carriers for pulmonary drug delivery, lubrication, and the development of high-efficiency emulsifiers. A central theme of his research is understanding phenomena that arise in realistic multicomponent mixtures, which are not predicted by the behavior of idealized single-component systems. Professor Tilton's research projects include studying colloidal forces in polymer/surfactant mixtures, where the complexation of polymers and surfactants profoundly alters colloidal forces, particularly affecting the strength and range of the colloidal depletion force. Another key area is the investigation of Marangoni transport in complex surfactant systems, exploring how surface tension gradients influence multiphase flows and how surfactant interactions can amplify or suppress these effects. This research also extends to developing self-dispersing aerosolized drug carriers that utilize Marangoni flows to improve the distribution of inhaled medications in the lungs. Additionally, his group works on interfacial engineering with responsive nanoparticulate polymer brushes, nanoscale objects composed of polymer chains attached to a central core, which serve as high-efficiency emulsifiers and responsive boundary lubrication agents with tunable friction and adhesion properties. Through his extensive research, Professor Tilton has contributed significantly to the understanding of interfacial phenomena in complex fluids, bridging fundamental science with practical applications in chemical engineering and related fields.

Research topics

• Chemistry
• Organic chemistry
• Chemical engineering
• Materials science
• Biochemistry
• Composite material
• Nanotechnology
• Horticulture
• Biophysics
• Biology
• Polymer chemistry
• Engineering

Selected publications

• Marangoni Instability Driven by Adsorption and Association of Oppositely Charged Surfactants at an Oil–Water Interface

  Industrial & Engineering Chemistry Research · 2025-07-18

  articleOpen accessSenior authorCorresponding

  When an aqueous buffer solution of the cationic surfactant tetradecyltrimethylammonium bromide near neutral pH is contacted with a solution of the fatty acid surfactant palmitic acid in tetradecane, convective flows appear spontaneously at the interface. This coincides with turbidity formation in the aqueous phase, consistent with aggregation between the headgroups of the two surfactants. Motivated by these experimental observations, a mathematical model is developed for transport, adsorption, and fluid flow in two immiscible contacting fluid phases, each containing a distinct surfactant. Linear stability analysis reveals that gradients generated by dynamic adsorption alone cannot drive instability, which only becomes possible if there is a process by which adsorbed surfactant is removed from the interface, as modeled by an interfacial reaction representing formation and desorption of aggregates in the experiments. A full numerical simulation shows that asymmetry in surfactant transport properties can inhibit convection due to the accumulation of surfactants on the interface.

  PublisherDOI

• Complex Wave Packet Dynamics Induced by Marangoni Stresses

  Industrial & Engineering Chemistry Research · 2025-07-11

  articleOpen access

  New features emerge during Marangoni spreading of a surfactant-laden drop on a thick film, where inertia plays a significant role in the hydrodynamics relative to the more commonly studied low inertia spreading phenomenon. We uncover these features using high-speed imaging and understand their dynamics using numerical simulations on subphases of varying viscosity and depths. Deposition of a drop of surfactant solution drives the formation of a packet of waves moving across the surface. Waves closest to the deposition point are partially covered with the surfactant layer and are directly affected by Marangoni stresses; waves at larger distances are not. A previously unobserved event, merging of peaks close to the deposition point, is detected at early times in experiments and replicated in simulation. Such an event does not occur if a pure fluid drop of the same composition as the subphase is deposited on the surface. At later times, the difference in speed between the surfactant front and the innermost wave causes splitting of the innermost wave, creating a wave covered with surfactant and another wave not covered with surfactant. These features are detectable for thicker subphases of low viscosity liquids, commonly found in laboratory studies and technological settings where Marangoni spreading is present.

  PublisherDOI

• A classification-based methodology for the estimation of binary surfactant critical micelle concentrations

  Digital Discovery · 2025-01-01

  articleOpen accessSenior author

  High throughput experiments with classification and regular solution theory is used to identify the critical micelle concentration in mixed surfactants.

  PublisherOA PDFDOI

• Diffusiophoretic Transport of Charged Colloids in Ionic Surfactant Gradients Entirely below versus Entirely above the Critical Micelle Concentration

  Langmuir · 2024-05-01 · 8 citations

  articleOpen accessSenior authorCorresponding

  When placed in an ionic surfactant gradient, charged colloids will undergo diffusiophoresis at a velocity, uDP = MDP∇ ln S, where MDP is the diffusiophoretic mobility and S is the surfactant concentration. The diffusiophoretic mobility depends in part on the charges and diffusivities of the surfactants and their counterions. Since micellization decreases surfactant diffusivity and alters charge distributions in a surfactant solution, MDP of charged colloids in ionic surfactant gradients may differ significantly when surfactant concentrations are above or below the critical micelle concentration (CMC). The role of micelles in driving diffusiophoresis is unclear, and a previously published model that accounts for micellization suggests the possibility of a change in the sign of MDP above the CMC [Warren, P. B.; . Soft Matter 2019, 15, 278–288]. In the current study, microfluidic channels were used to measure the transport of negatively charged polystyrene colloids in sodium dodecyl sulfate (SDS) surfactant gradients established at SDS concentrations that are either fully above or fully below the CMC. Interpretation of diffusiophoresis was aided by measurements of the colloid electrophoretic mobility as a function of SDS concentration. A numerical transport model incorporating the prior diffusiophoretic mobility model for ionic surfactant gradients was implemented to elucidate signatures of positive and negative diffusiophoretic mobilities and compare with experiments. The theoretically predicted sign of the diffusiophoretic mobility below the CMC was determined to be particularly sensitive to uncertainty in colloid and surfactant properties, while above the CMC, the mobility was consistently predicted to be positive in the SDS concentration range considered in the experiments conducted here. In contrast, experiments only showed signatures of a negative diffusiophoretic mobility for these negatively charged colloids with no change of sign. Colloid diffusiophoretic transport measured in micellar solutions was more extensive than that below the CMC with the same ∇ ln S.

  PublisherOA PDFDOI

• Towards realizing nano-enabled precision delivery in plants

  Nature Nanotechnology · 2024-06-06 · 110 citations

  reviewOpen access
  PublisherOA PDFDOI

• High Aspect Ratio Polymer Nanocarriers for Gene Delivery and Expression in Plants

  Nano Letters · 2024-12-31 · 13 citations

  articleOpen accessCorresponding

  Plant genetic engineering methods are critical for food security and biofuel production and to enable molecular farming. Here, we elucidated how polymeric high aspect ratio nanocarriers can enable DNA delivery to Nicotiana benthamiana plants and transient expression. We demonstrated that a nanocarrier with 20 nm width, 80 nm length, and a polymer-to-DNA ratio of N/P = 3.0 afforded the most efficient DNA delivery and expression among the parameter space investigated. Additionally, we showed that polymer–DNA complexes with a moderate positive charge of ∼14 mV favored penetration through the cell wall and membranes with the assistance of cell wall degrading enzymes. Together, these results establish a narrow window of aspect ratios and charges of the nanocarrier–DNA complex that enables DNA delivery to plants using polymeric nanocarriers. This fundamental nanocarrier structure–function relationship informs the design of soft-material nanocarriers for nucleic acid delivery in plant cells to facilitate a wide range of plant biotechnology applications.

  PublisherDOI

• Temperature-Responsive Bottlebrush Polymers Deliver a Stress-Regulating Agent <i>In Vivo</i> for Prolonged Plant Heat Stress Mitigation

  ACS Sustainable Chemistry & Engineering · 2023-02-14 · 25 citations

  articleOpen accessCorresponding

  Anticipated increases in the frequency and intensity of extreme temperatures will damage crops. Methods that efficiently deliver stress-regulating agents to crops can mitigate these effects. Here, we describe high aspect ratio polymer bottlebrushes for temperature-controlled agent delivery in plants. The foliar-applied bottlebrush polymers had near complete uptake into the leaf and resided in both the apoplastic regions of the leaf mesophyll and in cells surrounding the vasculature. Elevated temperature enhanced the in vivo release of spermidine (a stress-regulating agent) from the bottlebrushes, promoting tomato plant (Solanum lycopersicum) photosynthesis under heat and light stress. The bottlebrushes continued to provide protection against heat stress for at least 15 days after foliar application, whereas free spermidine did not. About 30% of the ∼80 nm short and ∼300 nm long bottlebrushes entered the phloem and moved to other plant organs, enabling heat-activated release of plant protection agents in phloem. These results indicate the ability of the polymer bottlebrushes to release encapsulated stress relief agents when triggered by heat to provide long-term protection to plants and the potential to manage plant phloem pathogens. Overall, this temperature-responsive delivery platform provides a new tool for protecting plants against climate-induced damage and yield loss.

  PublisherDOI

• Charge, Aspect Ratio, and Plant Species Affect Uptake Efficiency and Translocation of Polymeric Agrochemical Nanocarriers

  Environmental Science & Technology · 2023-05-25 · 67 citations

  articleOpen accessCorresponding

  An incomplete understanding of how agrochemical nanocarrier properties affect their uptake and translocation in plants limits their application for promoting sustainable agriculture. Herein, we investigated how the nanocarrier aspect ratio and charge affect uptake and translocation in monocot wheat (Triticum aestivum) and dicot tomato (Solanum lycopersicum) after foliar application. Leaf uptake and distribution to plant organs were quantified for polymer nanocarriers with the same diameter (∼10 nm) but different aspect ratios (low (L), medium (M), and high (H), 10–300 nm long) and charges (−50 to +15 mV). In tomato, anionic nanocarrier translocation (20.7 ± 6.7 wt %) was higher than for cationic nanocarriers (13.3 ± 4.1 wt %). In wheat, only anionic nanocarriers were transported (8.7 ± 3.8 wt %). Both low and high aspect ratio polymers translocated in tomato, but the longest nanocarrier did not translocate in wheat, suggesting a phloem transport size cutoff. Differences in translocation correlated with leaf uptake and interactions with mesophyll cells. The positive charge decreases nanocarrier penetration through the leaf epidermis and promotes uptake into mesophyll cells, decreasing apoplastic transport and phloem loading. These results suggest design parameters to provide agrochemical nanocarriers with rapid and complete leaf uptake and an ability to target agrochemicals to specific plant organs, with the potential to lower agrochemical use and the associated environmental impacts.

  PublisherOA PDFDOI

• Interaction of impinging marangoni fields

  Journal of Colloid and Interface Science · 2023-09-20 · 4 citations

  articleOpen accessSenior author

  HYPOTHESIS: Surface tension gradient driven Marangoni flows originating from multiple sources are important to many industrial and medical applications, but the theoretical literature focuses on single surfactant sources. Understanding how two spreading surfactant sources interact allows insights from single source experiments to be applied to multi-source applications. Two key features of multi-source spreading - source translation and source deformation - can be explained by transport modeling of a two-source system. MODELING: Numerical simulations of two oleic acid disks placed at varying initial separation distances on a glycerol subphase were performed using COMSOL Multiphysics and compared to spreading of a single surfactant source. FINDINGS: Interaction of two spreading sources can be split into three regimes: the independent regime - where each source is unaffected by the other, the interaction regime - where the presence of a second source alters one or more features of the spreading dynamics, and the quasi-one disk regime - where the two sources merge together. The translation of the sources, manifested as increasing separation distance between disk centers of mass, is driven by the flow fields within the subphase and the resultant surface deformation, while deformation of the sources occurs only once the surfactant fronts of the two sources meet.

  PublisherDOI

• Effect of polymer/surfactant complexation on diffusiophoresis of colloids in surfactant concentration gradients

  Journal of Colloid and Interface Science · 2023-03-24 · 14 citations

  articleOpen accessSenior authorCorresponding

  HYPOTHESIS: A concentration gradient of surfactants in the presence of polymers that non-covalently associate with surfactants will exhibit a continually varying distribution of complexes with different composition, charge, and size. Since diffusiophoresis of colloids suspended in a solute concentration gradient depends on the relaxation of the gradient and on the interactions between solutes and particles, polymer/surfactant complexation will alter the rate of diffusiophoresis driven by surfactant gradients relative to that observed in the same concentration gradient in the absence of polymers. EXPERIMENTS: A microfluidic device was used to measure diffusiophoresis of colloids suspended in solutions containing a gradient of sodium dodecylsulfate (SDS) in the presence or absence of a uniform concentration of Pluronic P123 poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) nonionic triblock copolymers. To interpret the effect of P123 on the rate of colloid diffusiophoresis, electrophoretic mobility and dynamic light scattering measurements of the colloid/solute systems were performed, and a numerical model was constructed to account for the effects of complexation on diffusiophoresis. FINDINGS: Polymer/surfactant complexation in solute gradients significantly enhanced diffusiophoretic transport of colloids. Large P123/SDS complexes formed at low SDS concentrations yielded low collective solute diffusion coefficients that prolonged the existence of strong concentration gradients relative to those without P123 to drive diffusiophoresis.

  PublisherDOI

Recent grants

• Marangoni Transport Synergism in Mixed Surfactant Systems

  NSF · $353k · 2017–2021

• Equilibrium and Dynamics of Polymer-Grafted Nanoparticles at Fluid Interfaces

  NSF · $337k · 2013–2017

• High Efficiency Nanoparticulate Emulsifiers

  NSF · $240k · 2007–2011

• Interfacial Activity of PEG-modified Proteins with Application to Sustained Release

  NSF · $294k · 2008–2012

• Friction Control by Adsorption of Polyelectrolyte-Grafted Nanoparticles

  NSF · $300k · 2011–2015

Frequent coauthors

• Gregory V. Lowry

  60 shared

• Michal G. Rose

  VA Connecticut Healthcare System

  54 shared

• Zaoli Jiang

  53 shared

• Edward Chu

  Montefiore Einstein Comprehensive Cancer Center

  53 shared

• Samuel So

  Stanford University

  52 shared

• Angeline Foo

  St. George Hospital

  52 shared

• Yun Yen

  51 shared

• Stephen Garoff

  Carnegie Mellon University

  37 shared

Labs

• Tilton GroupPI

  Complex fluid interfacial phenomena

Education

• B.S.

  University of Delaware

  1986

• M.S.

  Stanford University

  1987

• Ph.D.

  Stanford University

  1991

Awards & honors

• Victor K. LaMer Award for Outstanding Graduate Research in C…
• Fellow of the American Chemical Society
• Fellow of the American Institute for Medical and Biological…
• Excellence in Fluid Mechanics Research and Oral Presentation…
• Victor K. LaMer Award for Graduate Research in Colloid and S…